Casino random number generators (RNGs) are the software that decides every result in online slots and digital table games, picking each spin, roll or card from a stream of unpredictable numbers. In Great Britain, the Gambling Commission requires RNG output to be acceptably random and independently tested before a game goes live. A fair RNG still delivers the game's house edge, because that comes from the payouts.
An RNG produces a stream of numbers that are scaled and mapped onto results, such as one of 37 pockets or a reel position.
Gambling Commission standard RTS 7 requires output that is acceptably random, uniformly distributed and computationally infeasible to predict.
New games and RNGs can only be released once testing is complete and the report is with the Commission.
Compensated games and fake near misses are banned in UK online gambling.
A perfectly random RNG still carries the game's house edge: 2.70% on a single-zero roulette game.
UK standard
RTS 7: generation of random outcomes
Required quality
Acceptably random, uniformly distributed
Prediction
Must be computationally infeasible
Testing
Documentation, source code, statistical output
Approved test houses
Listed by the Gambling Commission
What casino random number generators do
An online roulette game has no wheel and an online blackjack game has no cards. Something has to stand in for the ball, the dice and the shuffle, and that something is a random number generator. Every time you press spin or deal, the game takes the next value from its RNG and turns it into a result: number 17, a pair of fives, a king of hearts or a particular set of reel positions.
Most RNGs in software are pseudo-random: an algorithm, started from a seed value, produces a long sequence of numbers that behaves like true randomness for all practical purposes. Some systems draw on physical sources of randomness too. What matters for players is not the method but the standard it meets, and in Great Britain that standard is set by the Gambling Commission's remote technical standard RTS 7 (opens in a new tab), covered below. This page is part of our casino maths section; for the wider question of fairness, see are online casino games fair.
From random number to result: scaling and mapping
An RNG's raw output is usually a very large number. A roulette game needs one of 37 outcomes, a dice game one of 6 per die, a card game one of 52 cards. Converting raw output into those outcomes is called scaling, and done carelessly it can quietly bias a game.
Worked example: why careless scaling breaks a wheel
Imagine, to keep the arithmetic simple, an RNG that produced whole numbers from 0 to 99, and a programmer who assigned them to 37 pockets by taking the remainder after dividing by 37. 100 = 2 × 37 + 26, so 26 pockets would receive three of the hundred values and 11 pockets only two. Those 26 pockets would land 3% of the time and the other 11 only 2%, instead of 2.70% each. A real RNG's range is vastly bigger, which shrinks the problem, but the principle is the reason the regulator cares.
The usual fix is to throw away values from the incomplete final block (here 74 to 99) and draw again, so every pocket gets exactly the same number of values. RTS 7 guidance allows for exactly this: random numbers should be used in the order received and not discarded unless they are outside the expected range, and any scaling must keep the output's random qualities. Then the numbers must be mapped onto outcomes according to the game's stated probabilities and paytable.
How an RNG stands in for physical equipment
Game
What the RNG decides
Outcomes it must treat fairly
Chance of each
European roulette
Winning pocket
37 pockets
1 in 37 (2.70%)
American roulette
Winning pocket
38 pockets
1 in 38 (2.63%)
Craps or sic bo
Face of each die
6 faces per die
1 in 6 per die
Blackjack or baccarat
Order of the shuffled cards
Every arrangement of the cards
Each card equally likely to be next
Slots
Stop position on each reel
Every position on the reel strip or symbol set
Set by the game's design and paytable
UK rules for casino random number generators
The Gambling Commission's Remote Gambling and Software Technical Standards (RTS) apply to games offered to players in Great Britain. RTS 7, on the generation of random outcomes, has three main requirements.
What RTS 7 asks of a casino RNG
Requirement
What it says
What it means for players
7A: random generation
Results must be 'acceptably random', provably so to a high degree of confidence, for example by generally accepted statistical tests. Output should be uniformly distributed over its range, not repeat a cycle, and be computationally infeasible to predict. Compensated (adaptive) games are not allowed.
No outcome is favoured, nothing can be forecast, and the game cannot adjust its odds to make up for past results.
7B: fair implementation
Games must follow their rules and prevailing payouts; random numbers are mapped to outcomes by the stated probabilities and paytables, used in order, and not discarded unless outside the expected range.
The maths you read in the help screen is the maths that runs.
7C: no misleading design
Designs must not mislead about the likelihood of outcomes, including 'substituting losing events with near-miss losing events'. Simulated wheels, dice and cards should have the same theoretical probabilities as the real thing, and several simulated devices should be independent.
A digital wheel behaves like a fair real wheel, and a near miss is not manufactured to tempt you.
Further guidance covers seeding and re-seeding, which must not introduce predictability, and requires that two instances of an RNG do not produce the same stream. Other standards sit alongside RTS 7: RTS 3 on showing rules and the chance of winning, and the game design rules on speed and features. For the full set, see our guide to the remote technical standards.
Who tests casino RNGs, and how
The Commission's testing strategy (opens in a new tab) says RNG testing covers three things: a review of documentation, a review of source code, and statistical testing of the raw output. It also says that all new games and RNGs can only be released once testing has been completed and the report provided to the Commission. Test reports go onto the operator's games register through the Commission's eServices system, and updates made through an in-house process are audited each year by an approved test house.
For certain licence types, the testing must be carried out by an independent test house on the Commission's approved list. The approved test houses (opens in a new tab) page named 14 labs as of October 2026, all approved for the remote technical standards, including eCOGRA, several Gaming Laboratories International (GLI) companies and BMM. Approvals differ by area, so a lab approved for remote games is not automatically approved for gaming machines, and the list changes over time. Our page on casino game testing labs goes through it, and how casino games are tested explains the process step by step.
A testing lab's logo on a website is not proof on its own. The reliable check is that the casino holds a Gambling Commission licence, which brings the testing rules with it.
A fair RNG still carries the house edge
A common misunderstanding is that a 'fair' game must be a break-even one. Fairness in the RNG sense means every outcome turns up at its true probability. The casino's margin comes from paying less than those probabilities justify. An RNG European roulette game must give every pocket a 1 in 37 chance, and it still pays 35 to 1 on a single number, so the house edge is 2.70% exactly as on a physical wheel. See true odds vs payout odds for how that gap is built.
The same goes for slots. The RNG chooses reel positions fairly, but the reel design and paytable set the RTP. A 96% RTP game has a 4% edge, so £1 spins for 500 spins (£500 staked) cost £20 on average. The RNG decides who gets which share of the returns; it does not decide how big that share is overall.
RNG blackjack and the shuffle
Wizard of Odds says most online casinos shuffle after every hand in RNG blackjack, and that card counting is useless when that happens. Shuffling every hand actually trims the house edge slightly compared with dealing deep into a shoe: by 0.02% in a six-deck game and 0.11% in a single-deck game, according to its online blackjack notes (opens in a new tab). The edge you face still depends on the table's rules, such as 3:2 or 6:5 on a blackjack.
RNG games vs live dealer games
Live casino games replace the RNG with physical equipment filmed in a studio: a real wheel, real cards, real dice. The Commission treats these as mechanical RNGs, and its guidance says players should not be able to interact with or touch the mechanism. Some games mix the two. Evolution's Lightning Baccarat adds one to five RNG-generated Lightning Cards with multipliers to a live deal, and the studio describes First Person Lightning Roulette (opens in a new tab) as an RNG roulette game.
RNG and live versions of the same game
Feature
RNG game
Live dealer game
Source of results
Software random number generator
Physical wheel, cards or dice
House edge, single-zero roulette
2.70%
2.70%
Card shuffling at blackjack
Usually every hand
Dealt from a physical shoe; shuffle method varies by table
Minimum game cycle in Great Britain
5 seconds for non-slot games; 2.5 seconds per slot spin
5 seconds per game cycle
How fairness is checked
Code review and statistical testing
Equipment and procedures, with no player contact with the mechanism
The maths is the same either way; the experience differs. Our comparison of live casino vs RNG games covers speed, limits and atmosphere, and RNG vs live roulette looks at the wheel game specifically.
Casino RNG myths
Advantages
Fact: RNG output must be computationally infeasible to predict, so no pattern or timing trick can forecast it.
Fact: compensated games are banned online in Great Britain, so a game cannot 'owe' you a win.
Fact: near misses may not be substituted for losing results to mislead players.
Drawbacks
Myth: 'slots pay more at night or at weekends'. The RNG is not tied to the clock.
Myth: 'the casino can switch a game to tight mode when you're winning'. Games must run as tested and registered.
Myth: 'RNG games are rigged and live games are honest', or the reverse. Both must meet Commission standards and carry the same edge for the same rules.
Belief in patterns is so widespread that it has names: the gambler's fallacy expects results to even out, and the hot hand fallacy expects streaks to continue. A properly tested RNG makes both irrelevant.
How to check an online casino game is fair
Check the casino holds a Gambling Commission licence. The operator's licence number should be on its site, and you can look it up on the Commission's public register.
Open the game's help or rules screen. UK rules require the rules and information on your chance of winning, such as the RTP or house edge, to be available before you play.
Note the game provider. Many studios publish their game rules and RTP figures on their own sites.
Be wary of sites that are not licensed for Great Britain. They offer none of the UKGC testing protections described here.
If something seems wrong, keep a record and complain to the casino first, then to its alternative dispute resolution provider.
Our guide on how to check a licence walks through the register, and who checks that online casino games are fair summarises the roles of the regulator and test houses. Remember that a fair game is still a paid-for form of entertainment: the house edge means losses are likely over time.
RNG games and the offers on this page
The partner offers here are mostly for RNG slots, and none affects how a game's random number generator works or the edge it carries. All are 18+ and for new customers, and terms apply. Casumo: a 50% bonus up to £100 on a first deposit of at least £20 for customers who opt in, wagered 10x within 7 days, with a £5 maximum bet and game, play and payment restrictions. 10bet: a 100% match up to £25 when the casino welcome bonus is selected at sign-up and the first deposit is made within 7 days; it is wagered 10x on slots within 3 days, bonus winnings are capped at £1,000, and Skrill and Neteller deposits are excluded.
Coral: after a qualifying deposit, bet £10 or more on slots for 100 free spins at 10p each on selected games, accepted within 48 hours and valid for 7 days, with no wagering on winnings. Neteller, PayPal, Paysafe, Skrill, prepaid cards and certain debit cards do not qualify. RNG and live table games are often excluded from bonuses or count less towards wagering, so check the terms before playing them.
Significant terms: 18+. New customers only. Opt-in required. 50% deposit bonus up to £100 on first deposit. Minimum deposit £20. 10x wagering on the bonus amount. Maximum bet or spin £5. Bonus valid for 7 days. Game, play and payment method restrictions apply. Full T&Cs apply. GamCare.org.uk
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Read our 10bet reviewLicence: Blue Star Planet Limited, Gambling Commission account 43173. Offer checked 5 October 2026.
Significant terms: 18+. New players only. Offer runs 01/10/2026 to 01/02/2027. Deposit (certain types excluded) and bet £10+ on slot games to get 100 free spins (selected games, £0.10 each, 48 hrs to accept, valid 7 days). No wagering on free spin winnings. Deposits by Neteller, PayPal, Paysafe, Skrill, prepaid cards and certain debit cards do not qualify. Restrictions and T&Cs apply. GamCare.org.uk
Read our Coral reviewLicence: LC International Limited, Gambling Commission account 54743. Offer checked 5 October 2026.
18+. Every casino game has a built-in house edge, so over time you should expect to lose what you stake. Set a budget before you play and stop when it’s gone: safer gambling tools and support. Free, confidential help: National Gambling Helpline 0808 8020 133.
Frequently asked questions
How do casino random number generators work?
A casino RNG is software that produces a continuous stream of unpredictable numbers, which the game scales and maps onto results. Press spin on a roulette game and the next number is converted into one of 37 pockets, each with an equal chance. In Great Britain the output must be acceptably random, uniformly distributed and computationally infeasible to predict under RTS 7. Our page on how casino games are tested explains how this is checked before release.
Are online casino RNGs rigged?
Games from Gambling Commission-licensed casinos must use RNGs that meet strict standards, and they cannot be released until testing is complete and reported to the Commission. Compensated games that adjust odds based on past results are banned, as are fake near misses. That does not mean you will break even: the house edge still applies because payouts are below true odds. Unlicensed sites offer none of this protection. See are online casino games fair for more.
Who tests casino RNGs in the UK?
Independent test houses approved by the Gambling Commission test RNGs and games for licensed operators. The Commission's list named 14 approved labs as of October 2026, including eCOGRA, several GLI companies and BMM, all approved for the remote technical standards. Testing covers documentation, source code and statistical analysis of the output. Approvals differ by area, so the Commission's own list is the place to check. Our casino game testing labs page covers each step.
Can you predict an RNG slot or roulette game?
No, a compliant RNG cannot be predicted. The Gambling Commission's standard says it should be computationally infeasible to predict the next number, and that seeding and re-seeding must not introduce predictability. Watching past results, timing your spins or choosing a time of day makes no difference to the outcome. Each result is independent of the last. The can you predict roulette numbers page tackles the most common prediction claims.
Is an RNG roulette game the same odds as a real wheel?
Yes, an RNG roulette game must reproduce the probabilities of the real device. RTS 7 guidance says a simulated wheel, dice or deck should have the same theoretical probabilities as the physical version. So a European RNG game gives each number a 1 in 37 chance and carries a 2.70% house edge, the same as a live single-zero wheel. The difference lies in speed and presentation. Compare the two on our RNG vs live roulette page.
Do RNG games have a memory of past results?
No, RNG games do not remember past results in any way that changes future odds. UK rules ban adaptive or compensated games in remote gambling, so a slot cannot increase its payouts after a dry spell or tighten up after a big win. Each spin draws a fresh random number. Some older land-based fruit machines historically worked differently, which is where the idea comes from. See do online slots have memory for the full answer.
Does a fair RNG mean the game has no house edge?
No, a fair RNG only means each outcome occurs at its true probability; the house edge comes from the payouts. An RNG European roulette game picks every pocket with a 1 in 37 chance but pays 35 to 1, so the edge is 2.70%. A slot's RNG chooses reel positions fairly, while the reel design and paytable set its RTP, such as 96%. Read RTP explained to see how return figures relate to cost.
What is the difference between an RNG game and a live casino game?
An RNG game uses software to generate results, while a live game uses physical equipment filmed in a studio, such as a real wheel or real cards. The Commission treats live equipment as a mechanical RNG and expects players to have no contact with it. Both carry the same edge for the same rules. RNG games usually shuffle cards every hand; live games use a shoe. Our live casino vs RNG page compares speed, limits and atmosphere.
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