How is advantage calculated exactly?

Advantage means rolling two d20 and taking the higher. The chance the result is at most k is (k/20) squared, so the chance of exactly k is (k/20) squared minus ((k-1)/20) squared. Disadvantage uses the lower die with the mirrored formula. Both are exact, not simulated.

How much does advantage improve my roll?

It depends on the target. Advantage helps most when you need a middling roll, adding up to about 25 percentage points near a 50% hit chance, and helps least at the extremes. The tool shows the exact point swing for your DC.

What is Elven Accuracy?

Elven Accuracy is a 5e feat that lets certain characters reroll one die when they have advantage, effectively rolling three d20 and taking the highest. The calculator models this as take-highest-of-three, which also roughly triples the natural-20 crit chance to about 14.3%.

How do natural 1 and natural 20 affect the result?

Following attack-roll rules, a natural 20 always hits regardless of the target and a natural 1 always misses. The hit percentages include this, which is why a high-modifier attack against a low DC still has a small miss chance from rolling a 1.

Does advantage change my critical hit chance?

Yes. A normal roll crits on a natural 20 5% of the time. Advantage raises that to about 9.75% because you have two chances to roll a 20, and Elven Accuracy raises it to about 14.26% with three dice.

D&D Advantage/Disadvantage Calculator

See exactly how much advantage is worth before you spend a resource to gain it. Enter your modifier and the target DC, and this tool reports the precise hit chance for normal, advantage, disadvantage, and Elven Accuracy rolls, along with the percentage-point swing and critical-hit odds.

How it works

A d20 roll under each condition has an exact distribution:

Normal — one d20, each face 1/20.
Advantage — two d20, take the higher. P(result ≤ k) = (k/20)², so P(result = k) = (k/20)² − ((k−1)/20)².
Disadvantage — two d20, take the lower, using the mirrored formula.
Elven Accuracy — three d20, take the highest, raising each term to the third power.

The tool sums the probabilities of every face that hits (face + modifier ≥ DC), while honoring the rule that a natural 20 always hits and a natural 1 always misses. It then shows the swing in percentage points relative to a normal roll.

Tips and example

Against a DC where you hit about half the time, advantage adds the most value, often around +20 to +25 percentage points. Near auto-hit or near-impossible targets it adds little.
Granting disadvantage to an enemy is symmetric to giving yourself advantage in magnitude, so debuffs that impose disadvantage are strong defensively.
The crit line shows why advantage builds love it: doubling your chances of a natural 20 from 5% to roughly 9.75% meaningfully boosts damage over a fight.