Damage per round calculator.

Answer

A +5 attack against AC 15 with a 1d8 + 3 weapon and two attacks a round hits 55.00% of the time and averages 4.35 damage per attack, 8.70 per round. With advantage the same attacks average 12.84 per round.

Worked example

Needing a natural 10 (10 + 5 = 15): faces 10 to 19 hit normally, 10 of 20 = 50%, and a natural 20 is a critical hit, 5%. A normal hit does 4.5 + 3 = 7.5. A crit rolls the die twice: 9 + 3 = 12. Per attack: 0.50 x 7.5 + 0.05 x 12 = 4.35. Two attacks: 8.70 per round.

How it works

Rules used. The Basic Rules say a natural 20 on an attack roll hits regardless of modifiers or armor class and is a critical hit, a natural 1 misses regardless, and on a critical hit you roll all of the attack's damage dice twice and add modifiers as normal. Advantage and disadvantage mean rolling two d20 and using the higher or lower.

The math. Expected damage per attack = P(hit, not crit) x (average dice + flat) + P(crit) x (2 x average dice + flat). A die with s sides averages (s + 1) / 2. Extra dice (Sneak Attack, smite) are rolled and doubled with the weapon dice on a crit, and the flat bonus is added once. The faces 2 to 19 hit when the roll plus your bonus meets or beats the armor class.

What is left out. Resistance, vulnerability, saving-throw spells, damage that only applies once per turn and wider critical ranges are not modeled. Treat the result as a baseline average, not a guarantee.

Sources and basis (opened 2026-10-08): Basic Rules, Chapter 9: Combat (D&D Beyond) for natural 20, natural 1 and critical hits; Basic Rules, Chapter 7: Using Ability Scores (D&D Beyond) for advantage and disadvantage.

FAQ

How do you calculate damage per round in D&D?

Multiply the average damage per attack by the number of attacks. Per attack, add the hit chance times average damage on a hit and the crit chance times average damage on a crit.

How are critical hits counted?

A natural 20 is a critical hit and always hits. All damage dice are rolled twice and the flat modifier is added once, so a crit does double the dice average plus the flat bonus.

Does advantage raise the crit chance?

Yes. The chance of at least one natural 20 in two d20 is 9.75% instead of 5%.

Why does damage per round fall so fast as armor class rises?

Each point of armor class removes 5% of the hit chance until only a natural 20 can hit, which leaves a 5% chance per attack.

Does this include saving-throw spells or resistance?

No. It models d20 attack rolls only. Halve the result by hand for resistance or work out saving-throw spells separately.