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How Slots Work

You’ve probably heard and read different explanations of what goes on inside slot machines. Some of the issues are quite complicated, and writers may oversimplify them; at the same time, there are a multitude of myths out there that have confused and misled players.

It never hurts to know the facts. So without getting too heavy into the mathematics or technical details, here’s a basic primer on how slot machines actually work.

Before we start, one note: The information in this article does not apply to every slot machines in the country that looks like a slot or video poker machine. While most slot and video poker machines in the US work the way this article describes, machines in some Indian casinos and some states controlled by the state Lottery work differently.

This information is accurate for machines in Nevada, Atlantic City, Mississippi and Indian casinos in many states. Most states that have “Class III” gaming compacts have machines that conform to the same standard. California and Arizona are examples. States that have Class II gaming compacts, such as Florida and North Carolina, have machines that work differently. All regulated slot machines in the country are random and fair; they just use different methods to choose winning or losing results.

It’s also worth mentioning that Indian gaming and its regulations are relatively new, and conditions change fairly frequently. Today there are 28 states with Native American casinos. More appear to be on the way.

A brief history of slot machines

In the beginning, slot machines were strictly mechanical. Gears whirled when the handle was pulled, and the reels spun and came to a halt. Each symbol on the reel had the same chance of appearing as any other. This is the way slots machine operated until the late 1980’s when computers began deciding where the reels would stop. For the past 15 years or so, the spinning reels on a slot machine have been just for show. The actual result is selected by a computer program called a Random Number Generator, or RNG for short.

With mechanical slots, each symbol had an equal chance of appearing in the payline. There were only so many symbols (including blanks) that could fit on a reel. The majority of reels had 20 or 22 symbols, or “stops.” The term “stop” was derived from the fact that the reel had to stop in one of 20 or 22 positions to show that symbol clearly on the payline. If there were 20 stops and 3 reels with 1 jackpot symbol on each reel, the chances of lining up three symbols for a jackpot were 1 in 8,000.

With these slot machines, the smaller the chance of hitting a top jackpot, the higher the jackpot could be. If a jackpot hit every 8,000 spins on average, on a 3-reel slot machine that accepted up to 5 coins, then the biggest payoff had to be less than 40,000 coins (5 coins X 8,000 spins). Otherwise, the slot machine wouldn’t make money over time. If a slot machine was designed to give smaller, more frequent payouts, that meant fewer coins were available for the top jackpot.

Then, in the 1970’s and 1980’s, game designers started to add more reels. With more reels there was less chance of hitting the big jackpot, so they were able to make the top prize larger. If a game had 20 stops and 5 reels, the chance of hitting the big one dropped to 1 in 3,200,000. Adding more reels made the games themselves much larger. Casinos would often place these huge slot machines, which contained 5, 7 and even 8 reels, right inside their doors. Players loved taking a shot at a prize that was often the size of a state lottery jackpot. But the slot machines were expensive, took up a lot of space, and were awkward to operate.

Then came the arrival of computer technology, the slot industry’s watershed event. An invention called the “virtual reel” freed up the number of possible outcomes from the physical limitations of the reels. The number of symbols you could fit on a reel was no longer a concern.

A game could now be programmed so that the odds of hitting a jackpot were, for instance, five million to one. If the Random Number Generator selected numbers for each reel that corresponded to the jackpot symbols, the reels would line up with those symbols and you’d win. Though the games basically looked the same as before, with three reels that spun and stopped, virtual reels allowed for gigantic, multi-million dollar jackpots. (The newer “Australian-style” video slots with 5 video reels are the latest advance in this type of slots machine. Everything from here on applies equally well to video as well as reel slots.)

Virtual Slots VS Reels Slots Machines

Essentially, the virtual reel creates an imaginary reel inside the computer’s brain. Because it’s imaginary, it can have as many stops as the programmer wishes it to have.

The slots machine you played last night with the million-dollar top jackpot might contain 3 imaginary reels, each with 147 virtual stops. Let’s say one of those 147 stops contains a jackpot symbol, and 20 stops are cherries. The physical reel in the machine might only have two cherry symbols. But anytime the RNG picks a number that corresponds to a cherry, the physical reels will stop on one of the two cherries. And it will always stop on the symbol that is “mapped” to the number selected, never on the “other” one. Each physical stop is assigned to a virtual stop.

If a slot machine has 147 virtual stops on each reel then there are 3,176,523 different combinations. The RNG will pick 3 numbers. In this case, each number is between 1 and 147. The first number maps to the position of the first physical reel, the second to the middle reel and the last to the third reel. Just imagine the RNG spinning three imaginary reels, each of which stops either on a blank space or a symbol. The computer then determines the physical stops that each virtual stop maps to, directs the slots machine to spin the reels, and has the reels stop in those positions. (The result of your spin is determined well in advance of the reels actually stopping.) Finally, the computer checks to see if the combination pays anything and, if so, how much.

Payback percentage (theoretical return) of reel slots

We’ve all seen neon signs like “This bank pays 99%” over a bank of slot machines. Although you might assume it’s a wild exaggeration, these numbers are in fact truthful. But that’s not to say they aren’t misleading. You see, slot machines are programmed to return a certain percent of the money wagered over an infinite number of plays. The key phrase is “over an infinite number of plays.” In reality, after a few million pulls the returns will be close to the theoretical return (but not exactly that amount).

The more you play, the more this payback percentage matters. If you play slot machines only a few times a year, you’re not going to see these long-term results. But if you play several times a week, you will. Over time, your bankroll will go further when you play a higher return machine.

Here’s another important point. In the short run, the slot machine’s hit frequency will probably mean more to your play than its payback percentage. High hit frequency slot machines generally give back lots of small pays (often less than your bet). This allows you to keep playing for a longer time. Low hit frequency machines usually pay off less often, but in larger amounts—so unless you still manage to hit a lot of small jackpots, or one larger jackpot, your bankroll won’t last as long.

To truly understand how a slots machine works, you first need to understand what the payback percentage is and how it’s calculated. In order to illustrate this, let’s return to the days of mechanical slot machines—when each symbol on each reel had exactly the same probability of appearing on the payline as any other symbol.

Let’s say our old-school mechanical slots machine has only one reel, with ten stops. Five of those stops are blank. One stop has the jackpot symbol, a pineapple. One has an orange, one has a plum, one has a watermelon and one has a cherry. These symbols, and how much they pay if they appear on the payline, are as follows:

Stop Symbol Pays

If you play 10 times and get each stop once, there will be 19 coins returned. This is the machine’s “cycle.” This is a term used to describe every possible combination of symbols on the reels. It does NOT mean that the slot machine is programmed to display each combination of symbols once, and then start over. It’s a mathematical fiction that enables us to calculate the payback percentage.

Let’s say it takes two coins to play. To play a cycle, you would pull the handle 10 times. At 2 coins per, you would wager 20 coins. As we saw earlier, the cycle pays 19 coins. So 19 / 20 = 95%. Our slot machine has a theoretical return of 95%. All slot machines can be analyzed exactly the same way. With virtual reels and millions of possible combinations on the imaginary reels, it takes a computer to make the calculation—but it’s done exactly the same way.

How can you be sure that slot machines pay back exactly what they’re supposed to? Well, in reality, they don’t. Ever. Over many millions of pulls they will return very close to the payback amount, but it’s a theoretical return, not an actual one.

Now we’ll make our slot machine virtual. Its computer has an imaginary reel with 100 stops. If we wanted it to pay back 98%, we’d calculate that a cycle (100 spins at $2 per) would cost 200 coins. 98% of 200 is 196. So we’ll make sure that the cycle returns 196 coins. We could give the top jackpot a large share of this, leaving fewer coins for smaller pays, or we could do the opposite. It depends on how we want our slots machine to play. If we add more reels and more stops, we have the kind of machine most players enjoy at their local casino.

Bonus Rounds

By now, you may be wondering how bonus rounds fit in. Just think of the bonus round activator as another winning combination. The bonus round has a fixed number of possible outcomes and a known probability of each one happening. Some bonus rounds are predetermined; others depend on the choices the player makes.

Let’s examine a predetermined bonus round. Say it has two possible outcomes: you’re either going to win five coins, or 25 coins. The RNG is programmed to pick a number between 1 and 10. If it picks 7, then 25 coins are won; any other number results in the 5 coin win. When the bonus cycle is complete, the total is 70 coins. Since there are 10 possible outcomes, the average is 70 / 10 or 7 coins.

Adding a bonus to a traditional slots machine might make you think you stand to win more money, but in reality it makes no difference. Other pays are reduced, or occur less often, to make up for the cost of the bonus. Bonus rounds add fun and excitement to slots, but they don’t make them pay out more. The theoretical return is set when the machine is first programmed, and you’ll get the programmed payback, more or less. If you’re lucky, you’ll make a profit; if you are unlucky, your credits will disappear fast. But hopefully you’ll have fun, at least.


Now that you know how slot machines work, you can see how silly it is to think that a machines is “due,” or “hot” or “cold.” You can hit a jackpot right after being paid for one. Every spin is a separate event; each is completely random. The chances of winning in the short term is left strictly to chance. Some people can play a slots machine that returns 85% and go home with a fistful of money, while others go broke playing a 99% game. It’s called gambling. So go out and have fun, and don’t bother trying to explain to the player next to you how the slot machines really work. They’d never believe you.


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