A random number generator (RNG) is the software that decides the result of every spin, card and roll in a digital casino game. Each time you press Spin, the game takes the RNG’s latest number and turns it into a result: reel positions, a card, a roulette pocket. The result is fixed the moment you play, and nothing before it, such as your last spin or your timing, affects it.
This page explains how casino RNGs work, who tests them, how RNG differs from RTP, which games use one, and which common beliefs about “hot” slots are wrong.
How a casino RNG works
Most online games use a pseudorandom number generator (PRNG). It is an algorithm that produces a long stream of numbers that pass statistical tests for randomness. It starts from a seed value and keeps cycling through numbers continuously, even when nobody is playing.
When you place a bet, the game takes the current number and maps it to an outcome. On a slot, each reel has a list of stops, and the number picks which stop lands in view. On a card game, it picks the next card from a virtually shuffled deck. Because the generator runs very fast and the exact moment of your click is unpredictable, there is no way to time a spin.
Pseudorandom and hardware RNGs
A hardware RNG draws on a physical source of noise, such as electronic noise in a circuit. It is truly random but slower and more expensive. Some systems use hardware noise to seed a PRNG, which combines unpredictability with speed. For players, the difference does not matter much. What matters is that the generator has been tested and the game cannot be adjusted on the fly.
Who tests casino RNGs?
Licensed casinos cannot simply claim their games are fair. Regulators such as the Malta Gaming Authority and the UK Gambling Commission require games and RNGs to be tested by approved independent labs before they go live. Well-known testing labs include GLI, BMM Testlabs, iTech Labs and eCOGRA.
Testing usually covers two things: the RNG itself (are the numbers statistically random and unpredictable?) and the game maths (does the game pay out as its rules and RTP say over millions of rounds?). Once certified, the game version is fixed, and changes need to be tested again.
You can check this yourself. A licensed casino shows its licence in the footer, and many display a testing seal. Our guide to online casino licences explains what each licence means for players.
RNG and RTP are not the same thing
The RNG decides each result at random. The return to player (RTP) is the share of all money staked that the game is designed to pay back over a very large number of rounds, for example 96%. The RTP comes from the game’s paytable and symbol layout, not from the RNG.
That is why a slot with a 96% RTP can pay nothing for 200 spins and then land a big win. The RNG produces random results, and only over millions of spins do they average out near the RTP. Many slots come in several RTP versions, so check the figure in the game’s info screen. Our slot RTP guide covers this in detail.
