Random Number Generator

Generate a truly random number between any minimum and maximum — whole numbers or decimals, with optional filters for even, odd, multiples of a number, or specific values to exclude. Every result comes from a cryptographically secure source with no bias toward any number in the range.

Why most "random" number code is subtly biased

The obvious way to generate a random number in a range is min + Math.random() * (max - min), or for integers, value % range. Both have real problems. Math.random() is not cryptographically secure and, in older engines, has measurably uneven bit distribution. The modulo approach has a subtler flaw: unless the range divides the generator's output space evenly, some numbers land more often than others — an effect called modulo bias.

This generator avoids both problems. It draws from crypto.getRandomValues(), the same cryptographically secure source browsers use for encryption keys, and uses rejection sampling to eliminate modulo bias entirely: any draw that would fall in the uneven leftover region is discarded and redrawn. Every number in your range ends up with exactly equal probability.

How rejection sampling works

The core algorithm

range = max − min + 1 limit = 2³² − (2³² mod range) draw a random 32-bit value; if it's ≥ limit, discard and draw again result = min + (accepted value mod range)

limit is always an exact multiple of range, so once a draw is below it, every possible remainder from 0 to range−1 has occurred the same number of times. Rejected draws happen rarely — for small ranges, well under 1% of draws — and cost nothing but a fast, invisible retry.

Decimals use the same algorithm, scaled

Rather than a separate method for non-integer results, decimal mode rounds your bounds to the chosen number of decimal places, multiplies by a power of ten to turn the range into whole numbers, runs the identical unbiased draw, and divides back down. A request for "0.4 to 85.8, 1 decimal place" becomes "4 to 858" internally — so decimal results inherit the same bias-free guarantee as whole numbers.

A note on very large ranges

JavaScript's Number type cannot represent every integer beyond ±9,007,199,254,740,991 (2⁵³−1). Beyond that point, some values become indistinguishable from their neighbors, which would silently make "random" results inaccurate. This calculator checks not just that your minimum and maximum are individually valid, but that the span between them also fits safely — a huge symmetric range like −9 quadrillion to +9 quadrillion can have two perfectly valid endpoints while the distance between them is still too large to generate accurately. If that happens, you'll see a clear message asking you to narrow the range.

Common uses

Use case	How to set it up
Picking a raffle or giveaway winner	Whole numbers, min 1 to your number of entries
Random sample for a survey or experiment	Whole numbers with an exclude list for anyone already sampled
Simulating a die or spinner	Whole numbers, e.g. 1–6 or 1–20
Random price, weight, or measurement for testing	Decimals, with the precision matching your real data
Team or task assignment	Whole numbers, min 1 to the number of people or tasks

Frequently asked questions

Is this generator truly random?

It uses your browser's cryptographically secure random number generator (the Web Crypto API), the same source used for encryption. Combined with rejection sampling to remove modulo bias, every number in your chosen range has exactly equal probability of being selected.

What is modulo bias?

It's a subtle skew that appears when a random number generator's output range doesn't divide evenly into the range you want. Numbers near the low end of your range end up very slightly more likely than numbers near the high end. It's small enough to be invisible in casual use, but it's real — this calculator avoids it entirely with rejection sampling instead of a plain modulo operation.

Can I generate a random decimal number?

Yes — switch to the Decimals tab, set your minimum, maximum, and how many decimal places you want. A range like 0.4 to 85.8 with 1 decimal place will return results like 47.3, evenly spread across that grid of possible values.

Can I generate a random number from a negative range?

Yes, both whole numbers and decimals support negative minimums and maximums — for example −50 to 50, or −10.5 to 10.5. If you accidentally enter the minimum larger than the maximum, they're swapped automatically.

How does excluding numbers work?

List the numbers to avoid, separated by commas. The generator draws as normal and simply discards and redraws any result that matches your excluded list, so the remaining numbers stay equally likely relative to each other.

Need a two-sided random outcome instead? Try the Coin Flipper or the Dice Roller.