Get your exact chance to hit or save, compare advantage vs. a flat bonus, and see which attack deals more damage.
1. Your roll
2. Target
AC, save DC, or difficulty
3. Your chance
55%
Roll 10 or higher
Succeeds 11 of 20 outcomes
Roll a d20, add your attack bonus, and check whether the total meets or beats the target's Armor Class. Since every face of the d20 is equally likely, your hit chance works out to (21 + bonus − AC) / 20. A +5 bonus against AC 15, for example, hits 55% of the time. A natural 20 always hits and a natural 1 always misses, no matter what the math says. Enter your bonus and their AC above for the exact number.
It depends how likely you already are to succeed. Advantage (roll twice, keep the higher) helps most when your odds are already close to 50/50, where it's worth roughly +5 to your total. Near the extremes, where you'll almost always hit or almost never hit, a flat bonus tends to matter more since advantage has less room to work with. Plug in your own bonus and target to see which wins for your specific roll.
A critical hit doubles your damage dice, not your modifier: 2d6 + 3 becomes 4d6 + 3 on a crit, not 4d6 + 6. Roll all of the attack's damage dice twice, add them together, then add your modifier once as normal.
Several small dice give a more consistent, predictable result, while one big die swings harder in both directions for the same average. 2d6 and 1d12 both average around 6.5–7 damage, but 2d6 rarely rolls very low or very high, while 1d12 can just as easily roll a 1 as a 12. Enter your own dice on the Compare Damage tab to see the exact average and how often each option beats a given damage threshold.
Both sides roll a d20 and add their relevant modifier (Stealth vs. Perception, say, or Athletics vs. Athletics), and the higher total wins. Ties typically go to whichever side isn't initiating the action (the hidden creature stays hidden, the grappler fails to break free), though some tables just reroll. Because both totals are random, the odds aren't as simple as beating a fixed number. The Contested Roll tab multiplies out both distributions for the exact win, loss, and tie percentages.
It's the exact probability distribution behind the single percentage above: every result your dice can produce, checked against your target plus five nearby DCs on either side, colored to show whether each one fails, meets, or beats that target. Use it to see how your odds change if the DM sets a slightly higher or lower DC, to judge whether a +1 item or feat is worth taking, or to read off the exact meet-or-beat percentage for any nearby target at a glance. Every number here comes from the full probability distribution, not simulated rolls, so it's exact.
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