Dice Roller
Roll a d20 (with advantage!), a handful of d6 or any XdY combo up to 20 dice — crits highlighted, totals summed, rolls fairer than any plastic cube in your drawer.
Spotted a mistake or have an idea? — errors get fixed and credited publicly.
Last reviewed: July 20, 2026 · Formulas verified by automated tests before every release.
What is the average roll of a d20?
10.5 — the average of any fair die is (faces + 1) ÷ 2, so a d4 averages 2.5, a d6 3.5, a d12 6.5 and a d20 10.5. For multiple dice, multiply: 5d20 averages 52.5, 6d4 averages 15. Roll any combo above — every result shows its total and average against expectation.
Average rolls, die by die
d4 → 2.5 · d6 → 3.5 · d8 → 4.5 · d10 → 5.5 · d12 → 6.5 · d20 → 10.5 · and XdY averages X × (Y+1)÷2 — so 3d4 sits at 7.5 and 100d20 at 1,050. Advantage on a d20 lifts the expected roll to ≈13.8, which is why it feels so much luckier than +1 bonuses. Building teams instead of characters? The team generator and random number generator run on the same fair randomness.
When a virtual die beats a real one
- Board game night — the dice always roll off the table; this one doesn't.
- D&D and tabletop RPGs — full set from d4 to d20 in your pocket.
- Classroom and family games — quick, visible to everyone on a shared screen.
- Decisions — odd = yes, even = no. We don't judge.
How to roll a d20 online
Pick d20 above (it's the default), set the number of dice, and hit Roll — or use ⚔ Advantage / 🛡 Disadvantage for the D&D 5e two-dice mechanic. A natural 20 lights up green; a 1 gets the condolences it deserves.
The XdY cheat sheet
| Notation | Range | Classic use |
|---|---|---|
| 1d20 | 1–20 | D&D attack rolls, checks, saves |
| 2d6 | 2–12 | Monopoly, Catan, greatsword damage |
| 4d6 | 4–24 | Rolling D&D stats (drop the lowest die) |
| 1d100 | 1–100 | Percentile rolls, loot tables |
| 8d6 | 8–48 | Fireball. Enough said 🔥 |
Advantage math, honestly
Advantage (roll two d20, keep the higher) is worth more than it feels: it raises the average roll from 10.5 to 13.82 and nearly doubles your chance of a natural 20 (from 5% to 9.75%). Disadvantage mirrors it downward. That's why the mechanic replaced stacks of small bonuses in D&D 5e — one reroll is worth roughly a +3 to +5 depending on the target number.
Fairness, honestly
Physical dice are surprisingly biased: worn corners and off-center pips skew results by measurable amounts. These rolls come from a cryptographic random source with rejection sampling — every face gets a mathematically equal chance, every time.
Sources, assumptions and limitations
- Formula: each die: uniform integer 1…Y via Web Crypto getRandomValues with rejection sampling (values above the largest multiple of Y are re-drawn, eliminating modulo bias)
- Assumptions: Cryptographically strong browser randomness (CSPRNG) — statistically indistinguishable from fair dice, though not a physical entropy source, which is why we say 'fair' and stop there.
- Not included: Server-side anything: rolls are generated and stay on your device; no results are stored or transmitted. Up to 20 dice per roll.
- Sources: MDN: Crypto.getRandomValues()
- Last reviewed: July 22, 2026 by the CalcNotebook team
Frequently asked questions
How do I roll a d20 with advantage or disadvantage?
Tap ⚔ Advantage (rolls two d20, keeps the higher) or 🛡 Disadvantage (keeps the lower) — both dice are shown, the kept one highlighted. It's the standard D&D 5e mechanic: advantage lifts the average roll from 10.5 to about 13.8.
What does a notation like 6d4 or 5d20 mean?
XdY = roll X dice with Y faces each: 6d4 is six four-sided dice (results 6–24), 5d20 is five twenty-sided dice. Pick the die, set the count with the slider, and the roll button shows exactly that notation.
Are the rolls fair?
Yes — each roll uses the browser's cryptographic random source with unbiased range mapping, so every face has exactly the same probability. Fairer than a slightly chipped physical die, honestly.
What do d4, d6, d8, d10, d12 and d20 mean?
The number is the count of faces: d6 is a standard cube, d20 is the twenty-sided die used in Dungeons & Dragons. "2d6" means rolling two six-sided dice and (usually) summing them.
Can I roll multiple dice at once?
Yes — up to 20 dice of the same type per roll, with the individual results, total and average shown. For mixed sets (like 1d20 + 2d6), roll each type separately.
Why is the average shown?
For game balance checks: the expected average of a die is (faces + 1) ÷ 2 — so 3.5 for a d6, 10.5 for a d20. If your night's rolls are far below that, it's luck, not the algorithm.