Random Number Generator

How the randomness works

Numbers are generated using the Web Crypto API's crypto.getRandomValues(), the same cryptographically secure randomness source used for encryption keys, with rejection sampling to avoid the small statistical bias that a naive Math.random() % range approach can introduce.

Common uses

• Picking a winner from a numbered list of entries
• Generating sample data for testing
• Settling a decision or assigning random order

Worked examples

• Lottery-style pick: range 1 to 49, 6 numbers, duplicates off. There are 13,983,816 possible sets of six, and the numbers come out in the order drawn, not sorted.
• Two dice: range 1 to 6, 2 numbers, duplicates on.
• Raffle: tickets numbered 1 to 250, 3 winners, duplicates off, so no ticket can win twice.
• Why the duplicate switch matters: drawing 5 numbers from 1 to 10 with duplicates allowed has a 69.76% chance of at least one repeat, because 1 − (10 × 9 × 8 × 7 × 6) ÷ 10^5 = 0.6976.

Rules of the tool

• Both ends of the range are included, the minimum must not exceed the maximum, and the results appear as a comma-separated list.
• The count is kept between 1 and 1,000, and a larger number you type is reset to 1,000. Asking for more unique numbers than the range holds, such as 20 from a range of 10, shows an error instead of repeating values.
• The minimum and maximum must be whole numbers; a decimal such as 1.5 shows a message, and so does a minimum above the maximum.
• Bounds can be any whole numbers from -9,007,199,254,740,991 to 9,007,199,254,740,991, so a range such as 1 to 5,000,000,000 works. Larger values can't be represented exactly and are rejected.
• With duplicates off, only the numbers already drawn are tracked, so even a very wide range stays fast.

Fair, but not an audit trail

Every value in the range has the same chance, and the random source is the one used for cryptographic keys. Nothing is recorded or verifiable afterwards, though, so for a prize draw, lottery or giveaway that must be provable, use a method the organizer or a regulator accepts. Rules for lotteries and sweepstakes differ by country and, in some places, by state or province.