Random number generator casino guide

What a random number generator does in a casino game

Every digital casino game needs a source of chance. In a physical casino that is a wheel, a shuffled shoe or a pair of dice; online it is a random number generator, or RNG. It produces a stream of numbers with no predictable pattern, and the game software turns those numbers into outcomes: which symbols land, which card comes next, which pocket the ball finds. The RNG glossary entry gives the short version.

Most casino RNGs are pseudo-random number generators: algorithms that start from a seed value and produce numbers that pass statistical tests for randomness. Done properly, it is computationally infeasible to predict the next number, even knowing the algorithm. Many systems mix in unpredictable inputs, such as hardware noise or timing data, when seeding.

The RNG runs on the game server, not on your phone or laptop. When you press spin, your device sends the request, the server draws the numbers, settles the round and sends back the result for the animation to display. That is why a dropped connection doesn’t change an outcome, and why ‘malfunction voids all pays and plays’ clauses refer to the server record.

From random number to spin result: how mapping works

A raw random number means nothing until it is mapped to the game. In a slot, each reel has a strip of symbols, often longer and more complex than what you see. The RNG picks a stop position on each strip, and the symbols at those positions form the result. The paytable then decides what, if anything, is paid. On a deck-based game, the RNG shuffles the virtual cards, typically before every hand.

Mapping is also where unfairness could creep in, which is why UK rules regulate it. The Commission’s standard RTS 7B requires random numbers to map to outcomes in line with the stated probabilities and pay tables, used in the order they are produced, and not discarded to steer results.

UK rules for casino random number generators

Random outcomes are governed by RTS 7, part of the Gambling Commission’s remote gambling and software technical standards (opens in a new tab), which apply to every operator licensed to offer casino games to people in Great Britain.

Two of these deserve attention because they answer the most common suspicion. The ban on adaptive, or ‘compensated’, games means a slot can’t tighten after a big win or loosen after a losing streak. The near-miss rule means a game can’t replace an ordinary loss with one that looks agonisingly close, such as two jackpot symbols and a third just above the line, more often than the true probabilities would produce. See our near-miss glossary entry and the wider game design rules.

How RNGs are tested before and after release

The Commission’s testing strategy says games and RNGs for remote casino licences must be tested by an independent test house from its approved list, and new games and RNGs can only be released once testing is complete. The process typically runs as follows.

1. The studio submits the RNG’s documentation and source code to an approved test house.
2. The test house reviews the code and runs statistical tests on raw RNG output and on scaled results or shuffled decks.
3. The game itself is checked: design, rules, theoretical RTP, and actual RTP through simulation and manual play.
4. Results go to the Commission before the game is released to UK players.
5. After release, updates that affect fairness must be retested before going live; minor updates follow change controls.
6. Each year, licensees responsible for game testing commission an audit by an approved test house, which includes checking that they have effective live RTP monitoring.

Our guides to how casino games are tested and the testing labs that do the work explain what a test certificate shows and which labs are involved.

Live dealer games: when the RNG is a real wheel

Live casino games don’t use a software RNG for the core result. The outcome comes from a physical device: a roulette wheel and ball, a dealer’s shuffled shoe, a set of dice or a game-show wheel, streamed from a studio. Some live game shows add RNG-generated multipliers on top, which fall under the software rules.

RTS 7 covers physical devices too. Their components must be made of materials that don’t deteriorate over time, the items used for selection must not be altered, and players must not be able to interact with or manipulate the mechanics. Our answer to are live dealers real and the live versus RNG comparison explain the differences in pace, cost and fairness checks.

Online RNG blackjack usually reshuffles before every hand, so card counting has no effect. Live tables use multi-deck shoes with regular shuffles; see card counting online for why that limits counting too.

A fair RNG still has a house edge

It’s easy to assume that a truly random game should be an even contest. It isn’t, because the randomness only decides which outcome occurs; the payouts decide what each outcome is worth. A perfectly random European roulette wheel still pays 35 to 1 on a 36 to 1 chance, giving the house 2.70% of every pound staked on average.

In slots, the same principle runs through the reel strips and paytable, set so that the game returns its RTP over the long run. The RNG ensures every spin is independent and unpredictable; it doesn’t make the game generous. Our guide to the house edge shows how that margin is built and what it costs in pounds.

Volatility is a design choice too, not an RNG setting. Two slots with identical RNGs can have very different swings depending on how their paytables concentrate wins, as covered in slot volatility explained.

Random number generator myths at online casinos

Each of these has its own explainer: do slots pay more at night, do casinos know if you are winning and can you predict roulette. The belief that a result is ‘due’ is the gambler’s fallacy. UK players can try demos only after age verification, a rule in place since May 2019; see the demo play guide.

How to check a casino’s games are independently tested

You can’t inspect an RNG yourself, but you can check the things that make testing compulsory. The key one is the licence: a site licensed by the Gambling Commission must use tested games and RNGs, and an unlicensed site has no such obligation.

• Find the licence holder and account number in the site footer, then search the Gambling Commission’s public register (opens in a new tab) by name, domain or number.
• Check the domain you’re using is listed against that licence.
• Open a game’s help screen for its rules, RTP and studio name.
• Look for the studio and its test house; many studios name their lab on their own sites.
• Avoid offshore sites that accept UK players without a UK licence.

Our walkthrough on how to check a casino licence shows the register in detail, and are online casinos rigged and how do I know if a slot is fair answer the questions people ask most. The risks of unlicensed sites are covered in offshore casinos explained.

Randomness, losses and safer play

Knowing that results are random is protective. It means no pattern, ritual or system can predict the next outcome, and that chasing losses in the hope of a ‘due’ win has no basis. Over time, the house edge means losses are the likely result of any casino session.

Since 31 October 2021, UK slots have shown your net position and session time, banned autoplay and set a minimum spin time of 2.5 seconds, with similar rules extended to other online casino games from 17 January 2025. Use them, along with deposit limits and time-outs. If gambling stops being fun, the National Gambling Helpline on 0808 8020 133 is free and open 24 hours a day. More explainers are in our online casino guides.

Licensed casinos using tested RNG games

Our three featured partners are licensed by the Gambling Commission, so their RNG games must meet the testing rules above. Coral’s bet £10, get 100 free spins (18+, new players, 10p spins on selected games, accept within 48 hours, valid seven days, no wagering on winnings; certain payment methods such as PayPal excluded) runs under LC International Limited’s licence, account 54743.

PlayOJO’s 80 free spins on Big Bass Bonanza (18+, new UK customers excluding Northern Ireland, first deposit of £10 or more, 10p each, winnings in cash) runs under Skill On Net Limited, account 39326. Casumo’s 50% bonus up to £100 (18+, opt-in, £20 minimum deposit, 10x bonus wagering within seven days, £5 max bet) runs under Recro Limited, account 61549. A tested RNG means fair randomness, not better odds.

18+. Gambling should be entertainment that costs money, not a way to make it. Set a deposit limit before you play: how to set deposit limits. Free, confidential help: National Gambling Helpline 0808 8020 133.