Spell Design (5e Guideline)
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Pages 283 and 284 of the Dungeon Master's Guide provide some basic advice for creating spells. This page builds on those guidelines and more thoroughly analyzes them. Many standards set on this page are based on SRD spells.
This page assumes you understand the content on the pages 5e SRD:About Spells and 5e SRD:Casting a Spell. It is also assumed you are generally familiar with how spells work in 5th edition (e.g., no "caster level"; concentration is a thing; cantrips scale with your character level and not your class level; etc).
- 1 Fundamental considerations
- 2 Class identity
- 3 Saving Throws and Attack Rolls
- 4 Balancing
- 5 Casting ability
Whenever possible, look for an SRD spell to use as a guideline. Chances are there is at least one spell that has something in common with what you have in mind.
In practice, simpler is generally better. A spell with a simpler effect is easier to remember without reference, and is generally easier to use. Complex spells aren't inherently bad, of course, but complexity for the sake of complexity is usually undesirable.
When creating a spell for existing classes (as opposed to a homebrew class you are making), take into consideration the style of spells available to that class. Clerics, for instance, don't go around casting explosive fireballs or arcane magic, just as wizards and sorcerers don't wield spells based on divinity or healing wounds. Summaries of each class's spellcasting identity are listed below.
Artificer spells tend to focus on item iteration, study of the arcane, and enhancing either their ability to do things or their allies ability to do things. They tend to have very little spells which directly aid in combat via healing or by dealing damage.
Bard spells are usually subtle and focus on interaction far more than the spells of other classes. Few bard spells are blatantly harmful to their targets, but those that inflict damage usually inflict either thunder damage, psychic damage, or damage based on a weapon. They notably lack any spells which strongly invoke elements like ice and fire, or any spells that conjure creatures—such spells would likely clash with the bard motif.
While bards can cast protective and healing spells, the best spells in this domain are usually reserved for clerics or paladins.
Clerics spells tend to focus on healing, exploration, and protection, with some emphasis being put on spells that relate to death and the divine. The best restorative spells are usually reserved for clerics.
Overall the class has very few damaging spells. Those which do inflict damage are usually less powerful than those available to warlocks and wizards, either because they have less range, affect a fewer number of creatures, or deal less damage outright. The few damaging spells the cleric can cast tend to deal fire, radiant, or necrotic damage.
In many ways, druid spells can be the most versatile of any spellcasting class. They can cast healing and protective spells nearly as well as a cleric, and have offensive options that are nearly as good as a wizard or sorcerer. However, the best of these spells are usually reserved for those classes. Druids have access to many exploration spells as well, but are most noticeably lacking in interaction spells—unless interacting with animals.
Although druid spells can be offensive, those that deal damage almost always do so over time. They dabble in spells that inflict cold, fire, and poison damage, but the most explosive ones are reserved for sorcerers and wizards. A druid can cast flaming sphere for example but not the more powerful fireball. In the same vein, a druid can cast healing spells but doesn't have something as potent as revivify.
Almost all spells available to druids invoke the natural world—through weather, water, plants, animals, or more esoteric phenomenon like moonlight or geological events.
Although it has as many spell slots as a ranger, a paladin's spells are noticeably more potent than those of its counterpart. It has several damaging options, casts protective spells, and can cast many healing spells like cure wounds, but notably has very few exploration and interaction spells. What interaction and exploration spells are available usually fit into the paladin motif of truth, purity, heroism, and justice.
Perhaps most defining of paladin-exclusive spells is that they often protect or empower allies that are physically close to the paladin, such as within 30 feet. Paladins also wield many exclusive spells that can be cast as bonus actions, and empower the first weapon attack they make that hits within the duration—these spells usually have "smite" in their name, and the Player's Handbook contains many such spells. Since smite spells require concentration, a paladin can't benefit from more than one on a given attack.
Paladin spells have a very short range, usually no more than 30 feet, or only activate when the paladin hits with a weapon attack. Compared to other spellcasting classes, paladins are very purposefully designed and encouraged to engage in melee combat instead of fighting at range.
When a paladin spell inflicts damage, it is usually radiant damage. They can dabble in other types, however, such as necrotic, psychic, thunder, and even force.
Like druids, ranger spells often invoke the natural world, but they often do so in a manner more similar to a huntsman than a naturalist. More so than any other class, ranger spells tend to be subtle. Even their attack spells don't come across as flashy, and are usually derived from weapons the ranger wields—such as hunter's mark or conjure volley, which can be found in the Player's Handbook.
A ranger spell should virtually never be flashy or invoke obtusely magical phenomenon like a fireball or a conjured elemental. Its spells are arguably the least "magic"-esque of all spellcasters in the SRD, as evidenced by the fact that it is the only one unable to learn spells available to literally every other spellcasting class, such as dispel magic.
Ranger spells often deal with exploration (though not with teleportation); and tracking animals, objects, or people. Several of their spells often deal with avoiding being tracked or detected, such as nondetection and silence, but they wield nothing as obtuse as invisibility.
Sorcerer spells are largely defined by wizard spells. The two classes share most spells, and the few that aren't shared between them are exclusive to wizards.
If a spell is exclusive to sorcerers and not available to wizards, it will likely pertain to the class features or archetypes of the sorcerer class.
Warlock spells are of special consideration, because unlike every other SRD class a warlock restores its spell slots after a short or long rest instead of only a long rest. Spells that could be abused if they can be cast too often, such as certain healing spells or spells with long durations, should not be made available to this class.
Fitting to its name, warlock spells are most often offensive or battle-oriented. Although they are not all blatant attacks, most of them can be used to gain the upper hand in a battle. Even when they are subtle, warlock spells are insidious—controlling the minds of others, invoking fear, or granting invisibility. The few spells a warlock wields that aren't harmful are usually spells that are common among spellcasters, such as dispel magic.
Aesthetically, warlock-exclusive spells often invoke the surreal, disturbing, or alien beings that could serve as the warlock's Otherworldly Patron. They may also invoke fey, or pertain to dreams, among other possibilities.
Wizards have perhaps the broadest domain of any spellcasting class. They have by far the widest variety of damaging spells, but also have protective spells, interaction spells, exploration spells, and more. Just about the only thing they can't do very well is restore hit points or alleviate negative conditions.
Wizard spells, particularly ones that inflict damage, are usually long range. Wizards are generally built to avoid melee combat.
Many wizard spells are also available to sorcerers. Virtually all battle-oriented spells are shared between them, though a wizard has access to some exploration and interaction spells as well. Those that aren't shared are usually either more subtle spells that encourage more long-term planning, such as alarm and guards and wards; or they deal with knowledge and intelligence, such as feeblemind and legend lore.
One important consideration of wizard spells is that in most campaigns, some of them can be cast by an eldritch knight (a fighter archetype in the Player's Handbook) or an arcane trickster (a rogue archetype also in the PHB). Specifically, EKs can cast spells of the abjuration and evocation schools up to 4th level, while ATs can cast spells of the enchantment and illusion schools up to 4th level. Because these archetypes' spell progression is so slow and their spell slots so limited, most spells should be fine for them. The only exceptions would be ones that overwhelmingly empower or augment the class features of these classes.
Saving Throws and Attack Rolls
Buffing or nonharmful spells like cure wounds and greater invisibility don't call for saving throws. By contrast, almost every single harmful spell either requires an attack roll to have any effect, or allows a saving throw to negate or mitigate the effect. The few that are harmful but always have an effect are usually damage spells, like magic missile, or aren't really all that harmful. The following sections list each type of attack roll or saving throw; for your harmful homebrew spell consider which of these best suits the spell you are creating.
Melee spell attack
Compare to spells like inflict wounds, flame blade, and shocking grasp. If you touch the target or hit it, the bad stuff happens. It doesn't matter how willful or resilient the target is. It will pretty much have the same effect as a goblin and an ancient red dragon. A minority of spells use this, and those that do usually imitate magic weapons or are directly damaging spells like inflict wounds. Clerics and druids have these, but fancier casters like sorcerers and wizards almost always have ranged spell attacks instead.
Ranged spell attack
See spells like chill touch, chromatic orb, guiding bolt, and ray of enfeeblement. Most often these spells involve sending a visible ray, orb, bolt, or another projectile of energy to hit the target from a distance. Like melee spell attacks, spells in this category primarily deal damage. If the projectile explodes or otherwise covers an area, it would probably be a Dexterity saving throw instead.
Obviously enough, if they can mitigate the effect primarily by being strong or stalwart, this is your go-to. In the SRD Strength saves are made primarily to avoid being physically knocked prone, avoid being grappled, and avoid being flung across the battlefield. If an effect is instead mitigated by the strength of one's internal composition, such as mitigating a poison or avoiding fatigue, a Constitution save might be better.
Generally, Strength is used for, "Opposing a force that would physically move or bind you."
See spells like fireball, prismatic spray, and faerie fire. Dexterity saves are also very commonly imposed by the breath attacks of dragons and dragonborns. More often than not, a Dexterity save represents an explosion of fire, lightning, cold, or other magic that bursts in a wide area that no one in that area could realistically avoid. Succeeding on a Dexterity save in this instance represents covering one's face, hiding behind a shield, or taking as much cover as possible. Fittingly, a successful Dexterity save only halves the damage, and anyone who fails it takes the full damage.
Of course, Dexterity saves can also be imposed from spells that agility or a quick reflex could mitigate. Although a Strength save is made to avoid being physically shoved prone, a Dexterity save could be used to navigate slippery terrain without falling prone (see the grease spell).
As a guideline, Dexterity is used for, "Dodging out of harm's way."
Constitution saving throw
Consider spells like blindness/deafness, finger of death, levitate, poison spray, symbol, and thunderwave. Constitution saves are possibly the most common saving throw imposed by spells. Usually any spell that can only be overcome with fortitude, a strong immune system, a healthy body, or just pure gusto will be a Constitution save. Spells which deal cold damage, poison damage, or to a lesser extent necrotic damage often impose this kind of saving throw. Debilitating physical conditions such as poison, blindness, deafness, other mitigated senses, or restricted body parts often rely on a successful Constitution save to end them.
An important consideration is how much the spell affects a creature made of stone, or a mindless but sturdy creature like a cockroach. If these kinds of creatures seem like they might be particularly resistant to the spell, then a Constitution save is probably your best bet. Otherwise, you may want to consider a Wisdom saving throw, or possibly a Dexterity saving throw.
At its core, Constitution saving throws are used for, "Enduring a disease, poison, or other hazard that saps vitality."
Intelligence saving throw
Feeblemind is the only SRD spell which calls for this kind of save, though detect thoughts calls for an Intelligence check. Psychic effects could call for this kind of save, as could any effect that relies on intelligence as opposed to willpower.
Since Intelligence is a numerical value of a creature's intellect, it should generally be used for, "Disbelieving certain illusions and resisting mental assaults that can be refuted with logic, sharp memory, or both. It should also be used whenever an effect can mess with how a creature's brain works."
See spells like charm person, dominate monster, fear, hold monster, polymorph, and scrying. Spells which affect the mind or consciousness almost always call for a Wisdom save. It is one of the most common types of saves from spells.
Wisdom is used for, "Resisting effects that charm, frighten, or otherwise assault your willpower."
See spells like bane, banishment, calm emotions, dispel evil and good, divine word, hallow, and zone of truth. Charisma saves aren't terribly common, but they cover a few distinct areas that Wisdom spells otherwise would. Any spell dealing with interplanar travel, whether it is forcing it or restricting it, usually calls for a Charisma save. Anything which primarily limits or restrict a creature's capability to express itself (including deception or intimidation) likely calls for a Charisma save. Some offensive cleric spells may call for Charisma saves as well, especially those which deal with divinity or repelling undead.
The most fundamental aspect of balancing a spell is determining an appropriate level. You never want to make a spell so good that someone would be a fool to not learn it over other options.
For example, if you want to create a spell that changes the appearance of any willing creature, it should probably be higher in level than disguise self and alter self. These spells can only affect you, the caster, and can change your appearance. A spell that could change the appearance of any willing creature for a similar duration is inherently more useful than either of these spells. Thus, it should probably be a higher level, or have some kind of drawback, so it isn't always an inherently better option than either of those spells.
Save vs Attack Roll
Another important aspect in determining a spell's power is what save is made against it. Monsters and NPCs typically have strong bonuses to Dexterity and Wisdom saving throws, followed closely by Constitution and Strength, and tend to only rarely have bonuses for Charisma or Intelligence saving throws. Player characters, meanwhile, will typically have good bonuses to Constitution and Dexterity Saving Throws, followed by Wisdom, Strength, and Charisma saving throws, and usually not have strong bonuses to Intelligence Saving Throws unless they are an Intelligence-based Caster, in which case they usually have poor Strength saving throws. This is because, at the point of generation, Constitution and Dexterity are the two most influential stats for all characters, because Constitution determines your hitpoints and Dexterity determines your AC and your Initiative bonus, which are typically considered valuable to players with level 1 characters. Wisdom is third because it determines Passive Perception as well as normal Perception, which makes it very useful to all classes, and is additionally the casting statistic for Clerics, Rangers, and Druids, and determines Ki saves for Monks. Strength is third because it is the stat for dealing damage with all non-finesse weapons and influences carrying capacity (and grappling), and is widely available as a proficiency bonus. Charisma is fourth because, while it doesn't often determine damage (unless you are a Warlock), it is the casting stat for four classes. Finally, Intelligence is last because it typically is not extremely strong except in Intelligence-based casters, of which there are only two.
Defining the Save helps decide your spell's role and balance. A spell that uses an Intelligence saving throw tends to be much less powerful if it is primarily designed to target Wizards, like an Intelligence Save against a Counterspell effect, but is much more powerful if it is designed to target Barbarians and other frontliners, such as an Int Save against a melee or strength debuff. Thus, the effects can be stronger if it targets wizards and should be weaker if it targets frontliners.
Finally, using a spell attack roll means that a spell can critically hit and deal twice damage, but can also miss against high AC. They are typically more useful against minions and miniboss monsters with lower ACs, and conversely against very high level boss monsters with Legendary Resistance.
|Cantrip||—||5½ (1d10)||—||3½ (1d6)|
|1st||11 (2d10)||13 ¾ (3d8)||7 (2d6)||8¾ (2d8)|
|2nd||16½ (3d10)||20 ⅝ (6d6)||14 (4d6)||17½ (5d6)|
|3rd||27½ (5d10)||33 ¾ (6d10)||21 (6d6)||26¼ (4d12)|
|4th||33 (6d10)||41¼ (10d8)||23½ (7d6)||29 ⅜ (8d6)|
|5th||44 (8d10)||55 (10d10)||28 (8d6)||35 (10d6)|
|6th||55 (10d10)||68 ¾ (15d8)||38½ (11d6)||48 ⅛ (14d6)|
|7th||60½ (11d10)||75 ⅝ (17d8)||42 (12d6)||52½ (15d6)|
|8th||66 (12d10)||82½ (15d10)||45½ (13d6)||56 ⅞ (16d6)|
|9th||82½ (15d10)||103⅛ (16d12)||49 (14d6)||61¼ (11d10)|
The adjacent table is derived from material in the Dungeon Master's Guide. If a spell has a cast time of "1 action" and a duration of Instantaneous, it should generally do damage according to its level according to the adjacent table. A spell with significant drawbacks can get away with doing a little more than the recommended damage, whereas one with powerful effects in addition to damage should probably deal less damage.
For each damage column, the first value represents ideal average damage for a spell of the given level. The value in parenthesis shows damage dice that, on average, will be equal or nearly equal to the first value. You can generally substitute any damage dice that will yield a similar average without significantly changing the spell's balance.
Note that even in the SRD, some spells defy these suggestions. This is most notable with 3rd level and 9th level spells available to sorcerers and wizards. Most classes get a boost to damage at 5th level; where many classes get an Extra Attack feature at this point, sorcerers and wizards instead get a few unusually powerful 3rd level spells like fireball. Meanwhile, these class's 9th level spells (see meteor swarm and wish) seem to follow completely different rules. When making spells for these classes at these levels, one may go a bit beyond the norms, but on D&D Wiki, homebrew spells that are greater than or equal to fireball and meteor swarm are frowned upon.
Damage over time
There are spells which can deal damage once every turn for a number of turns, and these can come in one of several different varieties. The damage dealt by these spells is still based on the damage table above.
- Stationary hazard. With this kind of spell, you create a cloud or other hazard that is fixed in space. Some examples of these are cloudkill, wall of fire, and incendiary cloud. Usually these spells require concentration. An important aspect of this spell is that it is fixed in space, so a creature will only be subject to the damage again if it doesn't remove itself from the hazard. Generally this type of spell should use the same damage as an instantaneous spell. A hazard which covers a large area or a very nuanced area might be better dealing a little less damage.
- Repeating action. This kind of spell lets you use your action to deal damage on your turn when you cast it, and lets you repeat that action on subsequent turns for the duration. Usually these spells require concentration. This kind of spell should deal half as much damage as a spell for its level, according to the Damage Table. A good example of this is flame blade. The flame blade spell lasts for a while, but each attack with it only deals an average damage of 10.5. Referencing the adjacent table, we can figure out a 2nd-level spell with a single target that does nothing on a failed attack roll should deal 21 damage. Of course, half of 21 is 10.5, meaning this is a virtually perfect amount of damage for the spell.
- Second hit. This kind of spell deals ⅔ of its damage instantaneously, then ⅓ of its damage at the end of the target's next turn. In the SRD, this kind of spell deals approximately as much damage as an instantaneous spell, albeit with the damage split between the two "hits." A good example of this is acid arrow.
- Repeating bonus action. This kind of spell is similar to repeating action, but on subsequent turns only requires a bonus action to use the effect again. Some examples include the spells arcane sword and flaming sphere. Based on these two spells, one can see at lower levels this kind of spell will deal about half as much as the recommended damage, whereas at high levels it may only deal ⅕. This difference is largely due to the fact player-characters' actions are designed to scale in damage much more rapidly than their bonus actions. As a rough guideline, this spell should deal ½ as much recommended at 1st or 2nd spell level; ⅓ as much at 3rd and 4th; ¼ as much at 5th and 6th; and ⅕ at levels higher than 6th.
- Automatic damage. A more powerful damage-over-time spell involves an action to cast the spell, and only requires concentration to deal damage on subsequent turns. Unless the spell level is very high, or the damage is very low, this kind of spell should afford the target(s) a saving throw every round to end the damaging effect. There are no spells in the SRD which fall into this category, and the only one in the Player's Handbook which does is searing smite, which only deals 1d6 damage and can be ended by any creature that uses its action to douse the fire. Suffice to say any spell which does automatic damage over time should have very low damage output compared to other spells of similar level.
Paladins have an array of spells which deal damage, but follow a different and distinct pattern. Casting one of these "smite" spells only requires a bonus action, lasts for 1 minute with concentration; if you hit with a weapon attack while it lasts, the spell does extra damage on top of that weapon attack, and then usually the spell ends.
This kind of spell has numerous benefits. A bonus action cast instead of an action cast means it has much less weight on your action economy. Triggering only on a weapon attack that hits mean the spell slot is rarely if ever wasted, as even if you miss the attack roll the spell is still there waiting for your next hit. Because of all these benefits, a paladin's smite spell usually deals much less damage on a hit compared to a normal damaging spell cast by a sorcerer or warlock.
Based on the paladin's Divine Smite feature, we can see a guideline for a smite is 9 (2d8) damage at 1st level, and an extra 1d8 for each level after this, to a maximum of 22.5 (5d8) damage. All smite spells in the Player's Handbook deal less damage than this feature, but trade that extra damage for benefits such as knocking the creature prone or making it frightened.
Smite spells are intentionally designed to be low in level, and don't scale to the extent normal damage spells do. Weapon damage and spell damage both scale as player levels increase, and consequently giving a character full progression in both domains can result in an overpowered character. Even with 5th level spell slots, a paladin is unable to outright deal more than 5d8 damage with any smite, with the sole exception of banishing smite, which deals 5d10 instead.
Hit point restoration
The damage table isn't as fine-tuned for this purpose, but you can use it as a rough estimate of how many hit points should be restored by a healing spell. Healing spells towards the lower end should restore a little bit less than the table suggests (see cure wounds), while those towards the high end should restore a little bit more (see heal).
According to the Dungeon Master's Guide, a cantrip shouldn't offer healing.
As a general guideline, the longer a spell's effect lasts, the weaker the effect should be.
This is most notable with spells that impose debilitating conditions, such as poisoned or stunned. Almost every spell that imposes disadvantage on attack rolls, or advantage on attack rolls against the target (or a condition that does so) affords the target creature a free saving throw on each of its turns to end the effect. If a spell inflicts a condition like this and doesn't afford regular saving throws, it should generally be at least 6th level.
By comparison, a spell that reduces speed or restricts reactions isn't considered as debilitating. Such a spell might require the target use an action to attempt a saving throw to end the effect, or in extreme cases not even afford a saving throw.
Far more often than not, a spell with a duration of more than instantaneous calls for concentration. "Debuffing" spells, in particular, should virtually always call for concentration.
Almost every spell that isn't instantaneous lasts for one of the following time periods:
- 1 round (until the start of your next turn)
- 1 minute (one encounter)
- 10 minutes (one exploration period including one encounter)
- 1 hour (a quick dungeon crawl)
- 8 hours (practically all day)
- 24 hours (until you cast this spell again tomorrow)
- Permanent / Until dispelled
There are very few reasons to make a spell with a lasting effect which doesn't fall into one of these periods of time. Using a different increment of time will generally make the spell more difficult to remember accurately, and thus more difficult to use.
Range and area of effect
Spells with an unusually long range or a particularly wide area of effect can make up for lesser effects somewhat.
Likewise, a spell that is particularly strong for a class that has it probably should have a small range or only affect one target. This is perhaps most noticeable with inflict wounds; it is one of the most damaging spells for its level available to a cleric, but it has a range of only touch.
Areas of Effect on a Grid
There are three ways 5E suggests adjudicating areas of effect on a grid, none of which agree with each other.
- Xanathar's template method is the least compatible with the rules for measuring grid distances, because it uses genuine geometric shapes, which are not compatible with either the standard or the option 1/1.5 rules for measuring grid distances across diagonals.
- Xanathar's token method violates the PHB rules for some shapes using the standard grid measuring rules - for example, using the token method, a 15 foot cone goes only 10 feet diagonally. Likewise, by definition, Create Bonfire should be able to hit two creatures 5 feet apart, but it can't using the token method - you have to use the template rules to let it do that.
- The DMG rules for snapping an AOE to a grid intersection makes cones virtually impossible to adjudicate, partially because cones aren't "circular". Other spells can also be adversely affected; for example, Create Bonfire creates a 5-foot cube, which the DMG rules combined with the PHB rules means you drop a cube onto a grid intersection, and then because the spell hasn't got a circular shape, it can't affect partial squares, so the spell can't hit anyone.
Furthermore, an area of effect centered on the caster is by definition centered on a grid square, not a grid intersection, which means these spells result in very different shapes.
Also note that under the standard grid rules, an X-foot radius sphere is a 2X cube, since cubes are expressed as their side length, and spheres are cube-shaped in the standard 5E grid rules. This means the DMG guidance for guessing how many targets are hit by an AOE are inconsistent - an X sphere or 2X cube is expected to hit X/5 targets if you use the sphere rules or 2X/5 if you use the cube rules, even though both spells have the same "footprint".
So be aware, any area of effect spell you make may play very differently on different tables, and e.g. a 5-foot radius sphere centered on the caster (such as Sword Burst) will hit a wildly different number of grid squares (technically 9, since Sword Burst can hit creatures in the caster's space) from a 5-foot radius sphere otherwise (usually 4 squares).
Consider multiclass balance
In a typical campaign, cantrips and 1st-level spells are not inherently exclusive to one class. Some races such as elves can gain cantrips regardless of their class, and the Magic Initiate feat (PH, pg. 168) lets anyone pick up two cantrips and a 1st-level spell. Aside from this, a player-character is usually allowed to multiclass into a class like wizard or cleric to pick up cantrips and spells, which can augment their capabilities in another class.
For most spells, this isn't problematic, but it can sometimes be disastrous. For example, a wizard cantrip that improves the damage of your unarmed strike might seem balanced at first glance, but consider: a high elf can take that cantrip as a racial feature at 1st level, then start off in the monk class. A monk already has strong unarmed strikes, and can make more unarmed strikes per turn than other classes. This could easily snowball into an unintentionally overpowered option.
Ideally, every spell uses a casting ability in one subtle but important way. As a general guideline, it should be less vital than an ability score for weapon attacks. An attack with a longsword, for example, adds Strength to both the attack roll and the damage roll. By contrast, an attack with a fire bolt spell only adds casting ability to the attack roll.
Avoid overusing the casting ability. If a spell imposes a spell attack or a saving throw, then the casting ability has already been used once. It shouldn't be added again, such as to the spell's damage. Casting ability should never be so vital that it could alter the appropriate level of a spell; for example shield of faith uses a flat +2 bonus because the difference between a +0 and a +5 bonus to AC is far too large.
When feasible, you should also avoid under-using the casting ability. A Wisdom-based spell should be at least a little better if you cast it with 20 Wisdom instead of 7 Wisdom. Cure wounds doesn't have an attack roll or a saving throw, but it still makes use of casting ability in a way that is significant to that spell. It isn't always feasible to implement casting ability in a way that works well, however; this is especially true for nonharmful, nonhealing spells like mage armor and lesser restoration. It's better to not use a casting ability than to use it poorly.