Skip to content
Randozi
All wheelsTools

d20 Advantage and Disadvantage Odds Calculator

Enter the total you need on a d20 and your modifier. The page gives the exact chance of reaching it with one roll, with the higher of two rolls and with the lower of two rolls, as fractions and as percentages, with three bars to compare them and the average of each.

Normal roll

30 %

6/20 = 3/10
average 10.5

Advantage

51 %

204/400 = 51/100
average 13.825

Disadvantage

9 %

36/400 = 9/100
average 7.175

30 %Normal51 %Advantage9 %Disadvantage

You need a natural roll of 15 or more on the d20.

Advantage is the higher of two d20 and disadvantage the lower of two. Special rules of a particular game, such as a natural 1 or 20, are not applied. Odds for play and study, not for betting.

How it works

  1. Enter the total you need to reach (1 to 20).
  2. Enter your modifier, positive or negative.
  3. Read the exact fraction and percentage for a normal roll, advantage and disadvantage.
  4. Compare the three bars and copy the summary.

What people use it for

  • Seeing what advantage is worth

    Compare the chance to hit a 15 with and without a second d20 before choosing an action.

  • Teaching probability

    Show why the best of two dice is not twice as likely, using exact fractions out of 400.

  • Balancing a homebrew rule

    Check how a modifier changes the gap between the three modes.

Questions

What are the formulas?
Let r = target − modifier be the roll needed, kept between 1 and 21. A normal roll succeeds with (21 − r) ÷ 20. With advantage you keep the higher of two d20, which fails only if both rolls are below r, so the chance is 1 − ((r − 1) ÷ 20)². With disadvantage you keep the lower, which succeeds only if both are at least r: ((21 − r) ÷ 20)². For a target of 15 and no modifier that is 6/20, 204/400 and 36/400.
What are the averages?
One d20 averages 10.5, the higher of two averages 13.825 and the lower of two averages 7.175. The modifier is added to each. It is the average of the roll plus the modifier, not limited by the target.
What if the target cannot be reached, or cannot be missed?
If the modifier makes the target impossible, every mode shows exactly 0 %; if any roll is enough, exactly 100 %. Special rules for a natural 1 or a natural 20 belong to particular games and are not applied here.
Is this tied to a game?
No. Advantage here means the higher of two d20 and disadvantage the lower of two. Check the rules of your own game. The odds are for play and study, not a help for betting.