How Online Slots Work: Reels, RNGs and Features
Online slots look simple, but there is a lot going on behind each spin. This guide explains how results are decided, what the features do and the rules every slot in Great Britain has to follow.
Random number generators and independent testing
Every online slot is driven by a random number generator (RNG), software that produces a constant stream of random numbers. The moment you press spin, the game takes the current numbers and maps them to positions on each reel. The spinning animation is just presentation: the result was decided the instant you pressed the button.
Slots offered in Great Britain have to be tested by independent test houses before release, and their RNGs are checked to make sure outcomes are genuinely random.
Two things follow from this:
- Every spin is independent. Previous results don't change the chance of the next outcome. A slot is never "due" a win, and a big win doesn't make the next spin any less likely to pay.
- There's no pattern to find. Changing your stake, stopping and starting or playing at a certain time of day doesn't affect the odds.
Reels, rows, paylines and ways to win
Slots are described by their grid, usually written as reels by rows: a 5x3 slot has five reels and three rows.
- Paylines are fixed paths across the grid. Matching symbols must land along an active payline, normally from the leftmost reel, to pay.
- Ways to win games pay matching symbols on adjacent reels in any row position, so a 5x3 grid offers 243 ways.
- Megaways slots change the number of symbols on each reel with every spin, giving up to 117,649 ways on some games.
More ways to win doesn't mean a slot pays more overall. The total return is set by its RTP, as explained below.
Symbols, wilds and scatters
- Standard symbols pay according to the paytable, usually with lower-value card symbols and higher-value themed symbols.
- Wilds substitute for other symbols to complete wins, and some expand, stick or carry multipliers.
- Scatters pay or trigger features wherever they land, rather than along a payline.
The paytable, in each game's information screen, shows exactly what every symbol pays at your stake.
Bonus rounds and free spins features
Most modern slots have at least one feature on top of the base game:
- Free spins rounds, triggered by scatters, often with extra wilds or rising multipliers.
- Hold and spin or cash collect features, where special symbols lock in place and award values.
- Cascading or tumbling reels, where winning symbols disappear and new ones drop in, allowing several wins from one spin.
A large share of a slot's total return is often paid through its features, which is one reason results can swing so much between sessions.
RTP and volatility
Two numbers describe how a slot pays over time:
- RTP (return to player) is the long-run percentage of stakes a game is designed to pay back. A 96% RTP slot returns £96 for every £100 staked, on average, over a very large number of spins. RTP explained goes into more detail.
- Volatility describes how those returns are spread: frequent small wins, or rarer bigger ones. See slot volatility explained.
The same slot can run at different RTP settings at different casinos, so it's worth checking. Our slots database shows the RTP each game runs at our featured brands.
UK rules for online slots
Online slots in Great Britain have to follow some of the strictest design rules anywhere:
- Stake limits. The maximum stake is £5 per spin for players aged 25 and over (from 9 April 2025) and £2 per spin for players aged 18 to 24 (from 21 May 2025).
- No speed features. Since 31 October 2021, slots can't offer autoplay, turbo or slam-stop features, and each spin must take at least 2.5 seconds.
- No false celebrations. Games can't celebrate a return that's equal to or less than your stake.
- Clear information. Slots must show how long you've been playing and your net position.
Setting limits
Slots are fast and every spin costs money, so it helps to know your budget before you start. Our cost of play calculator estimates what a session is likely to cost based on stake, spins and RTP, and every UK-licensed casino lets you set deposit limits and reality checks.
Looking for somewhere to play? These are the current casino welcome offers from our featured brands:
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You can also browse our slots hub for popular games and guides.
Keep it fun
Set a budget before you play, never chase losses and take regular breaks. Gambling should never be a way to make money. Free, confidential help is available from GamCare or the National Gambling Helpline on 0808 8020 133.
