House edge explained: how casinos make money on every game

Casinos make money because every game they offer is designed with a mathematical edge in their favour, known as the house edge. It is expressed as a percentage of each wager that the casino expects to keep over the long run, and it is built into the rules, odds and payouts before a single card is dealt or reel is spun.

What house edge actually means

House edge is a statistical average, not a guarantee on any single bet. If a game has a 5% house edge, the casino expects to retain £5 for every £100 wagered across many thousands of bets. Some players will win big in the short term, others will lose quickly, but the average outcome across all players and all bets converges towards that percentage.

This is simple probability at work. Casinos do not need to win every hand or spin. They need the maths to favour them slightly and consistently, then let volume do the rest.

How the edge gets built into the rules

The edge usually comes from paying out winning bets at odds slightly lower than the true probability of winning. A classic example is roulette. A single number in European roulette has a 1 in 37 chance of winning, which would be fair odds of 36 to 1. The casino pays 35 to 1 instead, and that small gap between true odds and payout odds is where the house edge comes from.

• European roulette (single zero): house edge of about 2.7%
• American roulette (double zero): house edge of about 5.26%
• Blackjack played with correct basic strategy: typically under 1%, though it rises sharply with poor play
• Slot machines: commonly 2% to 15%, corresponding to a return to player of roughly 85% to 98%

House edge versus RTP

House edge and return to player describe the same mathematics from opposite sides. A slot with a 96% RTP has a 4% house edge. Casinos and regulators usually publish RTP for slots because it is easier for players to interpret as a payback percentage, while table game odds are more often discussed in terms of house edge. For a fuller breakdown of how RTP is calculated and tested, see What is RTP? Return to player explained.

Both figures are long-run averages calculated over millions of simulated or recorded rounds, not predictions for any individual session.

Why the edge differs so much by game

Table games with fixed, well-documented rules tend to have the smallest and most stable house edges, because the odds are dictated by simple probability and do not change between venues in the same way. Side bets and insurance options in games like blackjack usually carry a far higher edge than the main bet, which is why they are often highlighted separately in strategy guides.

Slot machines sit at the other end of the scale. Their house edge is set by the game developer in the software and can vary between versions of the same title, between regulated markets, and sometimes between land-based and online releases. Regulators in most licensed markets require this figure to sit within set bounds and to be disclosed, though exact rules differ by jurisdiction.

Short-term variance versus long-term certainty

House edge only becomes reliable over very large sample sizes. A player can sit down and win several hands in a row even against a high house edge, just as a casino's overall GGR (gross gaming revenue, the amount retained after paying out winnings) is never identical day to day. Variance, the natural swings around the statistical average, is why individual results feel random even though the underlying maths is fixed.

Casinos manage this by operating at scale. With thousands of bets placed every hour across a casino floor or a betting platform, short-term swings cancel out and the house edge asserts itself as steady, predictable revenue.

Why this matters for players

Understanding house edge does not make a game winnable, but it does explain why no amount of strategy removes the underlying mathematical disadvantage. Games with a lower house edge, played correctly, simply lose money more slowly on average than games with a higher one. Readers can compare typical house edge figures across common games using the bonus calculator and house edge table.

Because losses are structurally built into every game, gambling should be treated as a form of entertainment with a cost attached, not a way to generate income. Anyone concerned about their own or someone else's gambling can find support services and further information on safer gambling and help lines.

The bottom line

House edge is not a trick or a hidden fee. It is openly built into the odds of every regulated casino game and is the mechanism by which casinos generate reliable revenue over time. Knowing roughly how large that edge is for a given game helps explain why results vary so much from player to player, even though the house always holds the mathematical advantage.