Guide · The icosahedron
d20 Dice Roller
Updated: June 2026
No die carries more drama than the twenty-sided d20. In Dungeons & Dragons and most modern role-playing games it decides whether you hit, dodge, persuade or fail, and a "natural 20" can turn a hopeless moment into a triumph. This roller gives you a fair d20 with modifiers, advantage and disadvantage built in.
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The single d20
A twenty-sided die is numbered 1 through 20, and on a fair die every face has the same 1 in 20 chance — exactly 5%. That makes the d20 the backbone of "roll high to succeed" systems: a target number sets the bar, and you simply need your roll, plus any modifier, to reach it. The average of a bare d20 is 10.5, so a difficulty around that line is roughly a coin-flip before modifiers. To roll one here, leave the count at 1, set sides to 20, and press Roll.
The natural 20 and natural 1
The two faces everyone watches for are 20 and 1. A natural 20 — a raw 20 on the die before modifiers — is the best possible roll, often a critical success or guaranteed hit; a natural 1 is the worst, frequently an automatic miss or fumble. Each has the same 5% chance, so on average you will see a 20 once every twenty rolls, and a 1 just as often. They feel rarer than they are because they matter so much, but the maths is plain: one in twenty, each way.
Advantage and disadvantage
Fifth-edition D&D replaced many fiddly modifiers with a clean idea: advantage means roll two d20 and keep the higher, while disadvantage means roll two and keep the lower. This tool does both directly — set the dice count to 2, sides to 20, and choose Keep Highest 1 for advantage or Keep Lowest 1 for disadvantage. The dropped die is shown crossed out so you can see exactly what happened.
The effect is bigger than people expect. Advantage lifts your average roll from 10.5 to about 13.8, and it makes a natural 20 nearly twice as likely (about 9.75%) while making a natural 1 far rarer. Disadvantage mirrors that downward. It is roughly equivalent to a hidden bonus or penalty of around +/-3 to +/-5 near the middle of the range.
| Roll | Average | Chance of 20 |
|---|---|---|
| 1d20 | 10.5 | 5% |
| Advantage | ≈13.8 | ≈9.75% |
| Disadvantage | ≈7.2 | ≈0.25% |
Adding your modifier
Most d20 rolls finish with a bonus: your skill, attack or save modifier. Type the modifier into the field, or just paste notation like 1d20+5 or 1d20-1, and the roller adds it to the die and reports the final total. Combine that with the crypto-secure source when you want a result no one at the table can question, all without sending a thing to a server.
Frequently asked questions
What are the odds of a natural 20?
5%, or 1 in 20. A natural 1 is also 5%.
How do I roll with advantage?
Roll 2 d20 and keep the highest 1. For disadvantage, keep the lowest 1.
How do I add a modifier?
Set a modifier such as +5, or type 1d20+5. The tool adds it to the die total.
What's the average d20 roll?
10.5 on a single die; about 13.8 with advantage and 7.2 with disadvantage.