How Do Casino Slot Machines Work?

Online Slots Reels With a Gears Background

If you’re going to play slot machines, you should understand how slot machines work.

That sounds obvious, but you’d be amazed at how many people have no idea how these games really work.

In this post I explain how slot machines work in simple terms that anyone can understand.

What Is a Slot Machine?

Real money slot machines are the basic gambling machine at most casinos, and slot machines generate 70% to 80% of most casinos’ revenue. In some casinos, they make up even more than that. They’re called slot machines because you used to put coins in a slot to play them.

Slot machines differ from other gambling machines because of the way the outcomes are generated. A video poker game is also a gambling machine, but it’s not a SLOT machine. Neither is a video blackjack game.

What makes a slot machine game a slot machine game are the spinning reels.

Row of Casino Slot Machines

In early mechanical versions of the games, you literally had big metal reels powered by springs and levers. They spun and landed on various stopping points when the kinetic energy that caused them to spin was all faded.

Modern slot machines use a computer program to determine the outcomes. The spinning reels are just for show, even on machines that LOOK like they’re using physical reels.

But many – if not most – modern slot machines just use a computer monitor with animated reels to determine outcomes.

How Does a Slot Machine Come Up With a Random Result?

Modern slot machines used a computer program to determine the outcomes. This program is called a “random number generator” or “RNG.”

Here’s how the random number generator works:

It constantly “thinks” of a range of numbers. Each number corresponds to a combination of symbols on the slot machine reels and the payline. When you press the spin button, the random number generator stops on that combination.

The manufactures are also able to use this technology to “weight” certain combinations more than others. When mechanical slot machines were all the rage, the odds were easy to calculate. If you had 10 stops or symbols on a reel, the probability of one of those stops landing was easy to calculate. It was 1/10.

But with a computer program, you could easily have a symbol that comes up once every 20 spins, another that comes up once every 5 spins, and another that comes up once very 10 spins.

These changes in probability can even happen from one identical machine sitting next to the other.

I’ll have more to say about that in this next point:

How Slot Machines Make Money for the Casino

All casino games give the house a mathematical edge. This is usually based on something simple – in blackjack, for example, the player has to play her hand first. In roulette, you have two green numbers, but the payout odds would be break-even if all the numbers were black or red.

With a slot machine, the house makes its money simply by paying out at lower odds than the odds of winning.

Each prize amount on a slot machine has a probability of happening. When you multiply that probability by the prize amount, you come up with the “return” for that combination.

Add all those returns together, and you get the total return for the machine.

That return is always less than 100%.

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Here’s an example:

You might have a 55% probability of getting no win at all. This would apply to all non-winning combinations, so the return for any of the non-winning combinations would be 55% X 0, or 0.

You might have a 20% probability of getting a single cherry symbol that doesn’t match anything else on the machine, and that might pay off at even money (1 for 1). The return for a cherry-blank-blank combination would be 20% X 1, or 20%.

You then might have another 20% probability of getting a 2 for 1 payoff for any combination that includes a bar symbol and no other matches. The return for a bar-blank-blank combination would be 20% X 2, or 40%.

If you had no other combinations, the overall return on this game would be 40% + 20% + 0%, or 60%.

This means that on average, over a staggeringly large number of spins, you’d wind up getting back 60% of the money you bet.

At $3 per spin over 1000 spins, you’d have bet a total of $3000.

The odds say that you would have gotten $1800 in winnings, for a loss of $1200.

Of course, a real slot machine pay table would have more than three possible prizes, and at least one of those prizes would be large. The average slot machine has a 1000 for 1 jackpot for its top prize.

How Do Bonus Games and Other Slot Machine Features Work?

Modern slot machines have a staggering array of special features, too. One of the most prominent of these features are the slot machine bonus games.

Here’s how that works:

Some combinations of symbols result in a mini-game that you get to play. It’s called a bonus game. It might be as simple as getting 10 free spins, each of which is guaranteed a winner. It might be more involved, like getting to play a Space Invaders type game where each alien you shoot has a prize amount associated with it.

Scatter symbols are also popular. A scatter symbol is a symbol that triggers some kind of payout regardless of where it appears on the screen.

For symbols to win on a slot machine, they need to match along one of the paylines. The traditional payline is the one that runs horizontally across the center, but most modern games have multiple paylines now.

The 8-liners in Texas are a good example. A screen has three rows of symbols, for a 3X3 grid. It’s called an 8-liner because there are eight paylines.

You have three horizontal paylines, three vertical paylines, and two diagonal paylines.

You must activate each payline with a bet, and it’s possible to win multiple prizes on multiple paylines.

Think of paylines as being similar to the various patterns that bingo variants have.

Scatter symbols trigger wins regardless of where it is on the screen.

Wild symbols are also popular. These are symbols which can fill in for another symbol that you would need to trigger a win on that payline.

If you play poker, you’re probably already familiar with the concept of wild cards. Just apply that concept to slot machines, and you understand how it works.

These are just a handful of the special features that are now available on modern slot machine games.

How Do You Win at Slots?

I wish I had a foolproof winning strategy for slot machines that I could share with you.

But I don’t.

No one else does, either, regardless of what they claim.

Slot machines are entirely random. You put your money in, and you take your chances. The only way to win on a slot machine is to get lucky.

You’ll find plenty of looney strategies that are supposed to improve your probability of winning. They’re all equally worthless.

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One example is to find slot machine games on the edges of the banks near the walkways. The idea is that the casino managers want to attract players to the slot machines, and they do this by putting the looser games near the walkways.

Of course, this isn’t true in modern casinos. Even if it were true, it wouldn’t mean you were going to beat the casino. Such games would still have a mathematical edge for the house that you couldn’t overcome in the long run.

Another popular strategy I see touted is to look for hot or cold machines. A hot machine is one that has paid out several times in a row. The idea is that the game has gotten hot and is going to keep paying out.

A cold machine, on the other hand, is one that hasn’t paid out in a while.

Some superstitious slots players think that a cold machine is “due” to pay out soon. Both of these are examples of the gambler’s fallacy in action. Each spin of the reels on a slot machine game is an independent event. The probabilities on each spin are the same.

If you have a 1 in 1500 probability of winning the top prize on a slot machine, that probability is the same regardless of whether the jackpot got hit on the previous spin.

It seems intuitive to think that if you just hit the jackpot, the probability of hitting it again on the next spin would be lower.

But that’s not the case.

It’s still 1 in 1500.

If you’re going to play slot machines, go into it with you eyes open and understand that it’s going to cost you money in the long run.

Don’t fall for lame, looney slot machine strategies.


Slot machines can be a lot of fun, but they’re like other casino games:

They’re more fun if you have some understanding of how they work.

After reading this post, you’re better educated that at least 80% of the slot machine playing public.