Roll a D20 / About
About Roll a D20
A focused dice tool for tabletop moments, quick checks and a little curiosity, with plain-language guides to what a D20 roll means.
What this website offers
Roll a D20 is a free website with one job: give you a fair twenty-sided die roll and explain the result clearly. There is no account, download or sign-up.
- The online D20 roller rolls one die, rolls with advantage or disadvantage, adds a modifier from −999 to +999, or rolls up to 100 dice at once.
- The D20 guide explains dice notation, modifiers, natural results and D20 Tests in D&D.
- The probability guide shows exact averages and success chances for standard, advantage and disadvantage rolls.
- The MTG guide explains how to resolve “roll a D20” effects in Magic: The Gathering.
Who it is for
The site is made for tabletop roleplaying players and game masters, Magic: The Gathering players using dice cards, and anyone who needs a quick, fair number from 1 to 20 without a physical die. The guides are written for beginners and assume no math background.
How the roller works
The roller uses your browser’s cryptographic randomness (the Web Crypto API) with rejection sampling, which removes the small bias that a simple remainder calculation would introduce. Each result is fixed before any animation starts, so the motion never decides the number.
Rolls are calculated on your device. Results and roll history are not sent to our server, and the history disappears when you reload or close the page.
How our content is produced
Sources
Rules explanations are written in our own words and checked against official publisher material: the 2024 D&D playing rules and the official MTG release notes for Adventures in the Forgotten Realms. Probability figures are calculated from the 20 equally likely faces of a fair die and the 400 equally likely pairs of two dice.
Tools and AI assistance
The site’s code and page copy were produced with the help of AI tools from a written design and content brief. The celestial die illustration was generated with an AI image tool and is decorative. We disclose this so you can judge the content for yourself.
Testing
The roller’s behavior is covered by automated tests, including the rejection-sampling boundary, advantage and disadvantage selection, modifiers, batch sums and input validation. The probability table is generated directly from the formulas shown on that page.
Editorial principles
- Explain, don’t hype. No technique can make a fair die roll higher, and we never suggest otherwise.
- Keep math and rules separate. A probability is not a game rule, and a summary is not the official text.
- Point to the authority. Current official rules, card text and your table’s decisions always come first.
- No invented credibility. We publish no fake reviews, ratings, testimonials or credentials.
- Independent content. We don’t sell links or accept payment to change what a guide says.
Reviews and corrections
Guides are reviewed when the underlying rules or cards change and when readers report a problem. If you find an error, contact us with the page address and what looks wrong. We check the report against the source and update the page if a correction is needed.
Independence and ownership
Roll a D20 is an independent website at rollad20.co. It is not affiliated with, sponsored or endorsed by Wizards of the Coast, Hasbro, or any voice assistant or game platform. Game names are used only to identify the subjects discussed. See the disclaimer for how to use the site’s results and guides.