Free tool
Free D&D Dice & Damage Calculator
Get your exact chance to hit or save, compare advantage against a flat bonus, and see which attack or spell deals more damage on average.
Based on the roll below — edit it to match yours.
Your chance to meet or beat 15
55.0%
With 1d20 + 5, you succeed on 55.0% of rolls.
You fail 45.0% of the time. You land exactly on 15 on 5.0% of rolls.
Enter your roll
What are you rolling?
Example: an attack roll of 1d20 + 5 against AC 15.
Advantage rolls twice and keeps the higher total. Disadvantage keeps the lower total.
Their defense
Chance to roll each total or higher
Your Odds at Every Possible Total
Each bar is cumulative: it's your chance of rolling that number or higher, not the odds of that exact number. The bar over 15 matches the big percentage above.
The numbers along the bottom are dice totals. Your chosen roll mode (normal, advantage, or disadvantage) is already factored into every bar.
How it works
Exact probability, not a simulation
This calculator builds the complete outcome distribution for your dice — every possible total and its exact probability — instead of rolling random samples. That makes results precise rather than approximate, including for advantage and disadvantage, contested rolls between two full distributions, and expected damage across misses, hits, and critical hits.
FAQ
Common Dice Probability Questions
How do you calculate the chance to hit in D&D 5e?
Add your attack bonus to a d20 roll and check if the total meets or beats the target’s Armor Class. With a flat bonus, each face of the d20 is equally likely, so your chance to hit is (21 + bonus − AC) / 20, capped between 5% and 95% for the natural 1 and natural 20 rules. Enter your bonus, die, and target AC above for the exact percentage.
Does advantage add more than a +2 or +3 bonus?
It depends on your target number. Advantage helps most when your chance to succeed is already close to 50%, and helps less at the extremes, where a flat bonus can matter more. Use the advantage-versus-bonus comparison on the Chance to Succeed tab to see which is better for your specific roll and target.
How much extra damage does a critical hit add?
A critical hit doubles the damage dice you roll, but not the flat modifier. For example, a critical hit with 2d6 + 3 rolls 4d6 + 3 instead of 2d6 + 3. Turn on “Include accuracy and saving throws” on the Compare Damage tab to see critical-hit odds folded into each option’s expected damage per turn.
How do I compare two weapons or spells for damage?
Enter each option’s dice and modifier on the Compare Damage tab. In simple mode, the calculator compares average damage when both options land. Turn on the accuracy toggle to also factor in attack bonus versus AC, save DC versus a target’s save bonus, critical hits, half damage on a successful save, and multiple uses per turn.
What is a contested roll and how do you calculate who wins?
A contested roll is when two characters roll against each other, such as Stealth versus Perception or opposed Athletics checks. The higher total wins, and equal totals tie. The Contested Roll tab multiplies both characters’ full probability distributions to give an exact win, loss, and tie percentage instead of a rough estimate.
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