Slot machines generated $3.39 billion in revenue in the US (American Gambling Association), up 4.6% from 2025. They’re arguably one of the most popular online casino games (there aren’t definitive statistics) in the world. They’re easy to play, and you don’t really have to think about what you’re doing other than pressing ‘spin’ because you can’t control the outcome.
Even though you can’t control the outcome, you’re always hoping for the lines to match up, and whether they do is partly down to the return to player (RTP) mechanism that dictates slot returns and payouts.
To help you understand the math behind the game engine, we’ve created a comprehensive guide to RTP and how slot returns are calculated.
How Return to Player Percentages Work
RTP is the expected percentage of all money wagered that a game returns to players over time. So, that doesn’t mean that a game with an RTP of 96% means you will win 96p back from every £1 spin. It also doesn’t mean you will keep 96% of your bankroll during each gaming session. You have to apply RTP across a substantial number of wagers rather than individual games.
That’s why in some games, you win nothing, and in other games, you win a decent amount that’s more than you expected it to be. If only every game were like that!
The corresponding theoretical house edge of a simple 96% RTP slot is:
100% − 96% = 4%. That means the game’s mathematical expectation is to retain approximately £4 for the house for every £100 of total turnover over sufficiently large samples. That £100 refers to money wagered, not necessarily money deposited.
Applying The Math Behind RTP to Slot Returns
Now you know RTP is an expected-value calculation. We will use a simple calculation to help you visualize it in practice.
Imagine a very simple $1 slot with only two possible results:
- 80% probability: $0 payout
- 20% probability: $4.80 payout
Expected return:
(0.80 × $0) + (0.20 × $4.80) = $0.96
Therefore:
$0.96 ÷ $1 × 100 = 96% RTP
We’ve simplified that for you, but if it feels complicated, think of it as multiplying every possible payout by the probability of receiving it and adding the results together. Real slots are actually so much more complicated. Their calculation can involve:
- Symbol frequencies
- Reel strips or virtual reel mapping
- Multiple paylines/ways to win
- Multipliers
- Free-spin features
- Bonus rounds
- Jackpots
- Different bet configurations
Then you have to think about volatility. Volatility in online casinos is how widely results can fluctuate around that mean.
Two slots can therefore both have 96% RTP and produce completely different player experiences. One might return many small prizes, and the other returns fewer prizes but occasionally delivers very large wins.
Does a Higher RTP Mean More Chances of Winning?
A higher RTP doesn’t mean you have more chances of winning, but it’s easy to see how people make that mistake. You need to remember that a higher RTP means higher expected monetary return over the long run. It doesn’t automatically mean that a winning result occurs more frequently.
And another rookie error is to think that multiple losses will eventually equal a win if you play a slot game for long enough. Slot games should produce outcomes according to their probability model without adaptive compensation.
And despite that, you are still better off going for a slot game with a higher RTP.
Slot games will always be fun to play. How often you’ll win or not and how much you win realistically depend on how lucky you are. The math is integrated into the game’s preexisting random number generator model, so every spin and outcome is technically always going to be random.