D20 Roller

D20 roller: roll a twenty-sided die and read the odds behind it

This page opens on a single d20. Press the d20 quick-roll button and you get one number between 1 and 20, shown large, with the roll added to a running history. The notation box is pre-filled with 1d20, so hitting Enter rolls the same thing. Everything else on the page — the other quick dice, the builder, the modifier field — exists so you can bend the d20 into whatever the moment needs: 1d20+7 for a skill check, 2d20 when you want to see both dice, 1d20-2 when something has gone badly wrong for your character.

The three ways to roll

• Quick roll buttons — one tap each for d4, d6, d8, d10, d12, d20 and d100. Each button rolls a single die of that size and resets the builder to match, so the d20 button always gives you a clean 1d20 with no leftover modifier from your last roll.
• The builder — three numeric fields (number of dice, sides, modifier) and a button that names exactly what it will roll, so you can see Roll 1d20+5 before committing. Dice count accepts 1 to 100 and sides accept 2 to 1000; the modifier can be positive or negative.
• Custom notation — a free-text field taking standard NdM with an optional modifier. Press Enter to roll it.

Results show the total in large type. When a roll involves more than one die, the individual faces appear underneath it as separate chips — that is what makes 2d20 useful here, because you can see both dice rather than just their sum. The history keeps your last 30 rolls with the notation, the individual dice in brackets and the total, and there is a Clear button when the list stops being useful.

The four things people type here

The notation box takes standard dice shorthand, but for a d20 page there are really only four inputs that matter, and one of them fails:

• 1d20 — one die. Note the count is not optional: bare d20 is rejected with an error message and your previous result stays on screen. This is the single most common stumble on this page.
• 1d20+5 — the check you actually make at a table. The chip underneath still shows the raw die face, so you can see whether you rolled a natural 20 or merely reached 20 with the bonus. That distinction decides a lot of rulings.
• 2d20 — two dice, both faces shown as chips. The big number is their sum, which is not what you want for advantage; read the chips instead.
• 4d6kh3 and 1d20+1d4 — both rejected. Keep-highest notation and multiple dice terms are not supported anywhere in this suite; roll the second die as a separate roll.

Advantage and disadvantage on this page

Be aware of one real limitation: this page has no labelled Advantage/Disadvantage buttons. Those live on the D&D dice roller, which is built for a whole character sheet. Here you roll 2d20, read the two chips, and take the higher face for advantage or the lower for disadvantage yourself. The displayed total will be the sum of the two dice, so ignore it when you are doing this — the chips are the numbers you want.

The maths is worth knowing either way, because advantage is far less uniform in value than people assume. If you need a target number t or better, a straight d20 succeeds with probability (21 - t)/20. With advantage you fail only if both dice fall short, so the chance becomes 1 - ((t-1)/20)².

Advantage is worth close to 25 percentage points when the check is near a coin flip, and under 5 points when you needed a natural 20 anyway. Expressed differently: the average of the higher of two d20 is 13.825 against a plain d20's 10.5, so advantage is worth roughly +3.3 on the roll on average — but that average hides the fact that it does almost nothing at the extremes and a great deal in the middle.

Natural 1 and natural 20

Each face of a fair d20 comes up 5% of the time, so a natural 20 and a natural 1 are equally likely and each appears about once every twenty rolls. Over a three-hour session with a hundred d20 rolls at the table, five of each is the expected count, and the odds of a table going a whole hundred rolls without a natural 20 are vanishingly small — roughly 0.95¹⁰⁰, about six in a thousand.

In fifth-edition D&D the special treatment of 1 and 20 applies to attack rolls (automatic hit, automatic miss) and death saving throws, not to ordinary ability checks and saves, where a 20 is simply the highest possible result and can still fail a high DC. Table rules vary and plenty of groups house-rule critical successes on checks; the die does not care either way. Rolling with advantage, the chance of seeing at least one 20 across the two dice is 1 - (19/20)² = 9.75%, and the chance of a double natural 1 is 1/400 = 0.25%.

Why the d20 feels swingier than 2d6

Because it is. Every face of a d20 is equally likely — the distribution is flat, so a 3 is exactly as probable as an 11. Two six-sided dice produce a peaked distribution where the middle result is six times more likely than either extreme. Systems built on 2d6 or 3d6 make character skill dominate; the d20 makes the die dominate.

That last row is the practical consequence. On a d20 every point of modifier is worth exactly 5 percentage points against any DC it can still affect — a +3 proficiency bonus is a flat +15 points whether the DC is 10 or 18. On a bell curve the same +1 is worth almost nothing at the tails and a great deal in the middle. It is why a +2 item feels reliably useful in a d20 system and situationally useful in a 2d6 one.

Working out a DC check

Success chance on a DC with a modifier m is (21 - (DC - m))/20, floored at 5% and capped at 95% wherever a natural 1 auto-fails and a natural 20 auto-succeeds. A worked case: a +5 modifier against DC 15 means you need a 10 or better on the die, which is (21-10)/20 = 55%. Bump the modifier to +7 and you need an 8 or better: (21-8)/20 = 65%. Each point of modifier moved it exactly 5 points, as expected.

Common target numbers, unmodified: DC 5 succeeds 80% of the time, DC 10 succeeds 55%, DC 15 succeeds 30%, DC 20 succeeds only 5%. Note DC 10 is 55% rather than 50% — "meeting" a DC succeeds, so you need a 10, not an 11. The off-by-one catches people constantly.

Success chance by modifier and DC

Because the d20 is flat, the whole table is one subtraction and a multiplication by five. Find your total modifier, find the DC, read the chance. Values are before any auto-success or auto-failure rule your table applies.

The 100% entry is the one to look at. With +10 against DC 10 the die literally cannot fail on arithmetic alone — every face from 1 to 20 clears it — which is why rulesets that auto-fail on a natural 1 change the answer to 95% and rulesets that do not leave it at certainty. Roll 1d20+10 here a few dozen times against a mental DC of 10 and you will never see a failure; that is not the tool being generous, it is the range.

The mirror case is the bottom-right corner. Even a +7 character facing DC 20 is a coin flip, and a +0 character facing DC 20 needs the single best face on the die. When a game master says "you can try, but it will be hard", 15% or 5% is usually what that sentence means numerically.

Where the numbers come from, and where they go

Rolls are generated in your browser from JavaScript's Math.random(), scaled to the die size — 1 + floor(random × sides), giving every face equal probability. There is no server call, no account, and nothing about your rolls leaves the page; the history lives in memory and disappears on refresh. That also means it works offline once loaded, which is handy at a table with poor signal.

The flip side, stated plainly: this is a pseudorandom generator, not a cryptographic one — the tool does not call crypto.getRandomValues. For a game, a classroom demonstration, or any decision where nobody is trying to predict your dice, that is entirely fine. For anything where an adversary would benefit from predicting the sequence, it is the wrong tool.

Common questions

Can I roll several d20 at once? Yes — set the dice count in the builder or type 5d20. You will get five faces as chips and their sum as the total. There is no option to total only the highest.

Is the die weighted or does it avoid repeats? Neither. Each roll is independent and uniform, so rolling three 20s in a row is possible and has probability 1/8000. The tool has no memory of previous rolls beyond displaying them.

Why did my roll history stop growing? It holds the most recent 30 entries; older rolls fall off the bottom.

Can I roll a d2 or a d3? Yes — the sides field accepts anything from 2 upward, so 1d3 and 1d2 work even though there is no quick button for them. The same goes for oddities like 1d7 or 1d30.