You’ve probably heard the phrase “the house always wins” when it comes to casino gambling. But what does it actually mean?

After all, people do hit jackpots, and casino games are supposed to be fair – so what guarantees the casino still comes out ahead?

The answer lies in a simple but powerful mathematical idea called “the house edge”: a small, systematic statistical advantage built into every casino game. It’s the invisible force that ensures the numbers will always tilt toward the house in the long run.

So, let’s unpack the science behind that edge: how it’s constructed, and how it plays out over repeated bets.

Roulette: the clearest place to see the house edge at work

Roulette looks like one of the fairest games in the casino. A spinning wheel with numbered pockets, half coloured red and half coloured black, and a single ball sent careening around the outside to eventually land in one pocket at random. If you bet the ball will land in a red pocket (or a black one), it feels like a 50–50 gamble.

But the real odds are a little bit different. In most Australian casinos you’ll find 38 pockets on the roulette wheel: 18 red, 18 black, and two “zero” pockets marked 0 and 00. (In Europe roulette wheels have 37 pockets, with only a single 0.)

The zero pockets are what creates the house edge. The casino pays out as if the odds were 50–50 – if you get the colour right, you get back double the amount you bet. But in reality, on a wheel with two zero pockets your chance of winning is 47.37%.

When you bet on a colour, the house has a 5.26% edge – meaning gamblers lose about five cents per dollar on average. A single-zero wheel is slightly kinder at 2.7%.

You don’t see the house edge in the course of a few spins. But casinos don’t rely on a few spins. Over thousands of bets, the law of large numbers takes over. This is a fundamental idea in probability that implies the more times you repeat a game with fixed odds, the closer your results get to the true mathematical average. The short-term ups and downs flatten out, and the house edge asserts itself with near certainty.

The law of large numbers is why casinos aren’t bothered by who wins this spin, or even tonight. They care about what happens over the next million bets.

The Gamblers’ Ruin problem

Another way to see why the house always wins is through the so-called Gambler’s Ruin problem.

The problem asks what happens if a player with a limited bankroll keeps betting against an opponent with effectively unlimited money (even in a fair game).

The mathematical answer is blunt: the gambler will eventually go broke.

In other words, even if the odds are perfectly even, the side with finite resources loses in the long run simply because random fluctuations will push them to zero at some point. Once you hit zero, the game stops, while the house is still standing.

Casinos, of course, stack the odds even further by giving themselves a small edge on every bet. That tiny disadvantage, combined with the fact the house never runs out of money, makes ruin mathematically inevitable.

The more bets you make, the worse your chances

Say you walk into a casino with a simple goal. You want to win $100, and you plan to quit as soon as you hit that target.

Your approach is to play roulette, betting $1 at a time on either red or black.

How much money do you need to bring to have a decent chance of reaching your $100 goal? A thousand dollars? A million? A billion?

Here’s the surprising truth: no amount of money is enough.

If you keep making $1 bets in a game with a house edge, you are practically certain to go broke before getting $100 ahead of where you started, even if you arrive with a fortune.

In fact, the probability of gaining $100 before losing $100 million with this strategy is less than 1 in 37,000.

You could walk in with life-changing wealth and still almost certainly never hit your modest $100 goal. (The full mathematical proof is beautifully demonstrated in the video below.)

Betting bigger may give you a fighting chance

So how do you create a real chance of success? You must either lower your target or change your strategy entirely.

If your target were only $10, you’d suddenly have over a 50% chance of going home happy, even if you started with just $25. A smaller goal means fewer bets, which means less opportunity for the house edge to grind you down.

Or you can flip the logic of Gambler’s Ruin: instead of making hundreds of small, disadvantageous bets, you can make one big bet.

If you put $100 on red all at once, your chance of success jumps to roughly 47%. This is far higher than the near-zero chance of trying to grind your way up with $1 bets.

The long-run strategy is mathematically doomed, while the short-run strategy at least gives you a fighting chance.

A small house edge adds up

Roulette is the clearest place to see the house edge, but the same structure runs through every casino game. Each one builds in a varying degree of statistical tilt or bias.

Some games, like roulette, have fixed, rule-based house edges that don’t change from one player to the next. But others, like blackjack, have a variable house edge that depends on how the game is played. But no game is exempt from the underlying structure.

Small edges don’t stay small when you expose yourself to thousands of bets. In the long run, the variance fades, and the outcome converges to the house’s advantage with almost certainty.

That’s why the house always wins. Because mathematics never takes a night off.