D&D Dice Probability: The Math Behind Your Rolls
Understanding dice probability makes you a better player. Here is what the numbers actually mean.
A single d20 has a 5% chance of rolling any specific number. A natural 20 happens 1 in 20 rolls. But the probability of rolling AT LEAST a certain number is what matters for skill checks and attack rolls.
D20 Probabilities
Need 10 or higher: 55% chance. Need 15 or higher: 30% chance. Need 18 or higher: 15% chance. Need a natural 20: 5% chance. Advantage (roll 2d20, take higher): need 10+ goes from 55% to 80%. Need 15+ goes from 30% to 51%. Advantage is roughly equivalent to a +4-5 bonus on the roll.
Damage Dice Averages
d4: average 2.5. d6: average 3.5. d8: average 4.5. d10: average 5.5. d12: average 6.5. 2d6: average 7 (more consistent than 1d12 — minimum 2 instead of 1, bell curve clusters around 7). Greatsword (2d6, avg 7) vs Greataxe (1d12, avg 6.5): the greatsword does more average damage but the greataxe has higher max damage and benefits more from critical hits.