Random number generator

How the randomness is made

Numbers come from your browser's cryptographic random source (crypto.getRandomValues), the same source used to make encryption keys. Nothing is sent to a server, and the page cannot be steered towards any number.

The browser hands over a random 53-bit number (a whole number from 0 to about 9 thousand million million). To turn it into a number in your range without favouring any of them, the page uses rejection sampling: if the random number falls in the small leftover slice above the last whole multiple of your range's size, it is thrown away and a new one is drawn. Simply taking the remainder would make the lowest numbers in some ranges slightly more likely; this method does not.

No repeats

With "No repeats" ticked, a number already picked is skipped and another is drawn, so every set of different numbers is equally likely. You cannot ask for more different numbers than the range holds: 1 to 10 has only 10.

Picking from a list

Each line of the list is one item, numbered from 1. The page picks item numbers in the same fair way. Blank lines are ignored. With "Each item at most once", the list is shuffled by the Fisher–Yates method and the first items are taken, which is the standard fair shuffle.

What it is good for, and what it is not

It suits games, classroom questions, choosing who goes first, sampling rows from a spreadsheet and picking a winner among friends. It is not for lotteries, raffles, prize draws or anything with legal or money consequences: those must use an approved, audited draw, and many places have laws about how they are run.