How this calculator works
Pool mode rolls every listed die. D20 modes roll one or two dice and retain one face before adding the modifier. A tie keeps the first die. The probability readout counts successful faces on an ideal fair d20 and combines two independent rolls when needed. Natural 1 and 20 overrides apply only in attack mode. Damage bonuses, rerolls, critical damage and other character features require their own game rules. The seed reproduces a sample rather than establishing the probability.
Formula
Check total = kept d20 + modifier. If one die succeeds with probability p, advantage succeeds with 1 - (1-p)^2 and disadvantage with p^2.
Worked example
Seed 1 gives d20 faces 7 and 9. With advantage and a +3 modifier, keep 9 for a total of 12. Against DC 15, nine faces succeed on a single die, so advantage has probability 1 - (11/20)^2 = 69.75%.
Advertisement
Frequently asked questions
Do several sources of advantage add more dice?
The basic rule still uses two dice. If advantage and disadvantage both apply, choose the normal one-d20 mode.
Does a natural 20 always pass an ability check?
The basic check and saving-throw model compares the modified total with the target. Automatic natural-20 success is applied here only to attack rolls.
Can I mix d4, d6 and d8 dice?
Yes. In pool mode enter 4,6,8. Enter each die separately and supply the total modifier once.