Random Number Generators in Gambling: What an RNG Does and Doesn’t Do

A random number generator is part of the system used to produce results in online slots, digital casino games, virtual events, bingo and some lotteries.

It does not remember that you have been losing. It does not decide that a particular account has won enough. It should not quietly change the odds because a game has paid out more than expected.

For gambling regulated in Great Britain, the Gambling Commission requires random outcomes to be acceptably random and prohibits adaptive behaviour that changes probabilities during play.

That does not make every result equally likely. The RNG supplies random inputs. The game’s mathematics decide what those inputs represent.

The RNG is only one part of the game

A software RNG produces a stream of numbers. The game then converts those numbers into recognised outcomes.

In a slot, that could involve mapping numbers to reel positions or symbols. In digital roulette, a number may correspond to one of the available results. In a virtual card game, random inputs may be used to shuffle or select cards.

The exact process depends on the product, but three components need to work together:

1. The RNG produces the random input.
2. The game maps that input to an outcome.
3. The rules and paytable determine whether the outcome wins and what it pays.

An RNG does not set RTP by itself. RTP comes from the complete probability model, including the available outcomes and the amounts attached to them.

Our guides to RTP and odds, probability and payouts explain those calculations in more detail.

Why software randomness is usually pseudorandom

Most software RNGs are pseudorandom number generators.

“Pseudorandom” does not mean obviously predictable or fake. It describes a deterministic process that begins with an input called a seed and produces a sequence that appears random.

The technical challenge is making the output unpredictable in practice. NIST defines a pseudorandom number generator as a deterministic computational process that takes one or more seeds and produces a sequence that appears random.

The Gambling Commission’s standard says it should be computationally infeasible to predict the next number without complete knowledge of the algorithm and seed. Seeds and reseeding must not introduce predictability.

Two instances of an RNG should not synchronise and produce the same stream. The output must also survive any scaling used to convert a large numerical range into the smaller set of outcomes required by the game.

What “acceptably random” requires

The Gambling Commission does not simply require a game to look unpredictable.

Its Remote Gambling and Software Technical Standards say that randomness must be demonstrable to a high degree of confidence using accepted statistical tests and methods.

The RNG and game should be capable of showing that:

• output is distributed across the expected range
• game outcomes follow their theoretical probabilities
• future output is unpredictable
• repeated output streams are avoided
• seeds do not create predictable patterns
• scaling and mapping preserve the required randomness

The standard also prohibits compensated or adaptive behaviour. A remote game cannot alter its probabilities because of what happened earlier or because the operator wants to change its short-term payout.

Bonus rounds and other features can use different rules where those rules are properly described and triggered by random events. That is different from secretly adjusting the normal game according to previous results.

Random does not mean evenly alternating

Random sequences can contain streaks and clusters.

A run of losses does not prove that the RNG has stopped working. A run of wins does not prove that the game has become generous. Random outcomes do not have to alternate in a way that looks balanced to a person watching them.

Where outcomes are independent and the rules remain fixed, the next result is not required to compensate for the previous sequence.

Long-run proportions can move towards their theoretical probabilities without any particular result becoming due next.

Our volatility and variance guide explains why short sequences can look very different from a game’s long-run average.

Can a game change its payout while you are playing?

Not without operating within the published rules and regulatory requirements.

The Gambling Commission says that game rules, paytables and outcome probabilities cannot be changed while a game is available for gambling, apart from changes already provided for in its rules.

A game with properly described bonus rounds, variable bingo participation or a progressive jackpot may change state according to its published design. That is not the same as an operator quietly reducing the chance of winning during play.

Where a game or paytable is changed outside those defined features, the game should be taken offline or suspended. Customers must be told that the rules, odds or payouts have changed.

How RNGs and games are tested

Testing is not limited to running a game several times and checking whether the results look varied.

The Gambling Commission’s testing procedure includes:

• reviewing the RNG’s technical documentation
• checking the algorithm or hardware for known weaknesses
• reviewing source code
• statistically testing raw RNG output
• testing scaled output and shuffled data
• checking game mathematics and theoretical RTP
• verifying how random inputs are mapped to displayed outcomes
• comparing the software with the rules and artwork shown to players

Approved test houses perform the independent testing required by the Commission. They must hold the relevant accreditation for the products they assess.

Not every cosmetic software change requires a complete external retest. A major change that may affect fairness, including changes to the RNG, mapping, game rules or relevant software implementation, requires external testing. Minor changes that do not affect fairness can be handled differently under the testing strategy.

The Gambling Commission does not personally approve every individual game or certify that faults can never occur. Licensees and software businesses remain responsible for meeting the standards.

What happens after release?

Testing before release is only part of the control system.

Licensees must monitor live RTP for both underpayments and overpayments. Actual RTP is compared with the expected figure over an appropriate volume of play. Monitoring should be frequent enough to identify unusual results rather than waiting for a fixed monthly check regardless of how much a game has been played.

A difference over a small sample is not automatically evidence of a fault. Volatility can produce wide short-term variation.

Monitoring looks for results outside the statistical tolerance expected for the game and volume of play. An unusual result can then be investigated to establish whether it reflects normal variation, a configuration problem or a software issue.

Are free-play games allowed to be more generous?

A free-play version should not quietly present a misleading picture of the real-money game.

The Gambling Commission says it should use the same RNG as the corresponding real-money game, another RNG meeting the required standard or another suitable source that does not introduce systematic bias.

The prize distribution should accurately represent the real-money version. A demonstration should not produce a higher-than-expected share of wins simply to make the game look more attractive.

Does pressing stop change a slot result?

Do not assume that pressing a button changes the mathematical result merely because it changes the animation.

The answer depends on the game’s certified design and published rules. On some games, a stop control may only shorten the visible reel animation after the result has already been determined.

The visual movement of reels should not be treated as evidence that a player can time or influence the underlying RNG. If a feature genuinely affects play, that should be explained in the game rules.

RNG games and live dealer games are different

A live roulette wheel or shuffled deck uses a physical process rather than relying entirely on a software RNG to generate the main result.

The video stream, betting interface and settlement system still use gambling software, but the outcome may come from the physical event.

Virtual roulette, automated card games and online slots are different. Their results are produced through software or another approved random process.

A product looking like a table game does not tell you how its result is generated. Check whether it is described as live, virtual or RNG-based.

Can a player verify an RNG?

A player cannot normally inspect the source code or reproduce the test house’s work.

You can still check:

• whether the operator is licensed to serve consumers in Great Britain
• whether the game rules and relevant probability information are available
• whether the operator and software supplier can be connected to Gambling Commission records
• whether regulatory action has been recorded
• whether the game information identifies its RTP and version where relevant

Use our gambling licence-check guide to connect the website, operator and Gambling Commission record.

A polished animation, long losing run or recent large win does not prove whether an RNG is compliant. The useful evidence comes from the game’s design, testing, monitoring and regulatory status.

An RNG supplies unpredictable inputs. The game’s rules turn them into wins and losses. Keeping those two parts separate makes RTP, volatility and probability much easier to understand.