D100 Dice Roller

Roll one or more virtual 100-sided dice. Each die returns a random whole number from 1 to 100, and you get the individual rolls, the total, and the average.

Quick Facts

Method
Uniform random integer from 1 to 100 per die
Each value is equally likely (1% each); the mean of one die is 50.5.

Your Results

Rolled
Rolls
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Each die, 1 to 100
Total
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Sum of all dice
Average
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Total ÷ number of dice

Ready

Choose how many dice and press Roll.

Understanding the d100

A d100 is a hundred-sided die: it produces a whole number from 1 to 100, and every value is equally likely. This roller simulates that by drawing a uniform random integer in the range 1–100 for each die. It is used in tabletop role-playing games such as Call of Cthulhu and older editions of Dungeons & Dragons (for percentile checks and random tables), and any time you need a fair one-in-a-hundred outcome.

How the roll is generated

For each die, the tool computes Math.floor(random × 100) + 1, which maps the interval [0, 1) into the integers 1 through 100. When you roll several dice, it reports each individual result, their total (the sum), and the average (total ÷ number of dice).

Percentile ("d100") dice in real life

A true 100-sided die exists (a "Zocchihedron"), but at the table a d100 is almost always rolled as percentile dice: two ten-sided dice, where one die is read as the tens digit (00, 10, 20, … 90) and the other as the ones digit (0–9). You add them, and the special reading of "00" + "0" counts as 100 rather than 0. The result is still a uniform number from 1 to 100.

The probability math

  • Each outcome: P(roll = k) = 1/100 = 1% for any k from 1 to 100.
  • Expected value (mean): E = (1 + 100) / 2 = 50.5. For N dice the expected total is N × 50.5.
  • Ranges: because outcomes are uniform, P(roll ≤ k) = k/100. So P(roll ≤ 30) = 30%, and P(roll > 95) = 5% — a common "critical" threshold.
  • Variance of one die: for a discrete uniform 1…100, Var = (100² − 1)/12 = 9999/12 ≈ 833.25, giving a standard deviation of about 28.87.

Common reference points

  • Roll ≥ 96 (top 5%): 5% chance — often used for rare successes or critical results.
  • Roll ≤ 5 (bottom 5%): 5% chance — often used for fumbles or rare failures.
  • Roll of exactly 100 (or 00): 1% chance.
  • Roll ≤ 50: 50% chance — a coin-flip threshold.

Frequently Asked Questions

What is a d100?
A d100 is a 100-sided die that produces a whole number from 1 to 100, each equally likely with a probability of 1/100 (1%). In tabletop games it is usually rolled as "percentile dice": two ten-sided dice, one read as the tens digit (00–90) and one as the ones digit (0–9), with 00 and 0 read together as 100.
What is the average roll of a d100?
The expected value (mean) of a single d100 is (1 + 100) / 2 = 50.5. Over many rolls the running average converges toward 50.5. For N dice the expected total is N × 50.5 — for example, three d100 average a total of 151.5.
Is this roller fair and random?
Each roll uses the browser's built-in random number generator to pick a uniform integer from 1 to 100, so every value has an equal 1% chance and each roll is independent of the last. It is suitable for games and simulations, but it is not a certified cryptographic randomness source.
How do I roll a d100 with two ten-sided dice?
Designate one d10 as the "tens" die (its faces read 00, 10, 20 … 90) and the other as the "ones" die (0–9). Add them: 40 + 7 = 47. If both dice show zero (00 and 0), that reads as 100, not 0.

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