This is part one of a series of guide-like posts on creating and modifying monsters in 5e D&D. This one covers calculating CR.

Creature Creation, Part 0: Introduction

Creature Creation, Part 2: Modifications

Encounter Difficulties

According to the Dungeon Master's Guide (DMG), there are four categories of combat encounters.

Easy encounters pose only minimal threat. At most, a couple party members might lose some HP. No significant resource expenditure--no short-rest/long-rest dependent resources or consumable items--unless the party has had many such since the last rest. A party should be able to handle 3-4 of these before needing a short rest, for a total of 8-10 per adventuring day without difficulty.

Medium encounters generally require the use of some resource--a spell slot or two, maybe a rage, maybe a healing potion. Still, no character should be in significant danger of being dropped to 0 HP outside of extreme bad luck (chain crits). An average party can handle 2-3 of these between each short rest, for 6-8 total in an adventuring day.

Hard encounters run a significant risk of dropping a character to 0 HP. The chance of actually killing a party member is slight, however. Either good tactics, very good luck, or significant resource expenditure will be needed. A party can usually take 1-2 per short rest at most, for 4-5 per adventuring day. Even then, the last few will be tight depending on luck and party composition.

Deadly encounters run a significant risk of killing one or more characters. Multiple characters will probably be reduced to 0 HP at least once, and there is a small chance of a TPK. More than 2 of these per adventuring day is pushing it.

To these four we might as well add Deadly++ (Boss) encounters. These are the end-of-campaign fights where the party must go all-out to kill or be killed. Having more than one of these per day (or having one of these at the end of a long day of regular fights) risks TPK (and severe WTF looks). These should be designed to allow players to dump everything into a damage nova.

What is CR and how do you calculate it?

Challenge Rating (CR) is a numerical indication of the approximate threat posed by an enemy. More precisely, it estimates how low of a level of a party it is an appropriate challenge for. A CR X creature is the strongest (most damage/health) creature that a party of 4 level X+1 PCs can expect to face in pairs without having a deadly encounter. In tiers 3 and 4 (levels 11+), this relationship actually shifts a bit--instead of level X+1 it becomes more like X+3

As a very imprecise measure, it mainly concentrates on two separate factors--offense and defense. A creature with a strong offense deals significant (relative to average PC health) damage per round; a creature with strong defense can last several rounds against an average party. As such, non-damaging abilities that do not increase survivability aren't factored into CR at all. The five factors that go into calculating CR are

1. Average Hit Points (HP)
2. Armor class (with a secondary contribution from proficient saving throws) (AC)
3. Average Damage per Round (DPR)
4. Attack bonus (or saving throw DC depending on main offense method) (ATK)
5. Traits or abilities that affect one of the first four factors. These show up as modifiers on the appropriate factor(s).

Calculating CR

CR is calculated as the mean of two parts: offensive CR (oCR) and defensive CR (dCR). In turn, oCR depends on DPR and ATK, while dCR depends on HP and AC.

Offensive CR

Calculating oCR depends on calculating DPR. This is the most fiddly and difficult part of the entire calculation. An important part is that this DPR represents a maximum average DPR, a ceiling on the damage output. Here are some rules:

• Plan out the first three rounds of a fight, trying to maximize the damage dealt.
• Assume all attacks hit and deal average damage.
• Include spells or other special abilities (like breath weapons) if they deal more damage than standard attacks. Assume that AoE abilities hit two targets and that both targets fail their saves and take average damage.
• Include all relevant traits (noted in the Traits section).

Once you have calculated the DPR, look at the table on page 274 of the DMG and find that DPR number in the Damage/Round column. The base oCR is the CR listed for that row. Also note the Attack Bonus and Save DC columns for that row--these are the guidelines for those numbers.

Next, consider how the creature attacks. If it's mostly physical, use the Attack Bonus number; otherwise use the save DC. Once adjusted for traits, this is the creature's ATK.

Take the difference between the creature's ATK and the guidelines and calculate the oCR adjustment = (ATK - guideline)/2. The final oCR = base + adjustment.

Defensive CR

The dCR is easier. First, adjust the HP and AC for defensive traits. Note that if the creature has three or more proficient saving throws, adjust the AC by +2; +4 for five or six proficient saving throws. Then find the adjusted HP of the monster in the DMG table and note the CR and the AC given for that row. Like the adjustment to oCR, the adjustment to dCR is given by half the difference--dCR adjustment = (AC - guidline)/2--and the final dCR by the sum of the base and the adjustment.

Final CR

The formal CR is calculated as (dCR + oCR)/2. What about rounding? The DMG says to round to the nearest CR step (usually integers), but I'm not so sure. The MM monsters suggest that there is more thought involved here--many of them are off from their calculated CRs by several steps.

A few considerations--
Damage output: Consider a weak player character (a wizard or sorcerer) of level = CR. Would a single round's damage output take such a character from full HP to 0 if everything connects? If so, the CR is too low and should be moved upward until that's no longer true. An example is the Ogre--it deals enough damage to KO a level 1 PC, so it's rounded up to CR 2. A rule of thumb is that for tier 1 or two, Level = CR + 1 is the earliest you could expect to run into multiple creatures of that CR and have a non-deadly fight. Since hard fights should (individually) have a low risk of permanent character death, a creature that can one-round KO a PC is significantly too dangerous (accounting for crits).

Abilities requiring high-level counters: Low level parties have few means of overcoming certain abilities. For example, immunity to low-level spells. Other examples include afflictions that permanently remove a character from play (like petrification) that require high-level spells (e.g. greater restoration) to remove.

Low Health: Because CR is an average, some monsters seem like a threat but aren't such in practice because they can be obliterated with ease. For example, the goblin has a calculated CR of 2 or more if it can hide consistently. On the other hand, it can't always do so and can be one-shot by most damage-focused level one characters on a good hit (7 HP ~ 1d8 + 2). This makes them glass cannons--able to hurt the party a bit but unlikely to survive very long. If a creature can be killed in a single round (or worse, attack) by a level appropriate PC (level CR + 1 or CR + 3 for T3/T4), especially if that wouldn't cost any resources beyond an action, its CR is probably too high. Adjust downward until this is no longer true.

For the first type (e.g. a rakshasa's immunity to spells of levels 5 or below), the minimum CR is in the appropriate tier (T3, in this case) and the other statistics can be adjusted to suit. The rakshasa is only a CR 5 threat without the magic immunity; with it, it's CR 13 to give magic users a chance against it. Another example would be the Archmage NPC--it's a CR 7 threat that is bumped up to CR 12 because of its globe of invulnerability spell (and the ability to cast 9th level spells--giving it a high level damaging spell drastically changes the threat picture).

For the second case, make sure that the ability requires multiple failed saves at low DCs to be completely removed from play and bump up the CR by one or two. The basilisk is only CR 2 by calculations, but is CR 3 due to the petrifying gaze ability.

If none of the considerations hold, round to the nearest CR step. Then adjust after play-testing (covered elsewhere).

Experience

There is a direct (but not linear) relationship between the CR of the creature and the experience related. It actually seems to be quadratic (related to CR²). Use the table on page 275 of the DMG for exact values.