Approximately how much damage should a concentration spell do each round to be balanced against instantaneous damage spells?

Question

The DMG (p. 283) provides a table for approximate damage for single target and multiple target spells by level:

Spell Level	One Target	Multiple Targets
Cantrip	1d10	1d6
1st	2d10	2d6
2nd	3d10	4d6
3rd	5d10	6d6
4th	6d10	7d6
5th	8d10	8d6
6th	10d10	11d6
7th	11d10	12d6
8th	12d10	13d6
9th	15d10	14d6

These are one-time damage figures from an attack or blast. The DMG further indicates that these attacks assume saving for half damage. If a saving throw (or missed spell attack?) leads to no damage, numbers can be increased by 25%.

My question is, what are appropriate numbers for a spell that can do additional damage in subsequent rounds, like moonbeam or Bigby's hand.

For example, a 2nd level spell should do approximately 3d10 (17) damage against a single target or 4d6 (14) against multiple targets. Moonbeam is a Concentration spell lasting for 1 minute (10 rounds), and can do 2d10 (11) damage to multiple targets (although the 5 foot radius means catching multiple targets in the area of effect is probably uncommon).

Certainly there are other spells in this category, and I'm not suggesting moonbeam is typical. But there seem to be a large number of spells with a duration of Concentration, up to 1 minute that do damage without requiring an action--either using a bonus action such as heat metal or spiritual weapon, or because they create an environmental effect, such as hunger of Hadar. What amount of damage would be typical or balanced for such concentration spells at each spell level?

Answer 1

There is no direct advice in the books

So we have to derive it from other parts of D&D.

A combat is 3 rounds

Now this isn't actually true; combats are often longer. But the impact of damage-later is usually less than damage-earlier; by taking the average over 3 rounds of combat, you get a good approximation of how much a "feature" is worth.

To back this up, this is how the DMG advises you balance monster damage.

Action value scales with level

For a given action, a character can either use an at-will resource, or burn a non-at-will resource.

Spellcasters at-will resource is typically a cantrip; in some cases, it is a weapon attack. If you do a comparison of the two, they tend to (very roughly) scale with character level.

When you have an concentration spell that requires burning an action each round, the value the spell provides is the difference between the spell's action value and the at-will value you could have gotten without spending any spell slots.

Using Firebolt as our baseline, this is 1d10 damage per tier (miss for nothing); compared to normal spells, this is 4.5 levels 1-4 (1st - 2nd level spells), 9 levels 5-10 (3rd-5th level spells), 13.5 levels 11-16 (6th-8th level spells), and 18 levels 17+ (9th level spells).

This will underestimate the value of the action. Because by level 17, you have enough low level spells that the actual comparison is either using your action on your concentration spell, or casting a low level spell (of which you have plenty). But at least it doesn't discount it completely.

But first, let's ignore the multi-target and subtract that "baseline damage".

Spell Level	One Target
1st	6.5 + 4.5
2nd	12 + 4.5
3rd	18.5 + 9
4th	24 + 9
5th	35 + 9
6th	41.5 + 13.5
7th	47 + 13.5
8th	52.5 + 13.5
9th	64.5 + 18

We then spread it over 3 rounds.

Spell Level	One Target	Per Round Damage
1st	6.5 + 4.5	6.7 = 2.2 + 4.5
2nd	12 + 4.5	8.5 = 3 + 4.5
3rd	18.5 + 9	15.2 = 6.2 + 9
4th	24 + 9	17 = 8 + 9
5th	35 + 9	20.7 = 11.7 + 9
6th	41.5 + 13.5	27.3 = 13.8 + 13.5
7th	47 + 13.5	29.2 = 15.7 + 13.5
8th	52.5 + 13.5	31 = 17.5 + 13.5
9th	64.5 + 18	39.8 = 21.8 + 18

Multiple target damage is very similar to single target damage if you assume the spell hits 1.5 foes; or, rather, assume it hits 2 foes on average, and damage on the 2nd foe is worth half as much as damage on the primary target. (The ratio varies)

If you treat moonbeam this way (and you should), its equivalent single-target damage is 16.5. This is closer to a 3rd level spell.

Concentration is another resource that these spell use. We could simply give concentration spells a 1 level bump; a 3 round 3rd level spell should do 15 damage per round, and moonbeam is pretty close to that.

For spells that don't use your action to repeat their damage, we don't get the boost from the damage you forgo from your action.

We can neglect the cost of the bonus action per round to keep things simple (most spells should have that cost).

Let's double check.

Test with actual spells.

Mordenkainen’s Sword is considered an awful spell. It does 3d10 damage, 0 on a miss, and requires concentration and a single action to cast. So its damage budget over 3 rounds is 64.5 + 18 / 3 or 27.8 × 1.25 = 34.8 damage per round. Instead it does 16.5. So Mordenkainen's Sword evaluates as really bad.

Sunbeam is considered a good spell. It is 6th level, does 6d8 (27) and blinded to multiple targets, is concentration, requires an action per turn, and does half damage on a successful save.

As multiple target spell, that is 40.5 single-target equivalent damage. My table above says a concentration action-per-round spell 6th level spell should do 29.2 single-target equivalent damage. So sunbeam is above par.

Summon Fiend is a recent 6th level spell. It does 3 attacks of ~16 damage per attack, 0 on a miss. It requires an action when you cast it. As a 6th level concentration spell, it has a single target 3 round with 0 damage on a miss damage budget of 36.5; it actually deals 48. Which makes it an above par spell.

Storm Sphere is a 4th level concentration spell. It does an initial 2d6 AOE damage (0 on a miss), then you can use a bonus action to deal 4d6 each turn on one target (0 on a miss, advantage first turn). (Foes could choose to stand in the storm and boost these values; I assume they won't.)

A 4th level 0 on a miss single target damage budget is 67.5. Storm Sphere does 7 × 1.5 = 10.5 initial, plus 54 over 3 rounds, for a total of 64. Pretty close.

Spirit Guardians is a 3rd level concentration spell that requires 1 action to deal damage. It does 3d8 multi-target damage with a save for half. So its single-target-equivalent damage budget is 60.75. Its one-target damage budget is 44, which makes it an above-par spell.

Probably more work needs to be done, but the general idea of "actions have value", "slots have value", "measure damage over 3 rounds" and "concentration has value (and risk)" I used above should give you a good ballpark to start in. And when looking at existing spells, really good ones tend to be at the top end of the range, and poor ones well under it.

Where the Rubber meets the Road

Note that the budget I assigned to "value of an action" is going to be rather low for any game with a 5 minute adventuring day. The spellcaster isn't likely to ever use cantrips, so cantrips are not the cost of the action.

If you expect 3 rounds per fight that "matter" over 8 fights per day, that is 24 rounds. A 11th level wizard has 16 spell slots and arcane recovery for another 1 or 2 slots. They cannot cast a different leveled spell every round of combat; so they may want a concentration spell that more efficiently uses their action than a cantrip.

The same wizard with 1 fight with 6 rounds that "matter" will want to use every round that matters in casting a spell with a level; 1 6th, 2 5th and 3 4th level slots. The opportunity cost for the action using a concentration spell repeatedly is higher for this wizard.

As you are working on spells for your home game, you should take into account (a) you don't want to write a spell that nobody will use, and (b) if you have 5 minute adventuring days, you should have a higher opportunity cost for using an action to fuel a spell.

Answer 2

Damage appropriate to the cost

For instantaneous spells that do damage, the cost is the expended Spell Slot (except for Cantrips) and the use of an Action (or sometimes a Bonus Action or Reaction). Note that the damage figures given are for 'pure' damage spells; spells that do something else as well usually do less damage - this should equally apply to ongoing effects.

If the ongoing effect has the same cost, then it should do about the same damage.

Costs and other factors

Spell slots: AFAIK, no ongoing effect spell consumes further spell slots so as a basic rule, they should do less damage than an instantaneous spell.

Concentration: This is a limited resource so requiring its use should allow the spell to do more damage than a spell that does not require concentration.

Save to end: Spells that allow a save to end the effect should do more damage than spells that will definitely last for the duration.

Subsequent attack rolls: A spell that requires an attack (or saving throw) on each subsequent turn should do more damage than one where the damage is automatic.

Uses an action: A spell that requires the use of your action on subsequent turns should do more damage than one that uses a bonus action which should do more than one that uses a reaction which should do more than one that uses no actions. Note that the cost of a bonus action depends on what else could be done with it - Rogues, Sorcerers, Fighters and Barbarians usually have bonus action options, Clerics usually don't: a Cleric spell that requires a bonus action is better than a Sorcerer spell that does.

Mobility: A spell whose effect moves automagically is better than one that requires an action to move which is better than one that is fixed. Similarly, a spell that can be moved 60 feet is better than one that can be moved 20. They should therefore increase in damage as the flexibility decreases.

Battlefield control: A spell that gives the target a choice of whether to take damage or suffer some other limitation (e.g. being unable to move) should do more damage.

Duration: A longer spell should do less damage. However, note that all spells have an effective duration of 1 round, one combat or multiple combats. A duration of 1 minute is effectively this combat only, durations of an hour or more cover multiple combats but they are usually a class' signature spell (e.g. Hex for the Warlock, Hunter's Mark for the Ranger) and these need special consideration.

Each spell requires its own judgement call

Once you've decided what the spell does, you can establish a damage range. If it's too little you can add more limitations and raise the damage, or vice-versa.

Then you playtest. Then you refine. Then you playtest again. Repeat as often as necessary.

Answer 3

The main factor to consider would be how long the Concentration spell would last; 2 rounds? 3 rounds? 10 rounds?

Let's take a Hypothetical, and try and work through it (warning: terrible math incoming)

Let's take the Fireball spell, and make it a Concentration spell.

A target takes 8d6 fire damage on a failed save, or half as much damage on a successful one.

So we're dealing with a spell that deals a base level of 8d6 (average of 24) damage in one round. If we were to turn it into a spell with a duration, it would have to depend on how long the spell lasts. If we said 4 rounds, then it would deal 2d6 per round, in order to be "balanced".

Some other things to consider is the potential that the spell can be interrupted. So you could buff it a bit in case it is cut short. Perhaps deal 3d6 per round, for up to 4 rounds, dealing a total of 12d6 (average of 36 damage). The other potential, since we're using the Fireball spell as a template, it allows the target(s) to make a saving throw to take half damage. This is pretty balanced on its own, since anyone within the template of the spell would automatically have to use an action to dodge, or take full damage.

So, all things considered (I've made some other adjustments to work with a cone instead of a sphere, etc):

Flamethrower

Level 3 evocation
Casting Time: 1 action
Range: 20 feet
Components: V S M (A tiny ball of bat guano and sulfur)
Duration: 4 rounds (Concentration)

A bright streak flashes from your pointing finger to a point you choose within range and then blossoms with a low roar into an explosion of flame. Once per round while the spell remains active, each creature in a 20-foot-range cone centered on your target must make a Dexterity saving throw. A target takes 3d6 fire damage on a failed save, or half as much damage on a successful one. The fire does not spread around corners. It ignites flammable objects in the area that aren’t being worn or carried.