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.

What is the D&D Advantage/Disadvantage Calculator?

Computes exact hit probabilities for d20 rolls with normal, advantage, disadvantage, and Elven Accuracy, given any modifier and target DC. See the percentage-point swing and crit odds. Essential for D&D 5e players valuing advantage. It runs free in your browser on Gera Tools, with nothing uploaded.

D&D Advantage/Disadvantage Calculator

Name: D&D Advantage/Disadvantage Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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.

Advantage in 5e — strategy and mechanics

The “expected roll” framing

A common shorthand is that advantage adds roughly +5 to your effective roll. This is approximately true near the middle of the distribution (the mean result on advantage is about 13.8 vs. 10.5 on a straight roll, a difference of roughly 3.3), but it obscures the context-dependence. Near the extremes — when you only miss on a 1 or only hit on a 20 — advantage adds much less. The percentage-point swing is a more honest measure: use the calculator to see the actual swing for your specific modifier and target.

Stacking advantage

In 5e, advantage does not stack. If two different sources give you advantage on the same roll, you still roll two dice, not three. (Elven Accuracy is the single exception: the feat effectively gives a third die when you already have advantage and meet the race/feat requirements.) This means the marginal value of a second advantage source is exactly zero, and spending resources or abilities to gain advantage when you already have it is wasted. Conversely, one source of disadvantage cancels one source of advantage regardless of how many advantage sources you have.

When is gaining advantage worth an action?

Bounded accuracy in 5e keeps ACs and DCs in a relatively tight range, which means advantage is nearly always valuable because it rarely reaches the “you were going to hit anyway” zone. However, there is a resource trade-off: using a bonus action, spell slot, or expending a feature to grant yourself advantage is only beneficial if the resulting increase in hit probability justifies the cost. At a 60% base hit chance, advantage raises it to about 84% — a 24 percentage-point improvement. At a 90% base hit chance, advantage raises it to about 99% — only a 9-point improvement that is rarely worth an action economy cost.

Disadvantage as a defensive tool

The same math applies to disadvantage. Imposing disadvantage on an enemy’s attack reduces their hit chance symmetrically to how advantage increases yours. If an enemy hits you on a 7 or higher (70% chance), imposing disadvantage drops them to about 49%, a 21-point reduction. This makes abilities and spells that impose disadvantage on attack rolls — Dodge action, Blur spell, Shield spell — very potent defensively, particularly when you expect to be attacked multiple times per round.

Elven Accuracy in practice

The Elven Accuracy feat (available to elves, half-elves, and certain other races in 5e) triggers when you have advantage on an attack roll using specific ability scores, letting you reroll one die — effectively taking the best of three d20 results. The critical hit rate under Elven Accuracy is approximately 14.3%, nearly three times the normal 5% rate. This makes it exceptionally powerful for builds that reliably have advantage — Assassin Rogues who always have advantage from Assassinate on surprised targets, or Paladins who can use Wrathful Smite to impose fear.