How do slot machines decide who wins?

How do slot machines decide who wins? The short answer

Every modern slot machine, from an online slot to a cabinet in a betting shop, runs a random number generator. When you press spin, the game takes the number or numbers it needs from that generator and converts them into reel positions. Those positions are checked against the paytable, and any winning combination is added to your balance. The spinning you then watch is presentation: the outcome existed before the first reel moved.

So the machine doesn't decide who wins. It decides what each spin shows, using numbers designed to be unpredictable, and then applies fixed rules to price that result. A player who has lost for an hour and a player who has just sat down face identical chances on their next spin. That single idea, set out in more depth on our how slot machines work hub, answers most of the other questions on this page.

How a slot machine decides a win, step by step

Here is the full chain of events for one spin of a typical video slot. Land-based machines follow the same pattern, with the RNG running inside the cabinet or on a linked server.

1. You set your stake. Online in Great Britain the cap is £5 a spin if you're 25 or over and £2 if you're 18–24; a Category C pub fruit machine takes up to £1 a game.
2. You press spin. The game takes the next output from its RNG, which produces numbers continuously whether anyone is playing or not.
3. Scaling. Each raw number is reduced to a range that fits the reel, for example 0 to 63 on a reel with 64 virtual stops.
4. Mapping. A lookup table turns each scaled value into a stop on the reel strip, and therefore into the symbols that will land on screen.
5. Evaluation. The software compares the symbols with the paylines or ways, totals any wins and checks for feature triggers such as three scatters.
6. Display. The reels animate and stop on the positions already chosen. Online, the whole game cycle must last at least 2.5 seconds before another spin can begin.
7. Settlement. Your balance updates and, on UK online slots, your net position for the session stays visible.

Steps two to five take a fraction of a second. Everything after that, including a third scatter that seems to hang in the air, is an animation of a decision that has already been made. The game cycle rules explain where one spin officially starts and ends.

From random number to reel stop: a worked slot machine example

Older mechanical machines had a fixed number of physical stops, so every stop was equally likely. Modern machines use virtual reels: the RNG picks from a longer list of positions, and several of those positions can point to the same symbol. That lets a designer make some symbols rarer than they look, a technique called weighting. The idea traces back to Inge Telnaes, who received US patent 4,448,419 in 1984 for a random number generator that selects reel stops.

With that reel used three times, three sevens need the 1-in-64 result on every reel: 1 ÷ 64 ÷ 64 ÷ 64, or 1 in 262,144 spins. A bell, by contrast, comes up three times as often as a seven on each reel, even though both occupy one physical stop. That is why a seven resting just above the payline looks close but isn't: the blank spaces either side of it are far more likely stops. The psychology is covered under near misses, and reel strips explains how strips are laid out.

These numbers are made up to show the method. Real games don't publish their reel weightings; the help screen gives the RTP and paytable instead.

How the paytable decides what a winning slot spin pays

Once the symbols are known, the paytable acts like a price list. It says how many matching symbols you need, where they need to land and how much each combination pays as a multiple of your stake. The 'where' differs between game families. On payline games such as Starburst, which pays both ways across 10 lines, symbols must sit on a fixed path. On cluster pays games they need to touch, and on scatter-pays games such as Gates of Olympus eight or more matching symbols anywhere on the grid make a win.

Wild symbols substitute for others before the evaluation runs, and scatter symbols usually trigger the bonus. Features are decided by further random numbers too: the Gambling Commission's technical standards allow bonus features only where they are based on random events. If you want to know exactly what triggers a bonus round on a given game, the rules screen is the place to look.

Where RTP fits: the slot sets the price, not the winner

Return to player is what the reel weightings and the paytable add up to over millions of spins. A 96% RTP means that, across a huge sample, £96 of every £100 staked flows back as wins and £4 stays with the operator. It says nothing about which individual spins or players get the £96. The table shows the long-run average cost at two common RTP levels.

In a real 100-spin session, one person may finish well ahead and the next lose most of what they staked. That spread around the average is volatility, and it is why a single evening tells you almost nothing about a game's RTP. The house edge is simply the other side of the same number: 100% minus the RTP.

Do pub fruit machines decide who wins differently?

Partly. A pub fruit machine is a Category C gaming machine with a £1 maximum stake and a £100 maximum prize. Many still use a random number generator for every result, but some UK machines are compensated: the software keeps track of what it has taken and paid out, and adjusts towards its target percentage, so earlier play can influence what happens next. The Commission's 2025 draft technical standards describe the display rule: a machine shows 'THIS GAME IS RANDOM', or states that the game is compensated and may be influenced by previous play. Our explainer on compensated machines goes further.

Even then, the machine isn't choosing a person. It has no idea whether the next pound comes from someone who fed it all evening or from someone who has just walked in from the bar. That's also why watching a machine to see when it might be 'full' doesn't work: you can't see its internal position, and it may return money through small wins and features over a long stretch. The pattern players notice is covered in why fruit machines pay out in bursts.

Who checks that UK slot machine results are fair

The Gambling Commission's remote technical standards require an online game's RNG output to be acceptably random and unpredictable, require random numbers to be mapped to outcomes in line with the stated probabilities, and ban adaptive behaviour, meaning any change to the probabilities during play. A simulated device must behave like the real thing: the Commission's own example is that a virtual coin must land heads with a probability of 0.5 every time.

Manufacturers of gaming machines, machine software and remote gambling software must use a Commission-approved test house, and Category B1, B2 and B3 machines need independent testing. On 2 March 2026 the Commission's list of approved test houses (opens in a new tab) named 14 firms. The Commission reviews test reports and audits rather than repeating the tests itself. Rules are enforced: in June 2026 Stakelogic paid £122,835 after some of its slots ran faster than the 2.5-second minimum (opens in a new tab). For the detail, see RNG certification and how machines are tested.

Myths about how slot machines pick winners

If you want the longer list, our guide to slot machine myths works through more of them, and are slot machines rigged? looks at the difference between a house edge and cheating.

UK rules that change how an online slot shows you a win

Since 31 October 2021, online slots offered to players in Great Britain have had to follow design rules that affect how a result is presented, though not how it is chosen. Autoplay is banned, each spin must take at least 2.5 seconds, turbo and slam-stop buttons are gone, and the game must show your net position and how long you've been playing. A game also can't celebrate a return equal to or below your stake. So if you stake 50p and get 30p back, there are no fanfares, because you've lost 20p. Background on the losses disguised as wins rule and the spin speed rule is on our rules pages.

These rules don't change the RNG or the paytable. What they change is how easy it is to misread who is winning: you, or the game.

What this means each time you spin a slot machine

• Decide your budget before you start, and treat it as the cost of the entertainment. Our bankroll guide shows how to size it.
• Check the RTP and rules in the help screen; some games come in several RTP versions.
• Expect losing runs. They're part of the maths, not a sign that something is about to change.
• Use deposit limits, time-outs and reality checks; the slot safety tools guide explains each one.
• If play stops being fun, the National Gambling Helpline is free on 0808 8020 133, and GAMSTOP (opens in a new tab) blocks UK-licensed gambling sites.

Where to play online slots with tested RNGs

Any slot at a UK-licensed casino runs on an RNG that has been through an approved test house, so the way wins are decided is the same at each of the casinos we list. What differs is the welcome offer. Coral's is bet £10 on slots and get 100 free spins worth 10p each on selected games, accepted within 48 hours and valid for 7 days, with no wagering on winnings. Ladbrokes gives 50 free spins a day for 10 days (up to 500, 10p each, no wagering) after you play £10 on slots. PlayOJO gives 80 no-wagering spins at 10p each on Big Bass Bonanza with a first deposit of at least £10.

Free spins are settled by the same RNG as paid spins: 100 spins at 10p is £10 of spins, and at a 96% RTP the long-run return would be about £9.60, though any one batch can land well above or below that. All three offers are for new customers aged 18+, some payment methods are excluded and full terms apply. Game availability changes, so search the casino's lobby for a title rather than assume it's there.

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.