Online slots: how they work, RTP, volatility and features

An online slot is a random number generator (RNG) with a picture on top. When you press spin, the result is already decided; the spinning reels are an animation. Everything else, from paylines to bonus rounds, is a way of turning that random number into a payout. This page explains the parts that affect your money.

How a slot decides a spin

1. You set a stake and press spin.
2. The RNG picks a number for each reel (or the whole grid).
3. The game maps those numbers to symbols using its reel strips, the long lists of symbols each reel can stop on.
4. The pay table is checked. If symbols line up in a way the game pays, you win that multiple of your stake.
5. Each spin is independent. A slot has no memory. A run of losses does not make a win more likely, and a big win does not make the next one less likely.

Reels, paylines and ways

How symbols "line up" depends on the game's layout:

The number of lines or ways does not change the RTP. A 117,649-ways game is not more generous than a 20-line game; it just spreads its return differently. See Megaways slots and cluster pays slots for the detail.

RTP: what the number means

RTP (return to player) is the share of all money staked on a game that it is designed to pay back over a very large number of spins. The rest is the house edge.

RTP describes millions of spins, not your session. Over an evening you can finish well ahead or lose everything you brought; the RTP only tells you which way the average leans. In Great Britain, the Commission's technical standards require the average theoretical RTP to be easy to find before you play (RTS 3). Our RTP and volatility guide goes further.

Volatility and hit frequency

Two slots can both return 96% and feel nothing alike.

• Hit frequency is how often a spin pays anything. A game with a 25% hit frequency pays on about one spin in four, though many of those "wins" are smaller than the stake.
• Volatility describes how the return is spread. Low-volatility games pay small amounts often. High-volatility games pay little most of the time and save much of their RTP for rare, large wins, usually inside a bonus round.

High volatility means a bigger spread of results. With a fixed budget, you are more likely to lose it all before a big win lands, and also more likely to see a large win if one does come. Neither is better value; they are different risks at the same price.

Bonus features

Most modern slots keep a large share of their RTP in features:

• Bonus spins rounds triggered by scatter symbols, often with multipliers or extra wilds.
• Cascades or tumbles, where winning symbols vanish and new ones drop in.
• Hold-and-win respins, where coin symbols lock and the respin counter resets (hold and win slots).
• Progressive jackpots, fed by a slice of every stake (progressive jackpot slots).
• Feature buys, where you pay a set price to go straight to the bonus. Not offered in GB (bonus buy slots).

Because so much of the return sits in features, the base game on its own often returns well below the headline RTP.

RTP versions: the same game at different prices

Many studios supply one game at several RTP settings and let each casino choose. iGaming Business reported in April 2026 that operators can deploy versions of the same game at roughly 96%, 94% or 92%, and that some UK operators were moving to lower settings.

The graphics, features and maximum win look identical. The cost does not. Dropping from 96% to 92% doubles the average cost of play, from £4 to £8 per £100 staked. The only way to tell which version you are on is the RTP shown in that casino's game rules, so read it every time.

Myths and systems

• "It's due to pay." Each spin is independent. Nothing builds up.
• "Slots are tighter at busy times." A certified game's maths does not change by time of day.
• "Raising your stake after losses improves your odds." It only raises how much you lose on average per spin. Staking systems change the size of your bets, not the edge.
• "Near misses mean you were close." A near miss is just another losing outcome.

Rules in Great Britain

GB has the tightest slot design rules of our markets. All of these come from the UK Gambling Commission:

• No autoplay. Customers must commit to each game cycle individually (RTS 8), and the Commission's summary of changes states that providing auto-play for slots is not permitted.
• Minimum 2.5 seconds per spin. It must be at least 2.5 seconds from the start of one game cycle to the next (RTS 14D).
• No turbo, quick spin or slam stop, and no playing several slots at once (RTS 14C and 14E).
• No celebrating losses. A return equal to or less than your stake cannot be presented with win-style sounds or animations.
• Net position and session time must be shown on screen: your winnings minus losses since the session began, and the elapsed time (summary of changes, RTS 2E and 13C).

These changes were announced in February 2021 and took effect on 31 October 2021, as SBC News reported at the time.

• Feature buys. In January 2020 the Commission warned operators that bonus buy-ins did not meet its technical standards, and the six operators it found offering them removed the feature (iGaming Business).
• Stake limits. Online slots are capped at £5 a spin for players aged 25 and over (from 9 April 2025) and £2 a spin for players aged 18 to 24 (from 21 May 2025), per the Commission's stake limit guidance.

The 2.5-second rule also caps speed. At most 1,440 spins fit in an hour (3,600 seconds ÷ 2.5). Our calculation: at £1 a spin on a 96% slot, that is £1,440 staked and an average cost of £57.60 an hour; on a 92% version, £115.20.

Canada and US states set their own rules, and many GB restrictions do not apply there. Check the game's help screen for what is on offer where you play.

Demo play

Most studios offer a demo mode with play credits. Use it to learn the features, the bet steps and how the bonus triggers. Keep two limits in mind: a few hundred demo spins say almost nothing about volatility, and a studio's demo is not necessarily the same RTP version your casino runs for real money.

Where to play

We list only licensed sites, and show each market's rules. See UK casinos, Ontario casinos, New Jersey casinos, Pennsylvania casinos and Michigan casinos, or browse all casino markets. For the studios behind the games, see our providers hub, and read how we rate sites.