Monster Attack & Damage Modifier Calculator
Calculate a monster's attack bonus and damage modifier based on its ability score, proficiency bonus, and challenge rating (CR). Uses standard D&D 5e rules.
Fill in the fields and click Calculate.
Formula
Ability Modifier = ⌊(Ability Score − 10) / 2⌋
Proficiency Bonus (by CR): CR 0–4 → +2 | CR 5–8 → +3 | CR 9–12 → +4 | CR 13–16 → +5 | CR 17–20 → +6 | CR 21–24 → +7 | CR 25–28 → +8 | CR 29–30 → +9
Attack Bonus = Ability Modifier + Proficiency Bonus (if proficient) + Extra Bonus
Damage Modifier = Ability Modifier + Extra Flat Damage
Average Damage = (Number of Dice × (Die Size + 1) / 2) + Damage Modifier
Assumptions & References
- Based on D&D 5th Edition rules (Player's Handbook & Monster Manual).
- Ability scores range from 1 to 30; modifier = ⌊(score − 10) / 2⌋.
- Proficiency bonus is determined by the monster's CR, not player level.
- Attack bonus applies to the d20 roll to hit; damage modifier applies to the damage roll.
- Average damage assumes average die rolls ((die size + 1) / 2 per die).
- Extra bonuses cover magic weapons, class features, or special abilities.
- This calculator does not account for advantage/disadvantage or critical hits.