This calculator follows the "Creating a Monster" method in the 2014 Dungeon Master’s Guide (pages 273 to 279). It gives you an estimate, not a verdict: the DMG itself says the only way to know whether a monster is fun is to playtest it. Use the result as a starting point, then adjust the CR after you see the monster at the table.
Step 1: defensive CR
Find the monster’s hit points in the Hit Points column of the table below. That row is its starting defensive CR. Then compare its Armor Class with the AC on that row. For every 2 points of difference, move the CR one step up (higher AC) or down (lower AC). A 1-point difference changes nothing. Steps go through the fractions, so one step down from CR 1 is CR 1/2.
Step 2: offensive CR
Work out the monster’s damage per round. If it does different things on different turns, add up the damage of its first three rounds and divide by 3. A breath weapon it can use once in those three rounds counts for one round, and an area effect usually hits two targets, so double it. Find that number in the Damage/Round column, then compare the monster’s attack bonus with that row and move one step for every 2 points of difference. If the monster mostly forces saving throws, compare its save DC instead.
Step 3: final CR and XP
Average the defensive and offensive CRs and round to the nearest CR. When the average falls exactly halfway, this calculator rounds up, which matches the DMG’s own example (defensive 2 and offensive 3 make CR 3). The final CR sets the monster’s XP and proficiency bonus.
Effective hit points and AC
Some defenses count as extra hit points or AC when you work out CR, without changing the stat block:
- •Resistances or immunities to several damage types (especially nonmagical bludgeoning, piercing and slashing) multiply hit points: ×2 at an expected CR of 1 to 4, ×1.5 for resistance (×2 immunity) at 5 to 10, ×1.25 (×1.5) at 11 to 16, and ×1 (×1.25) at 17 or higher.
- •Vulnerability to several common damage types halves effective hit points.
- •Saving throw proficiencies: three or four add 2 to effective AC, five or more add 4.
- •Flying plus a ranged attack adds 2 to effective AC if the expected CR is 10 or lower.
- •Traits from the DMG’s monster features table: Magic Resistance +2 AC, Pack Tactics +1 attack bonus, Regeneration adds 3 × the hit points it regains each round.
Worked example: a homebrew bog hag
Say you build a swamp hag with 52 hit points, AC 15, proficiency in Constitution, Wisdom and Charisma saves, and two claw attacks at +5 to hit for 9 damage each.
- •Defense. 52 HP sits on the CR 1/2 row, which expects AC 13. Three save proficiencies make her effective AC 17, which is 4 above, so move up two steps: 1/2 → 1 → CR 2.
- •Offense. Two claws for 9 each is 18 damage per round, the CR 2 row, which expects +3 to hit. Her +5 is 2 higher, so move up one step: CR 3.
- •Final. The average of 2 and 3 is 2.5, which rounds up to CR 3: 700 XP, proficiency bonus +2.
Drop her to AC 13 and the defensive CR falls to 1, the average to 2, and she becomes a CR 2 monster worth 450 XP.
Checked against published monsters
The method lands exactly on the printed CR for SRD monsters with no CR-changing traits. The axe beak (AC 11, 19 HP, +4 for 6) comes out at CR 1/4, the camel at CR 1/8, the giant lizard at CR 1/4, and the hill giant (AC 13, 105 HP, two greatclubs at +8 for 18) at defensive 3 and offensive 6, which averages 4.5 and rounds up to its printed CR 5. Monsters with spells, legendary actions or unusual traits drift further, because the DMG asks you to judge those by hand.
2014 and 2024 monsters
SRD 5.2.1 (the 2024 rules) doesn’t include a monster-building table. Its stat blocks list XP and proficiency bonus for each CR, and those match the 2014 values used here. You can run a 2024 stat block through the calculator the same way; just treat the result as a rough guide, since the 2024 monsters were not built from this table.
Next: balance the whole fight
CR rates one monster. To see how a group of monsters stacks up against your party, add them to the encounter difficulty calculator. Need a stat block to test? Draft one with the AI stat block generator and check its CR here, or browse SRD monsters by challenge rating to compare yours with a published one. The XP calculator splits the reward afterwards.
Sources: Dungeon Master’s Guide (2014), "Creating a Monster" and the Monster Statistics by Challenge Rating table (game numbers only); SRD 5.1 and SRD 5.2.1 by Wizards of the Coast (CC-BY-4.0) for the published monster stats, XP and proficiency bonus.