An online slot's result is decided by a random number generator before the reels stop. Its published RTP is a theoretical average over a very large number of spins and says nothing reliable about a single session. Volatility describes how much results swing. Paylines, ways to win and features change how wins are distributed, not the fact that the game keeps a margin. Online slots face legal restrictions in the Maldives.
On this page
Online slots are one game family, and this page stays inside it: what actually decides a spin, and what the numbers printed next to a slot do and do not promise.
It is the narrow view. Where our casino games explainer sets the families side by side, everything below concerns the software game on its own.
What decides a spin
Every spin's outcome comes from a random number generator (RNG). Great Britain's Gambling Commission requires RNG output to be acceptably random, uniformly distributed across its range and unpredictable without knowledge of the algorithm and seed1.
The same standard says games that simulate a physical device, such as spinning reels, must reflect the probabilities of the real thing, and that a game's rules, payouts and probabilities may not be changed while it is available to play1.
The practical point: the animation on screen does not decide anything. It shows an outcome the RNG has produced.
Return to player: what the published figure means
Return to player (RTP) is the proportion of amounts staked that a game returns as prizes, and it is always an average over a very large number of games2.
The Gambling Commission explains that a fully random game needs a million or more game cycles for its return to stabilise near the target. After 8,000 games, the actual figure would most likely be considerably above or below it2.
A hypothetical example: a slot with a 96% RTP returns, on average across an enormous number of spins, 96 units for every 100 staked. A single evening on that game can end with nothing returned at all.
What published RTP does not mean
- It is not the amount a player gets back. RTP does not apply to a single player or session3.
- It does not make a game "due". There is no pattern, cycle or due win3.
- It does not describe how often a game pays. That is closer to volatility, covered below.
- On an unregulated site, there is no independent way to confirm the figure shown.
In regulated markets such as Great Britain, information such as RTP or house edge must be available before a customer gambles4. Our guide to what RTP means covers the maths further.
Where RTP figures come from
Game developers publish a figure for each title. Pragmatic Play, for instance, lists an RTP of 96.50% for Gates of Olympus on its own game page5. In regulated markets, the rules must also describe how the game works and how winners are determined before anyone stakes money4.
A figure repeated on a promotional website is only as reliable as the site repeating it. The provider's own page, or the game's information screen on a regulated platform, is the place such numbers originate.
Individual titles in our slots database record the figure the provider publishes, and the studio profiles cover the companies behind them.
Volatility
Volatility refers to the variability of bet outcomes6. A low-volatility slot tends to return smaller amounts more often; a high-volatility slot can go through long stretches of losses between rarer, larger wins. Both can have the same RTP.
Researchers who studied 4,281 online slot players in the UK found that the effect of volatility on player behaviour was nuanced and varied between groups6. Volatility is a description of risk, not a measure of quality. See slot volatility explained.
Paylines, ways and pays-anywhere
Slots use different rules to decide which symbol combinations win.
- Paylines. Fixed lines across the reels. A win needs matching symbols along an active line.
- Ways to win. Matching symbols on adjacent reels count regardless of their row. Big Time Gaming's Megaways mechanic, for example, offers up to 117,649 ways on six reels, with the number of symbols per reel changing between spins7.
- Pays anywhere. Some games pay for enough matching symbols anywhere on the screen. Pragmatic Play's Gates of Olympus works this way on six tumbling reels5.
More ways to win does not mean a better return. It changes how wins are spread across spins. The return and the operator's margin are one number seen from two sides, as our guide to the house edge sets out.
Common features
- Free spins. Triggered by scatter symbols. In Gates of Olympus, four or more scatters award 15 free spins5.
- Multipliers. Symbols or rounds that multiply a win. Gates of Olympus uses random multiplier symbols worth up to 500x5.
- Tumbling or cascading reels. Winning symbols are removed and new ones fall into place, allowing several wins from one spin5.
Features are part of the RTP calculation. They redistribute the game's expected return rather than adding to it.
Near misses
A near miss, two jackpot symbols and a third just off the line, can feel meaningful. The Victorian Gambling and Casino Control Commission warns that near misses can trick the brain into thinking a win is close, and that "almost wins" don't mean a win is coming3.
Slots in the game database
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Video slot - RTP
- RTP (return to player) is the percentage of all money staked on a game that it is designed to pay back as winnings, averaged over a very large number of plays. It does not predict the result of any single session.
- Volatility
- A measure of how widely a game's results vary around its average return, commonly represented by the standard deviation. High-volatility games pay larger wins less often; low-volatility games pay smaller wins more often.
- Random number generator (RNG)
- Hardware or software that generates the random numbers used to decide game outcomes. In regulated markets, RNG output is tested to show that it is statistically random and unpredictable.
- Paylines
- Fixed patterns across a slot's reels along which matching symbols must land to form a win. The number of paylines, and whether they can be changed, is set by the game's rules.