Dungeons & Dragons
d20 probability check
Enter a DC and your modifier, pick how you're rolling, and see your chance to succeed. Open Advanced for bonus dice and natural-20 rules.
0%
chance to succeed
- 0%Normal
- 0%Advantage
- 0%Disadvantage
Advanced options
How it works
This d20 check calculator turns a DC and your modifier into a clear success percentage. Set the difficulty, add your bonus, choose normal, advantage, or disadvantage, and it tells you exactly how often the roll lands.
It models the d20 the way the rules do: advantage or disadvantage rolls two dice and keeps the better or worse result. Handy for picking a DC as a DM, or weighing your odds before you commit to a risky check.
Example. Need a DC 15 check with a +6 modifier? You succeed on a 9 or higher, which is 60% normally. With advantage that climbs to 84%.
FAQ
How does advantage change my chances?
Advantage rolls two d20s and keeps the higher, worth roughly +3 to +5 on average depending on the DC. The biggest swing is on middling DCs, where the outcome is most uncertain.
Does it account for natural 20s and 1s?
Yes. The calculation treats the d20 as a fair 1 to 20 roll, so the full range, including natural 20 and natural 1, is reflected in the percentage.
What is a fair DC to set?
As a rough guide, DC 10 is easy, 15 is moderate, and 20 is hard for a typical character. Plug in your players' likely modifiers to see what each DC really means at your table.
Does it work for attack rolls and saving throws too?
Yes. Any roll that is a d20 plus a modifier against a target number works the same way: use the attack bonus against the target's AC, or the save bonus against the spell DC, and the percentage applies just as it does for ability checks.