How random number generators work and how they are tested

Almost every online casino game that depends on chance uses a random number generator (RNG), the software that decides a card, a spin or a crash point. This guide explains what an RNG is, how it is checked and why earlier results say nothing about the next one.

Pseudo-random, not magic

Most game RNGs are pseudo-random: mathematical procedures that turn a starting value, called a seed, into a long stream of numbers that look random and pass statistical tests. The same seed and procedure give the same stream, so the seed must be unpredictable.

Well-designed systems take the seed from an unpredictable source, such as timing noise in the hardware, and use generators built so that the next output cannot be worked out from earlier ones. The aim is that nobody outside the system can know its internal state.

The raw numbers are then converted into a game outcome, such as one of the 37 pockets on a single-zero roulette wheel or the order of a shuffled deck. The conversion must give every outcome exactly its correct probability. A careless one can add a small bias, which testers look for.

How an RNG is tested

Independent testing laboratories check gaming software from the outside. In general terms, they review the generator's design and source code, run very large samples of its output through statistical tests, and compare the game's behaviour with its published rules.

In many places a licence requires certification by an approved laboratory, and requirements differ by jurisdiction. A certificate is a check of one version of the software against one set of standards. It cannot tell anyone how a session will go, and it does not remove the house edge.

Why a result cannot be worked out from earlier ones

Each draw is independent. A pseudo-random generator has an internal state, but a well-designed one gives no practical way to turn earlier outputs into the next. A run of reds, a series of low crash points or a card that has not appeared for a while does not change the next outcome. Believing it does is called the gambler's fallacy.

Random also does not mean favourable. The house edge comes from the payout table, which pays slightly less than the true odds, not from a rigged generator. Over many rounds, players lose on average.

Be wary of anyone selling software or paid advice that claims to read an RNG. A properly built one cannot be read that way, and no betting system changes the odds of a random game.

This guide is for information only and is for adults aged 18 or over, or the legal age where you live. If gambling stops being fun, take a break and consider contacting a local support service.

18+ only. This article is for information, not advice to gamble. Responsible gambling.