When it comes to spellcasting and CR, the Dungeon Master’s Guide leaves you on your own essentially, telling you only to make sure you take into account spells that increase the AC or hit points of a monster, and spells that do more damage than a normal attack from that monster. I struggled with getting this right for a while and having finally figured out a decent method, I want to share it in the hopes it will save other DMs time.
Getting Started. You’re going to want to go through your monster’s prepared spell list and make note of several things. For damage spells, note how much damage on average they each do per turn. If the spell is an area of effect spell, multiply the amount of damage it deals to each creature that fails its saving throw by 2. Note any spells that are cast as a bonus action, because it can cast these spells in addition to a cantrip or physical attack. Also note any spells with several hours or longer duration such as mage armor because the caster, if it can benefit from them, will probably keep them up. Spells with a duration of 1 minute to an hour will probably be cast before battle if the spellcaster is aware a battle might be imminent. Figure out which of these spells will be cast before the first round and deduct the appropriate number of spell slots of the appropriate levels from the monster’s total for the purposes of calculating how many spells of each level the monster can cast. Don’t deduct spell slots for things like mage armor if the monster’s AC is already higher than it would be with the spell. Lastly note any healing spells or spells that temporarily boost AC or hit points such as cure wounds or shield.
Physically Weak vs. Physically Strong Casters. If your caster is a human wizard with only a single quarterstaff attack, it’s not going to deal more damage on average over the first three rounds than casting a cantrip like fire bolt, so we can ignore the physical damage entirely. If your caster is something like an ancient red dragon, than it will probably deal more damage per turn with multiattack than many of its spells so unless it can cast high level spells it probably won’t see much of a change to its CR.
Damage Spells and CR. The most straightforward of spells as far as they influence CR are the damage spells. Now that you have noted the average damage per turn of the creature’s spells, look at the spells that deal more damage than the total of the creature’s physical attacks it can make on one turn. If the top damage spell can be cast three times and it is not weaker than the multiattack, use its damage as the base damage per turn. If it can only be cast once or twice, average it out with the next most powerful damage spell or two. For instance if it can cast its top damage spell (average damage = x) once and its second best damage spell twice (average damage = y) then its average damage per turn would be (x + 2y)/3. If it can only cast the second spell once and has to use a third spell, then just average the damage of the three spells, or if it deals more damage with its physical attacks than any of the spells substitute the total damage of physical attacks for one of the spells. Some damage spells of a low level deal more damage at higher levels that other spells of those levels. You may want to note how much more damage it does per spell level above the first.
Action Economy and CR. Some spells that influence CR will be ignored if the monster is powerful enough. Since creatures typically only get one action per turn, a spell like cure wounds that would supplant a chunk of damage output from the creature (though a more impactful spell like heal should be considered) will be ignored unless the creature is going to be low enough CR where the amount healed is 50% of its hit points or more. Bonus action spells for the most part can be used every turn, so they will almost always impact the CR, so you can typically count healing word for example unless the creature has a more useful bonus action that influences CR. Lastly take into account reaction spells like shield if the creature doesn’t have a better reaction ability. Ignore the possibility of opportunity attacks.
Example 1. The mage NPC from the Monster Manual has the following damage spells (its average damage for each is in parentheses): fire bolt (11), magic missile (10 @ 1st level, +3 per spell level), fireball (56 @ 3rd level, +7 per spell level), ice storm (46 @ 4th level, +9 per spell level), and cone of cold (72 @ 5th level, +9 per spell level). The cone of cold deals 2 more damage at 5th level than the fireball, but fireball deals more damage at 4th level than ice storm, so we assume the mage will cast cone of cold once and fireball twice since its attack only deals 4 damage per turn. On the first round it deals 72 damage, on the second and third it deals 63 each, so the average is 66 damage per round. The mage’s only other spells that influence CR are fly, mage armor, misty step, and shield. We’re going to assume the mage has mage armor up already and casts shield once per round as a reaction which neatly uses up its 1st level spell slots. For mage armor, we just make the mage’s base AC 15, and for shield I adjust the base AC by 2 (or for a similar spell or effect that gives a brief bonus to AC you could take the bonus and divide by 2). If we assume the mage knows battle is about to happen and casts fly on itself before the first round, its AC is adjusted by 2 if it is meant to face characters of level 10 or less. Lastly, it can use misty step as a bonus action to get away from melee attackers without attacks of opportunity, but since it is already flying, it is more likely to use misty step to escape if it loses too many hit points. For this reason, I’m not going to adjust anything for misty step. My calculations put the mage at a CR of 5, shy of its CR 6 from the Monster Manual. The reason for this is many of the Monster Manual stat blocks have a CR that is adjusted based on playtesting. In my own experience, however, the mage’s low hit points mean that it can probably be killed in one round by a party of level 6 characters. By comparison, the flameskull which is far more difficult with its immunities and resistances (including magic resistance) is only a CR 4 despite the same number of hit points. In my opinion if there is any adjusting to do, the flameskull should be CR 6 and the mage CR 4. With a pure calculation using this method, both are CR 5. I think the reason for this is that WotC’s method for calculating CR overvalues damage and undervalues defenses at low challenge ratings, ignoring the fact that many creatures are weak enough to die in the first round against an appropriate level party of PCs.
Example 2. The priest NPC from the Monster Manual has the following damage spells (its average damage for each is in parentheses): sacred flame (9) and guiding bolt (14 @ level 1, +3 per spell level). Spiritual weapon can be used as a bonus action (7 @ level 2, +7 per two spell levels). We can assume spirit guardians (27 @ level 3, +9 per spell level) is cast before combat if the priest is aware of an incoming threat. The optimal method of attack would be sacred flame and spiritual weapon on turn one, a third level guiding bolt on turn two, and a second level guiding bolt on turn three, with the damage from spirit guardians on all three rounds, and an attack from spiritual weapon on turns two and three. The priest deals an average of 43 damage on turn one, 54 damage on turn two, and 51 on turn three for a three-round average of 49. The other spell that the priest may use is sanctuary, which uses a bonus action, however its best course of action is probably to use spiritual weapon. If the priest uses sanctuary in place of spiritual weapon, I’d reduce the average damage by 7 and increase AC by 2. The result is a CR of 3 either way, so it really doesn’t matter. The stat block gives the priest a CR of 2, which again, is due to the CR calculation method being less accurate for creatures with CR of 10 or less because of how quickly they can be killed regardless of how much damage they do. WotC adjusted many of these CRs after playtesting, but during playtesting people are going to run creatures less than optimally and it’s a slippery slope if you adjust CR based on that because someone who runs the NPC optimally might slaughter a group of appropriate level and wonder what happened. Similarly, if your group encounters a group of 5 priests and you use the CR in the MM, the encounter challenge rating is 9. If you use my calculation method, the encounter CR is 12, a significant difference. I’m not recommending you recalculate all the CRs in the Monster Manual, but if you create your own monster don’t worry about adjusting CR after playtesting. However, if you encounter a lot of issues with one of the monsters in the MM (*cough* flameskull) feel free to recalculate it. I recommend you err on the side of the CR being one too high than being one too low.
I plan on adding a list of spells and their effect on CR on this site, but it will be on a page rather than a blog post to make it easy to find as a resource.