How do casinos always win?

How do casinos always win? The short answer

Casinos win because the price of every bet is fixed in their favour. When a bet pays less than its true odds, the difference stays with the house on average, and that difference is called the house edge. It's the same idea as a bookmaker's margin, just built into the rules of the game instead of being added to a price.

The word "always" needs a footnote. A casino loses individual bets all day long, and some players walk away ahead. What the casino can rely on is the total: once enough money has been staked, its takings settle very close to the edge multiplied by the turnover. Britain's licensed online casino games kept £5.7bn of players' stakes after paying out winnings in the year to March 2026, according to the Gambling Commission's industry statistics.

The house edge: paying less than the true odds

Take a straight-up number on a single-zero wheel. There are 37 pockets, so the true odds against your number are 36 to 1, but the bet pays 35 to 1. Bet £1 on each of the 37 numbers and you stake £37; the winner returns £36 (£35 winnings plus your £1 back). The missing £1 out of £37 is a 2.70% edge, and it applies to every bet on the European layout, as our guide to true odds against payouts explains.

The textbook formula from Hannum and Cabot's Casino Math (opens in a new tab) puts it neatly: if a bet pays x to 1 and the true odds are y to 1, the edge is (y − x) ÷ (y + 1). For the straight-up bet that's (36 − 35) ÷ 37 = 2.70%. On a double-zero wheel the true odds become 37 to 1 with the same 35 to 1 payout, so the edge almost doubles to 2 ÷ 38 = 5.26%.

Slots do the same thing in a less visible way. A slot's RTP is the share of stakes it's designed to pay back over its lifetime, so a 96% RTP game keeps 4% on average. The pay table and the reel maths set that figure before anyone spins.

House edge by game: what £100 staked costs on average

Because expected loss is simply amount staked multiplied by the edge, you can compare games in pounds. These figures come from standard game maths and the Wizard of Odds house edge table (opens in a new tab); our fuller house edge by game page adds more bets.

Notice that £100 staked isn't the same as £100 deposited. If you start with £20 and keep re-betting your winnings, you can easily stake £100 or more in a session, which is why the cost per hour matters more than the edge alone. At £10 a spin and 60 spins an hour on European roulette, that's £600 staked and an average cost of about £16.20 an hour.

Why volume lets the casino win in the long run

One bet is a coin flip with a slight lean. A million bets are a business. The reason is the law of large numbers: as the number of bets grows, the actual share of money kept gets closer and closer to the theoretical edge.

The spread of results grows too, but only with the square root of the number of bets, while the expected profit grows in direct proportion. The table uses £10 even-money bets on a single-zero wheel, where each bet has an expected value of −27p and a standard deviation of about £10 (the arithmetic is in our variance and standard deviation guide).

A busy online casino handles far more than a million bets, spread over thousands of players. Some of those players win, and the casino pays them gladly, because the combined total keeps tracking the edge.

Casinos don't win every bet, and some players finish ahead

Short sessions are dominated by luck, not maths. A player who places 50 roulette bets might be £100 up or £150 down, and both are normal. A high-volatility slot can pay a large prize to one person while hundreds of others pay for it. What a player can't do is keep that lean going in their favour forever, because every extra bet adds another slice of edge.

That's also why the long-run picture for a single gambler is the same as for the casino, only reversed. The idea of gambler's ruin says a player with a limited bankroll in a negative-edge game will eventually run out if they keep playing, whatever staking plan they use. The sensible response is to treat play as paid entertainment with a budget, not as a way to make money.

How online casinos win without rigging games (UK rules)

A common belief is that online casinos must cheat to stay profitable. They don't need to, and in Britain they aren't allowed to. The Commission's random outcome standard, RTS 7 (opens in a new tab), requires results to be acceptably random and bans adaptive or "compensated" games that shift the odds to recover past payouts. New games and random number generators must be tested by an approved test house before release, as our page on how casino games are tested explains.

Operators must also tell you the odds. Under the Commission's game information standard, players must be able to see the RTP, the house edge or the probability of winning before they commit. The edge is out in the open; the profit comes from it being applied fairly, every time.

Other rules that protect the casino's margin

Some casino rules exist mainly to stop the edge being worn away. They're worth knowing because they explain features players sometimes find odd:

• Commission on Banker bets. In baccarat the Banker hand wins slightly more often, so a 5% commission is taken from Banker wins to keep that bet below fair odds; see why casinos charge commission on banker bets.
• The zero. Without the green pocket, roulette even-money bets would be fair; the zero is the whole edge.
• Table limits. Maximum stakes cap how far a doubling system such as the Martingale can chase a loss, which is one reason those systems fail.
• Bonus wagering. Bonus funds must be staked several times (no more than 10x the bonus in Great Britain since 19 January 2026), and the house edge applies to every pound of that wagering.
• RTP versions. Many slots come in more than one RTP setting, and the casino picks which one to offer.

Myths about how casinos win

That last point is worth unpacking. A casino can estimate your theoretical loss from your average stake, the number of bets and the edge: £10 a spin, 60 spins an hour for three hours at 2.70% comes to £48.60. Rewards are budgeted against figures like that, which is why they never outweigh the edge for a typical player. More detail sits in our explainer on how casinos make money.

How to work out what a casino game will cost you

You can turn the house edge into a budget figure in a minute. Use these steps, or our house edge calculator:

1. Find the game's RTP or house edge in its help screen or rules page.
2. Decide your stake per bet or spin.
3. Estimate how many bets you'll make: a slot with a 2.5-second minimum spin can run far more rounds an hour than a live table.
4. Multiply stake × bets × house edge to get your average cost.
5. Set a budget you're comfortable losing, and expect real results to swing well above or below the average in any one session.

Where to play at UK-licensed casinos

Every casino below holds a Gambling Commission licence and runs games with a published edge, so the maths on this page applies at all of them. Their welcome offers are mainly for slots, and table or live games are often excluded or count for less towards wagering, so check the terms first. 18+ and terms apply.

Casumo (Recro Limited, account 61549) gives new customers who opt in a 50% bonus up to £100 on a first deposit of at least £20; the bonus must be wagered 10x within 7 days, with a £5 maximum bet while it's active and game, play and payment restrictions. 10bet (Blue Star Planet Limited, account 43173) offers new UK customers a 100% match up to £25 if they choose the casino bonus at sign-up and deposit within 7 days, wagered 10x on slots within 3 days, with bonus winnings capped at £1,000 and Skrill or Neteller deposits excluded.

At Coral (LC International Limited, account 54743), new players who deposit and bet £10 or more on slots between 1 October 2026 and 1 February 2027 get 100 free spins worth 10p each on selected games, to accept within 48 hours and use within 7 days, with no wagering on spin winnings; Neteller, PayPal, Paysafe, Skrill, prepaid cards and certain debit cards don't qualify. Remember that the edge applies to bonus play as well.

18+. Every casino game has a built-in house edge, so over time you should expect to lose what you stake. Set a budget before you play and stop when it’s gone: safer gambling tools and support. Free, confidential help: National Gambling Helpline 0808 8020 133.