D&D Advantage Calculator

Calculate the exact probability of hitting a target AC or DC with Advantage, Disadvantage, and modifiers in D&D 5e.

Success Probability
79.75%
Need to roll 10+ on d20 with Advantage
Normal Probability
55.00%
Advantage Delta
+24.75%
Probability Visualization
79.75%
0% 50% 100%
Crit Chance (Nat 20)
9.75%
Crit Fail Chance (Nat 1)
0.25%

How to Use the D&D Advantage Calculator

  1. Enter the Target DC or AC — this is the Difficulty Class you need to meet or exceed, or the Armor Class of the creature you are attacking.
  2. Set your total modifier — this includes your ability modifier, proficiency bonus, and any other bonuses that apply to the roll.
  3. Choose your roll type — Normal (single d20), Advantage (roll 2d20, keep highest), or Disadvantage (roll 2d20, keep lowest).

Understanding Advantage and Disadvantage in D&D 5e

Advantage and Disadvantage are core mechanics in Dungeons & Dragons 5th Edition that replace the stacking numerical bonuses of previous editions. When you have Advantage, you roll two twenty-sided dice and use the higher result. When you have Disadvantage, you roll two d20s and use the lower result. This elegant system keeps the math simple at the table while providing a significant statistical impact.

The Mathematics Behind Advantage

A normal d20 roll produces a flat, uniform distribution — every number from 1 to 20 has an equal 5% chance. Advantage fundamentally changes this distribution. The probability of succeeding on a roll with Advantage is calculated by finding the probability that both dice fail and subtracting from 100%:

P(success) = 1 - ((targetRoll - 1) / 20)2

For Disadvantage, both dice must succeed:

P(success) = ((21 - targetRoll) / 20)2

When Advantage Matters Most

Advantage provides the greatest benefit when you need to roll around a 10 or 11 on the die. At the midpoint, Advantage swings your success rate from 55% to roughly 80% — a 25 percentage point increase, equivalent to about a +5 bonus. When you only need a 2, Advantage barely helps (95% to 99.75%). When you need a 20, it nearly doubles your odds (5% to 9.75%) but the absolute gain is small.

Critical Hits and Natural 1s

Critical hits (natural 20s) have a 5% base chance on a normal roll. With Advantage, this jumps to 9.75% — nearly one in ten rolls. With Disadvantage, the chance plummets to just 0.25%, or roughly 1 in 400 rolls. Natural 1s follow the inverse pattern: 5% normally, 0.25% with Advantage, 9.75% with Disadvantage.

Common Sources of Advantage

In D&D 5e, you gain Advantage from numerous sources: attacking an unseen target, using the Help action, a Rogue's Steady Aim feature, attacking a prone creature in melee, the Reckless Attack barbarian feature, and many spells like Faerie Fire and Guiding Bolt. Understanding when you have Advantage helps you make tactically optimal decisions during combat encounters.

Frequently Asked Questions

When you have Advantage, you roll two d20s and take the higher result. Disadvantage is the opposite — roll two d20s and take the lower. This shifts the flat 5%-per-face distribution into a curved probability, significantly boosting or reducing your chances of hitting high numbers.
A normal d20 roll averages 10.5. With Advantage, the average increases to approximately 13.82. With Disadvantage, the average drops to approximately 7.18. This is roughly equivalent to a +3.3 bonus on a normal roll.
Advantage provides the biggest benefit when you need to roll around 10-11 on the die. At that midpoint, Advantage boosts your success chance from 55% to about 80% — a 25 percentage point improvement. The benefit is smaller at the extremes (needing a 2 or a 19).
Yes. In D&D 5e, if you have any source of Advantage and any source of Disadvantage at the same time, regardless of how many sources of each, they cancel out and you roll normally. Two Advantages and one Disadvantage still results in a normal roll.
A natural 20 (critical hit) has a 5% chance on a normal roll. With Advantage, the chance of rolling at least one natural 20 jumps to 9.75% — nearly double. With Disadvantage, the critical hit chance drops to just 0.25% (1 in 400).