D20 Dice Roller

Why the d20 sits at the centre of so many games

A twenty-sided die has a flat distribution: every face from 1 to 20 is equally likely. That flatness is what makes it a good resolution die. Each point of modifier moves your success chance by exactly five percentage points, everywhere on the scale, which makes bonuses easy to reason about both for players and for whoever wrote the rules.

Contrast that with adding several dice together. Three six-sided dice also span a range, but the middle results are vastly more common than the extremes, so a +1 bonus is worth far more near the centre than at the edges. Flat is predictable, and predictable is what a core mechanic needs.

What advantage actually does

Rolling twice and keeping the higher is a neat piece of design because it changes the odds without changing any number on the character sheet. There is no arithmetic to remember mid-roll, and it stacks with modifiers cleanly.

The average moves from 10.5 to roughly 13.83, but the average is the least interesting part. What matters is the effect at your target number, and that varies a great deal. If you need an 11 or better — a fifty-fifty check — advantage takes you from 50% to 75%, a swing of twenty-five points and the largest benefit available. If you need a 2 or better, you were succeeding 95% of the time already and advantage adds under five points.

The practical consequence is that advantage is worth most when the outcome is genuinely uncertain, and worth little when the check was nearly decided either way. Players tend to assume it is a flat bonus of around five; it is not, and knowing where it is strong is a real tactical consideration.

Disadvantage is not symmetric in feel

Mathematically disadvantage mirrors advantage exactly, dropping the average to about 7.17. It feels worse than advantage feels good, for the same reason losses generally register more strongly than equivalent gains — and because failing a check usually has a more visible consequence at the table than succeeding.

Most rule sets have the two cancel rather than accumulate: any number of advantages and any number of disadvantages together produce a normal roll. That keeps the bookkeeping trivial, which is the point of the mechanic.

Natural 20s and natural 1s

Many games treat the raw extremes specially — an automatic success or a critical hit on a natural 20, an automatic failure or fumble on a natural 1. The word natural is doing real work: it means the die itself, before modifiers.

A character with a +9 bonus rolling an 11 reaches a total of 20, and that is not a natural 20. The tool reports the raw die alongside the total for exactly this reason, so the distinction is visible rather than something to reconstruct.

Showing the discarded die

When you roll with advantage the tool shows both dice, not just the one it kept. This is a small thing that matters for trust: a result which reports only the kept value asks you to take it on faith that two were rolled at all. Showing the pair makes the mechanic auditable, in the same way a physical roll is auditable because everyone can see both dice on the table.

The face shown is the face drawn

The result is decided by a cryptographic draw before any animation exists, and the animation is then aimed at that outcome. The die you see settle is the die that was rolled — the timing of the animation has no influence on the number, and cannot.

Privacy

Rolls are generated in your browser. Nothing is transmitted, stored or logged.

Rolling several checks at once

Some situations call for a batch of independent d20 rolls — initiative for a group of creatures, saving throws against an area effect, a handful of skill checks resolved together.

Raising the dice count rolls them independently, which is the correct behaviour: each creature’s result should have nothing to do with any other. Reading them off in order and applying them to your list in the same order keeps the bookkeeping straightforward.

What this does not do is apply different modifiers to different dice, since the modifier applies to the total. Where each roll needs its own bonus, rolling the raw dice here and adding the modifiers yourself is usually quicker than rolling one at a time.