Random Number Generator

How to use the Random Number Generator

1. Set the minimum and maximum, and how many numbers you want.
2. Switch on "No duplicates" for a draw, raffle or lottery selection.
3. Press Generate. Results of more than twelve numbers also appear as a copyable list.

Where the randomness comes from

This tool uses crypto.getRandomValues, the browser's cryptographically secure generator, which draws from entropy collected by your operating system. That is a meaningful upgrade over Math.random, whose output is predictable if an attacker observes enough of it.

For a dice roll it makes no practical difference. For anything with a prize attached, or where someone might benefit from predicting the outcome, it matters.

Avoiding modulo bias

The obvious way to get a number from 1 to 6 is random % 6 + 1. It is subtly wrong. If the underlying generator produces values 0 to 9, then 0–5 map to each result once but 6–9 wrap around to give 1–4 a second chance. Those outcomes become more likely.

The fix is rejection sampling: discard any raw value that falls in the uneven tail and draw again. This tool does that, so every number in your range is equally likely. Most quick implementations skip it, which is why homemade random pickers often show a measurable skew over thousands of draws.

Unique selection done properly

Picking unique numbers by drawing repeatedly and discarding duplicates works badly when you want most of a range — selecting 45 unique numbers from 1 to 50 spends most of its time rejecting collisions.

Instead, this tool builds the full range and runs a partial Fisher–Yates shuffle, taking as many values as you asked for. Every possible combination is equally likely, and it runs in constant time per number regardless of how much of the range you want.

Frequently asked questions