RTP, and what it does not tell you

A game's return to player is the proportion of all money staked on it that comes back to players as winnings, averaged over an extremely large number of rounds. A game with an RTP of 96% returns, on average, 96 units for every 100 staked. The missing 4 units are the house edge, which is simply the same figure viewed from the operator's side: RTP and house edge always sum to 100%.

The figure is theoretical, in the strict sense that it is calculated from the rules of the game rather than measured from what happened. For a game whose outcomes are fully defined, like roulette, it can be derived exactly on paper. For a slot, it comes from the mathematical model of the reels, the symbol weights and the bonus features, which the developer designs to produce a target return.

European roulette has 37 pockets, numbered 0 to 36. A bet on a single number pays 35 to 1, meaning you receive 35 units of profit plus your own stake back. The true odds of hitting it are 1 in 37.

Stake one unit on a single number for 37 spins, and you expect to win once. That win returns 36 units in total, your stake plus 35 profit, against 37 units staked. So 36 divided by 37 gives 0.973: an RTP of 97.30% and a house edge of 2.70%.

American roulette adds a second zero, giving 38 pockets, while still paying 35 to 1. The same calculation gives 36 divided by 38, or 0.9474: an RTP of 94.74% and a house edge of 5.26%, nearly double. The payout did not change; the number of ways to lose did. This is where a house edge always comes from, the gap between the true odds of an event and the odds at which the game pays it.

RTP converges only across enormous numbers of rounds. A slot's stated 96% is an average over millions of spins, pooled across everyone playing. Over a few hundred spins, the distribution around that average is wide enough that almost any result is unremarkable, which is why players routinely lose their whole stake on a high-RTP game and occasionally win large amounts on a low-RTP one.

It also does not mean you keep 96% of your deposit. The percentage applies to every stake, including stakes funded by winnings. Deposit 100, and if you keep playing your winnings back on a 96% game, the edge applies again to each new bet: the total amount you wager, not the amount you deposited, is what the percentage bites into.

Finally, a game is never due. Each spin, roll or deal is independent of the last, so a game that has paid nothing for an hour is exactly as likely to pay on the next round as it was on the first.

Independent testing laboratories verify two distinct things before a game is licensed: that the random number generator produces statistically unpredictable outcomes, and that the game's realised return over very large simulated volumes matches the theoretical RTP the developer declares. In Great Britain, the Gambling Commission's remote technical standards also require that information about the chances of winning, including the house edge or RTP, is available to a player before they commit.

The same game can ship with different RTP settings, which operators may select from, so the figure shown inside the game is the one that applies to you, not the figure quoted on a comparison page. Two further cases move the number: on games involving decisions, such as blackjack or video poker, the quoted return assumes optimal play and drops with every mistake; and on progressive jackpots, part of the return sits in a jackpot that most players will never win, so the return excluding the jackpot is lower than the headline.