Can casinos adjust the RTP on slot machines? The RNG, and the game rules it is confused with

Can casinos adjust the RTP on slot machines? The short answer

Can casinos adjust the RTP on slot machines? Not on an online slot while it is open for play in Great Britain. The Gambling Commission’s rule RTS 7D says a game’s rules, payouts and outcome probabilities may not be changed while it is available, except as its own rules provide.

The RTP (return to player) is the share of stakes a game is designed to pay back over a very large number of plays. It does not sit in the random number generator (RNG) at all. It comes from the game rules: the paytable, the symbols on each reel and the feature rules.

So this entry defines the RNG first, then the neighbour it is most often confused with.

Sources: Gambling Commission: RTS 7; Gambling Commission: testing strategy Annex A.

1. Random number generator (RNG)

A random number generator is ‘any item of hardware or software which is used to generate random numbers with the intended property of statistical randomness’. That is the Commission’s definition, in section 2 of its remote technical standards.

Two more steps sit between the number and the screen. First, scaling converts the output into the range a game needs, such as 1 to 52 for a pack of cards.

Then mapping selects an outcome using that result. The Commission’s own example is a reel strip symbol.

Sources: Gambling Commission: RTS definitions.

2. Not to be confused with: the game rules that set the RTP

The game rules are the paytable, the symbol distribution and the feature rules. The Commission’s testing strategy says that ‘collectively the game rules determine the overall game RTP’. The RNG only supplies the numbers those rules read.

The Commission’s own worked example shows the split. A fruit machine has three reels of 20 positions, so there are 8,000 combinations.

At 50p a game, one pass through all 8,000 costs £4,000. It pays £3,600 in prizes, so the RTP is 90%.

Now put a third cherry on reel 3 in place of one blank. The cherry prizes rise from £160 to £240, so total prizes become £3,680 and the RTP becomes 92%.

The RNG did not change. The reel strip did.

Sources: Gambling Commission: testing strategy Annex A; Gambling Commission: how machines meet their %RTP.

3. Can the RTP change while a game is open?

For online games, RTS 7 sets two limits. Requirement 7A says ‘adaptive behaviour (that is, a compensated game) is not permitted’. Requirement 7D bars changes to the rules, payouts and outcome probabilities while a game is available, unless the rules provide for them.

A change is made with the game taken offline or suspended. The altered game then shows a notice, such as ‘rules changed’ or ‘new odds’.

But some movement is written into the rules and is allowed. A progressive jackpot, one that grows from a share of stakes, may rise, and bingo odds may depend on the number of players.

Sources: Gambling Commission: RTS 7.

4. Not to be confused with: a compensated machine

A compensated machine is a land-based gaming machine that adjusts as it goes. The Commission’s player guide says these machines ‘vary the chance of winning a prize depending upon results from previous games played’. On a random machine, the odds ‘remain constant’.

This is the one case where a game steers towards its figure. But it steers to meet its stated RTP, not to change it.

Online, RTS 7A rules it out. A gaming machine must also say whether it is compensated or random.

Sources: Gambling Commission: return to player; Gambling Commission: how machines meet their %RTP; Gambling Commission: RTS 7.

5. Who checks a change, and when

Every new online game is tested by an approved test house before release. The test report goes to the Commission. A major change needs external retesting, and the Commission counts ‘any change to the RNG, scaling and mapping, or game rules’ as one.

While a game is live, the licensee must monitor its actual RTP against the advertised figure. The Commission gives the sum as win divided by turnover, the total staked. An annual games testing audit then checks that changes were put in the right category.

Sources: Gambling Commission: testing strategy summary; Gambling Commission: testing strategy Annex A; Gambling Commission: live RTP monitoring.

6. In a game’s information screen

RTS 3C requires information about the chances of winning to be easily available before a customer commits to gamble. It must include the RTP percentage or the probability of winning events. That figure is the one the tests and the monitoring are measured against.

A word such as ‘random’ or ‘tested’ on that screen names a mechanism. So ask which mechanism and which evidence the claim refers to.

It is not an assurance about withdrawals, customer support or every product the operator supplies. A precise definition narrows the question; it does not certify the service.

Sources: Gambling Commission: RTS 3.

7. Usage note

In ordinary speech, ‘the RNG’ often stands for the whole game. In this entry it means the number source only. The RTP belongs to the rules.

Example: A £10 stake at 92% RTP has an expected return of £9.20 over a very large number of plays; one session can land far from that. Support: GamCare, 0808 8020 133.

Sources: Gambling Commission: return to player; GamCare: talk to us now.

See also

For the percentage itself, see the entry on 99% RTP. For the words that travel with a game’s statistics, such as how widely results swing, see volatility and variance. For the long form of game testing, and for why a studio’s RTP range does not name the version in a casino, see Studio Ledger.

Sources read — checked 23 September 2026

Eight Gambling Commission pages and one GamCare page, each opened on 23 September 2026. Three are parts of the remote technical standards: the definitions, RTS 3 and RTS 7. Three are parts of the testing strategy: the summary, live RTP monitoring and Annex A.

The other two Commission pages are its player guide to return to player and its licensing-authority feature article on how gaming machines meet their RTP. The helpline number is from GamCare’s own contact page.

Sources: Gambling Commission: RTS definitions; Gambling Commission: RTS 3; Gambling Commission: RTS 7; Gambling Commission: testing strategy summary; Gambling Commission: live RTP monitoring; Gambling Commission: testing strategy Annex A; Gambling Commission: return to player; Gambling Commission: how machines meet their %RTP; GamCare: talk to us now.