How Slot Machines Work: the RNG, RTP and Volatility Explained

The RNG is software that produces sequences with no usable pattern. For online games offered in Great Britain, the Gambling Commission’s technical standards require outcomes to be generated randomly and the rules and the likelihood of winning to be described to the player.^{1 (source)} Games are tested against those standards before they go live.

Because the result exists before the reels stop, nothing you do during the animation changes it. Stopping the reels early, choosing when to press, or the speed of your clicks all have no effect on what you win.

RTP is the slot version of the house edge: the two always add up to 100%. A 96% RTP means a 4% house edge, which is higher than blackjack played well or European roulette, and lower than some side bets. Our house edge guide compares slots with the table games.

The figure comes from the maths of the game: every possible reel combination, its probability and its payout, added up. It is a design figure, checked by testing, and it is only reached over an enormous number of spins. In a session of a few hundred spins, a slot can pay back much more or much less than its RTP, and with high-volatility games the gap is usually large.

You can find a game’s RTP in its help or paytable screen. Some games are released in several versions with different RTPs, and a version with a lower figure costs more per spin at the same stake, so it is worth checking the screen of the game in front of you rather than a general figure for the title.

Volatility is not about cost. Over a long run, both games keep the same share of stakes; what differs is the path. A high-volatility game spends a larger share of its RTP on rare big prizes, so most sessions end below the average and a few end far above it.

Many game providers publish a volatility rating in the game information, sometimes as a scale of one to five. There is no standard definition behind those ratings, so treat them as a description from the maker rather than a measured figure.

On a payline slot, each line is a path across the reels, and a win needs matching symbols from the leftmost reel along that path. The stake is spread across the lines, so a £1 spin on a 20-line game stakes 5p per line. A “ways” game counts every combination of positions on adjacent reels, so a grid of five reels and three rows has 3 × 3 × 3 × 3 × 3 = 243 ways to win.

More ways or lines do not mean a better game. The paytable is set so that the overall return matches the RTP whatever the number of lines; more lines simply mean more frequent, smaller wins. When a win pays less than the stake, the game is still taking money, which is why UK rules now limit how such results are presented online.

A feature is triggered by a random event, usually a set number of scatter symbols, and it pays from the same overall budget as everything else. A game with a rich free-spins round often pays less in the base game to make room for it, which is one reason feature-heavy slots tend to be more volatile.

Progressive jackpots pool a small part of every stake across one game or a network of games until someone wins. The contribution to the pool is part of the game’s RTP, so the base game pays back correspondingly less. A very large jackpot does not change the odds of winning it on your next spin.

The figures above are the legal caps for each category.^{4 (source)} Category B1 machines are found in casinos; category C machines are the familiar fruit machines in pubs and adult arcades; category D covers low-stake machines such as some coin pushers and crane grabs. Which category a venue may offer depends on its licence or permit.

Fruit machines in pubs and arcades have their own gameplay traditions, such as holds, nudges and feature trails. Online slots have no prize cap by category, but online play has its own rules on stake size and speed, set out below.

The speed and design rules came into force on 31 October 2021. They set the 2.5-second minimum game cycle, banned autoplay and features that speed up play or give the illusion of control over the outcome, and stopped games celebrating a spin that pays back the stake or less with sounds or graphics suggesting a win. Games must also show your total losses or wins and the time played during a session.^{2 (source)}

The stake limits followed in 2025: £5 per spin for adults aged 25 and over from 9 April, and £2 per spin for players aged 18 to 24 from 21 May.^{3 (source)} Operators must check age and identity before anyone can deposit or play, which is how the lower limit is applied to younger players.

These rules apply to every licensed site serving players in Great Britain, wherever the operator is based. Northern Ireland has separate gambling law. Free help is available at any time from the National Gambling Helpline, and our safer gambling page lists the tools every licensed site must offer, such as deposit limits and GAMSTOP.

Before you pick an online table, our UK casino comparison shows which licensed casinos carry the most blackjack and roulette tables. 18+

The “due” myth comes from how random streaks look. In a long run of spins, dry spells and bunches of wins happen naturally, and it is easy to read a pattern into them. The next spin is drawn the same way regardless, so a long run without a feature changes nothing.

Timing myths are the same. The RNG runs continuously and the result depends on the instant the spin is registered, which no player can control to any useful degree. Likewise, changing your stake does not change your chances, unless the game’s help screen says a feature or RTP setting depends on the stake.

RTP and volatility are the only two facts that describe a slot, and both are on the game information screen.