How does a slot machine know when to pay?

How does a slot machine know when to pay? The short answer

A modern slot machine knows when to pay because it calculates every result from a number and then checks that result against a table of prizes. It has no sense of whose turn it is, no record of how long you have been sitting there and no plan to reward patience. The core component is the random number generator, or RNG, which produces numbers continuously. When you press spin, the machine takes the current number, or a set of them, and converts it into reel positions. If those positions form a winning combination on the paytable, the machine pays.

Everything you see afterwards, the reels spinning, slowing and landing, is a presentation of a result that already exists. That is why our answer to is stopping the reels a skill is no for online slots.

Step by step: what happens inside a slot machine on each spin

1. You place a stake. The machine deducts it from your credit.
2. The RNG supplies numbers. It runs all the time, even when nobody is playing, so the number taken depends on the exact instant of the spin.
3. Numbers become reel stops. Each number is scaled to a position on a virtual reel map for each reel.
4. Stops become symbols. The map tells the software which symbol sits at each chosen position, and which symbols appear above and below it.
5. The paytable is checked. The software scans the paylines or ways for combinations, adds any scatter pays and checks for feature triggers.
6. The win is credited. The total is added to your balance and the reels animate to show the result.

Our guide to how slot machines work puts this sequence alongside the hardware, and how slot machines decide who wins looks at it from the player's side.

How virtual reels let a machine pay rarely and still look fair

Physical reels have a limited number of positions, often around 20 symbols. If each position were equally likely, a three-reel machine would have only 20 × 20 × 20 = 8,000 combinations, which caps how large and rare a top prize can be. The solution, patented by Norwegian mathematician Inge Telnaes in 1984, was the virtual reel: a larger list of positions in software, mapped onto the visible symbols.

With 256 virtual stops per reel, a three-reel machine has 256 × 256 × 256 = 16,777,216 combinations. The designer then decides how many of those virtual stops point at each symbol. That is reel weighting, and it is how the machine ‘knows’ a jackpot should be rare without ever remembering anything.

In this example the seven appears on the visible strip as often as a bell, yet the software lets the reel stop on it only once in 64 spins. Blanks next to sevens are often given extra weight, which creates frequent near misses that mean nothing about the next spin.

How the paytable tells the machine what to pay

The paytable is the list of every winning combination and its prize, usually expressed as a multiple of the line bet or total bet. The machine pays exactly what the paytable says for the combination the RNG produced. It cannot decide to pay more because you've been unlucky.

Combine the paytable with the reel weighting and you get the game's maths model, recorded in a document called a par sheet. Multiply each prize by its probability and add them up, and you get the return to player. Worked example: if a 10p game returns 96%, then 1,000 spins, or £100 staked, cost about £4 on average over a very long run. Any single session will land well above or below that.

The UK exception: compensated fruit machines

Some British fruit machines work differently. A compensated machine keeps a running account of money in and money out and adjusts what it offers so that it stays near its target percentage. After a long spell of taking money, it may become more generous with features; after a big payout, it may tighten. In that sense a compensated machine does ‘know’ when it is behind its target.

This is disclosed, not hidden. Draft machine standards published by the Gambling Commission in January 2025 say a machine must display ‘THIS GAME IS RANDOM’ or, if compensated, state that it is compensated and that results may be influenced by previous play. Even so, you can't see the internal account, so a compensated machine gives you no usable signal about when to play. Our page on loose and tight slot machines explains why.

Random slot machines vs compensated machines compared

How an online slot knows when to pay

An online slot works on the same principle, but the RNG and maths run on a game server, not inside a cabinet. Your screen sends the spin request, the server calculates the result and returns it, and your browser or app animates it. The Commission's remote technical standards (opens in a new tab) govern how those games behave in Great Britain. Since 31 October 2021, UK slots must take at least 2.5 seconds per spin and must not celebrate a return equal to or below the stake. The technical set-up is covered under online slot servers.

Who checks that the machine pays as it should

Manufacturers of gaming machines and online game software must use an accredited, Commission-approved test house. The Commission's list of approved test houses (opens in a new tab) named 14 firms as of 2 March 2026, including eCOGRA, GLI, BMM and Quinel. They check that the RNG is random and that the RTP matches the maths model. The RNG certification page explains what the tests involve.

Myths about how slot machines know when to pay

• ‘It pays when it's full.’ A random machine has no fill level. A compensated machine tracks its balance, but you can't read it.
• ‘It knows I'm on a losing streak.’ The RNG has no idea who is playing.
• ‘The casino can switch it on to pay.’ In Great Britain, game maths is tested and fixed in certified software; settings and versions are controlled.
• ‘A near miss means it's warming up.’ Near misses are a product of weighting, not a signal.

Our slot machine myths guide covers many more, and are slot machines rigged looks at fairness in detail.

Where to play RNG online slots in the UK

All online slots at our partner casinos use certified random number generators, so each spin is independent. For new UK customers aged 18+ (excluding Northern Ireland), PlayOJO gives 80 free spins on Big Bass Bonanza for a first deposit of £10 or more, each worth 10p, with winnings paid in cash and no wagering. Coral and Ladbrokes run bet-£10 spins offers, shown below. Terms apply, and game choice varies by casino, so search the lobby. More answers are in our questions hub and offers pages.

18+. Gambling should be entertainment that costs money, not a way to make it. Decide a budget before you play: plan a slot budget. Free, confidential help: National Gambling Helpline 0808 8020 133.