Random Number Generator

The range is inclusive at both ends, and the bias is one part in a billion

A range of 1 to 100 here means 100 possible numbers, both ends included. That sounds obvious and it is the single most common off-by-one in anything that picks a random number, so it is worth stating.

Each number comes from a cryptographically random 32-bit value, reduced into your range with a remainder. That carries a small and well-known bias: unless the size of your range divides 4,294,967,296 exactly, some values are backed by one more source value than the others. For a range of 100, the remainder is 96, so 96 of the 100 values are favoured by about one part in a billion. For a range of 2 the remainder is zero and there is no bias at all.

That is far too small to matter for a prize draw, a sample or a test fixture. It is not acceptable for generating a secret, but neither is this page: a password or a key should come from a tool built for it, and this site has a password generator for that.

Unique mode gets slow at the end, for a reason worth knowing

With Unique ticked, the page draws a number, throws it away if it has seen it before, and draws again. That is simple and correct, and it gets expensive as the range fills up.

Asking for 100 unique numbers from 1 to 100 takes about 519 draws on average, not 100. Asking for all 500 from a range of 500 takes about 3,396. The last few are the problem: when 499 of 500 have been used, each draw has a one in 500 chance of being the one you still need. Statisticians call this the coupon collector’s problem.

Leave yourself headroom and it stops being an issue. Drawing 500 unique numbers from a range of 1,000 takes about 693 draws, barely more than the 500 you asked for. If you want a full shuffled list rather than a sample, you want a shuffle, not repeated drawing.

Asking for more unique numbers than the range contains is refused outright, with "Unique count exceeds range size.", rather than looping forever.

Things the page quietly fixes or quietly limits

• A minimum above the maximum is swapped, not refused. Enter 100 and 1 and you get numbers from 1 to 100. This is a convenience, and it means a typo in one field will not produce an error you can see.
• The count is capped at 500, silently, and a count below 1 becomes 1.
• Only whole numbers. There are no decimals, and a decimal you type is truncated by the parser rather than rounded.
• Non-numeric input is refused with "Enter valid integers." rather than being guessed at.
• Nothing leaves your browser, and nothing is stored between visits, so a list you need to keep must be copied out before you close the page.

Sources

• The browser’s cryptographic random source: crypto.getRandomValues(). developer.mozilla.org
• An example of range generation that removes the bias by rejection rather than modulo: Node.js crypto.randomInt(). nodejs.org/api/crypto
• Why the fallback is not good enough for secrets: Math.random(). developer.mozilla.org

The figures on this page were computed from this tool’s own code on 9 October 2026. Where a probability is marked as simulated, it came from 20,000 simulated runs and is an estimate, not an exact value.