Advantage and disadvantage calculator.

Answer

With a +5 modifier against a target of 15 on an attack roll, you succeed on 55.00% of rolls normally, 79.75% with advantage and 30.25% with disadvantage. Advantage adds 24.75 percentage points here; it never adds more than 25 points.

Worked example

Attack at +5 against AC 15 needs a natural 10 or higher (10 + 5 = 15). Faces 10 to 20 succeed: 11 of 20 = 55.00%. With advantage you fail only if both d20s fail: 1 - 0.45 x 0.45 = 79.75%. With disadvantage both must succeed: 0.55 x 0.55 = 30.25%.

How it works

The rule. The Basic Rules say that with advantage or disadvantage you roll a second d20 and use the higher roll (advantage) or the lower roll (disadvantage). If a roll has both, you are considered to have neither and roll one d20.

The math. If one d20 succeeds with probability p, advantage succeeds with 1 - (1 - p)^2 and disadvantage with p^2. The swing from advantage is p x (1 - p), which peaks at 25 percentage points when p is 50%.

Attack rolls versus checks. For attack rolls the Basic Rules say a natural 20 hits regardless of modifiers or armor class and a natural 1 misses regardless, so no attack is below 5% or above 95%. The check or saving throw mode applies no automatic result, so a target that is out of reach stays at 0% or 100%. Some tables use house rules; this tool models the two modes above.

Average roll. One d20 averages 10.5, the higher of two averages 13.825 and the lower of two averages 7.175. A natural 20 comes up on 5.00% of single rolls, 9.75% with advantage and 0.25% with disadvantage.

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

FAQ

How much does advantage increase your chance to hit?

By p x (1 - p) where p is your normal chance, at most 25 percentage points. It helps most when you would succeed about half the time and least when success is nearly certain or nearly impossible.

What are the odds of hitting with advantage if I need a 20?

9.75%. You need at least one natural 20 in two rolls: 1 - 0.95 x 0.95.

Do advantage and disadvantage cancel out?

Yes. The Basic Rules say that if a roll has both advantage and disadvantage you have neither and roll one d20, even if several things impose disadvantage and only one grants advantage.

What is the average d20 roll with advantage?

The average is 13.825, compared with 10.5 for a single d20. With disadvantage it is 7.175.

Does stacking advantage help?

Not under the Basic Rules, which say that if two favorable situations grant advantage you still roll only one additional d20.