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How Do Slot Machines Work?

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Modern slot machines use computer technology known as Random Number Generator (RNG) software to determine which symbols are selected to produce winning or losing outcomes.

Slot machines are probably the most easy-to-understand and arguably the most fun way of making money in a casino. Land-based and online casinos often have hundreds if not thousands of slot machines available in all sorts and sizes, covering all kinds of themes like Ancient Egypt, Vikings, and fairy tales. 

Nowadays there are even movie-based, comic-based, and series-based slots like the Jurassic Park slot by Microgaming, the Iron Man 2 slot by Playtech, and the Narcos slot by NetEnt. Apart from these modern video slots, some online casino software providers are already cranking out virtual reality casino games.

Despite the fact that new land-based and online slots are released on a daily basis and millions of people across the globe love playing these casino games, few people know how slot machines actually work. 

In this guide, we will be explaining to you how slot machines work and what to look out for before deciding which slot machine to play. Be sure to buckle up, as it’s about to get wild!

How Do Slot Machines Work?

Almost everyone has encountered a slot machine or two at least once in their lives. Whether you have seen them in movies, bars, casinos, or online, they all work pretty much the same. 

The first slot machines were invented, sold, and marketed by Charles Fey, a German mechanic in the early 1900s. These slot machines had three reels with printed images and other symbols on them like horseshoes and bell icons. Back then slots had a single line drawn over a glass pane, also known as the payline. 

The idea behind these machines was quite simple: the player has to try to get three matching symbols on this payline in order to win. The first online video slots used the same principle as these traditional slot machines designed by Fey, also known as fruit machines, coin machines, gaming machines, one-armed bandits, and simply as slots.  

Over the course of the years, these slots were expanded with new features and concepts that made the games even more exciting, entertaining, and potentially more profitable for their players. 

Slot machines from back in the day with a minimum amount of reels and symbols like bells, cherries, apples, and BAR symbols still exist today and are known as classic or retro slots. Many players love the nostalgic feeling of playing these kinds of slot machines and some of them like Megabucks, Total Overdrive, Sevens and Bars, Hit it Hard, Fruit Zen, Lightning Joker, and Pay Day are still among the most popular slots today. 

However, the standard slot machine layout consisting of 3 reels has been replaced by a 5×3 layout, meaning that modern land-based and online slots now mostly have a playing field of 5 reels and 3 rows. Modern video slots also tend to have special symbols such as wilds, scatters, and multipliers.

Apart from the newly added special symbols, most modern slot machines also have more paylines than just one. Most slots nowadays have 10 to 20, but there are also Megaways slots with a hundred to hundreds of thousands of paylines using a special mathematical mechanism developed by Nik Robinson, founder and CEO of Big Time Gaming, one of the world’s leading casino software providers.

How Do Slot Machine Reels Work?

Every slot machine obviously uses reels that spin around to determine whether the player wins or loses money. It doesn’t matter how many reels a slot machine has, the principle behind the inner workings is still the same as it was over a century ago. 

Every reel spins independently and isn’t affected in any way by its neighboring reels. The amount of reels a certain slot has has no influence on your chances of winning. 

In essence, every symbol on the reels of a slot machine is ‘’weighted’’. This weighting simply determines how likely it is that a particular symbol will land in a particular place. Heavier weighted symbols are usually the higher value symbols. These are the symbols that rarely appear on the reels, so the chance of a high-paying winning combination is lower.

In the event that a certain combination has such a heavy weight that it takes a relatively long time before it falls again, it is not unusual for a casino or casino website to tie a huge jackpot to this. 

The way the reels are “weighted” in a land-based slot machine, happens with online slot machines by the means of an algorithm. An algorithm in an online slot machine also called a Random Number Generator, is a combination of a random number sequence that ensures that every outcome is completely random.

Symbols that are ‘heavier’ in a land-based slot machine are less often the outcome of this number series with online slots. Slots in an online casino work in a mathematically very complex way that ensures that the outcome of games is actually random and therefore also fair.

The Inner Workings of Slot Machine Random Number Generators

A Random Number Generator, often abbreviated to RNG, is a particular software that every slot machine runs on. The RNG ensures that every game offers a completely fair and random outcome.

Each time the reels spin, the Random Number Generator helps create a unique sequence of symbols. In fact, RNGs are the only way casinos can use to implement risk in a slot in the fairest way possible. For example, no one can predict which order will be produced on the next spin and the outcome will always be random.

In a standard slot game with 5 reels and 3 rows, the number of possible combinations is already in the billions. RNG software ensures that each of these billions of possible combinations has the exact same chance of appearing on the next spin, regardless of previous results or outside influences. This creates a fair game.

The Random Number Generators of slot machines are regularly tested by various authorities. An example of an authority that checks these RNGs is, for example, BMM Testlabs.

BMM Testlabs extensively monitors and tests slots to ensure players that slot machines work properly and that the outcomes are completely random and fair.

The Influence of RTP, Volatility, and Hit Frequency on How Slots Work

Every slot machine has a Return to Player (RTP), level of volatility, and a hit frequency. All these features are in favor of the casino and make sure the casino has a mathematical advantage over the player. Without this house edge, it wouldn’t make sense for casinos to install slot machines in their place or host online slots on their site. 

How often you win when playing slots depends, among other things, on the Return to Player (RTP) and the hit frequency and we are happy to explain how these work. 

Slot machine Return to Player

RTP stands for Return to Player and is always displayed as a percentage. It indicates what percentage of your bankroll you will win on average at a particular slot machine over the course of time. Each RTP percentage is calculated by looking at the results of thousands or even millions of spins from a slot machine.

It is important to point out that the RTP cannot guarantee the amount that you will actually win as a player by playing the slot. Instead, it’s more of an indication of how much of your total wagers you can expect to earn back in the long run.

Let’s take a slot machine with an RTP of 96% as an example. In this case, you can assume that the online casino will win 4% of your total bets on this particular slot after playing for a while.

This percentage is also known as the house edge. Slots with an RTP of 100% or higher do not exist, as there would then be a negative house edge. Once more, the RTP is only an indication of how much you will lose to the casino on average and it certainly does not mean that you can no longer win money at all.

The outcome of each spin is still completely random. 100 spins from one player can produce vastly different results than 100 spins from another player.

Slot Machine Hit Frequency

Hit frequency plays an important role in how slots work. The hit frequency indicates how often a winning combination will appear on the reels. A slot with a hit frequency of 25% will produce a win every four spins on average.

Most slots have a hit frequency between 10 and 30%, but you need more than just this value to determine whether a particular slot is worth it. It is best to combine the following three values: the RTP value, hit frequency, and average winnings you receive. The average winnings you receive depend on the volatility of a slot. 

Slot Machine Volatility

Low volatility slots often pay out small prizes. The chance that you will gamble away your total bankroll is small, but the chance that you will hit a jackpot is also small. Slot machines with high volatility are less likely to pay out prizes, but, on the other hand, pay out higher amounts compared to low volatility slots. 

It is useful to look up the volatility, return to player, and hit frequency of a slot machine before playing. Based on this information, you can assess whether a slot machine suits you as a player.

Unfortunately, the volatility of a slot machine is not always indicated. Unlike the payout percentage, this is usually not mentioned in the game manual when you hit the ‘information’ button. Sometimes you simply don’t know what type of slot machine you have in front of you. 

Because of this, you often hear players complain that they win small prizes too often, or that there are far too few prizes. In most cases, they are playing a slot with a volatility that does not suit their playing style.

Special Symbols on Slot Machines

If you’ve read our how do slot machines work guide thus far, you might get the idea that you can only win by getting at least three matching symbols in a row across a payline. While this was actually the case prior to online gambling, this has now changed considerably.

Thanks to a number of different special symbols, including Wilds, Scatters, Multipliers, and more, winning is not only easier but also a lot more fun. The modern slot machines that you come across in land-based and online casinos all work with different types of symbols. 

Below we explain the most common special symbols slot machines work with nowadays. This will give you a better understanding of how slots work.

Wild symbols

Wild symbols are your best friends in any slot machine that features them since a Wild symbol has a special function. With a Wild, it is possible to win, while you only have two matching symbols on a payline.

How do Wild symbols work? Very simple: a Wild symbol can substitute any other symbol on the same payline. Let’s suppose you have two cherries, melons, or any other type of symbol that your chosen slot has and a Wild symbol. This counts as a winning combination and you are therefore entitled to a payout.

With Wilds, it is even possible to win by only having one symbol on the reels, but then you have to get two Wilds on the same payline. The nice thing about the Wild symbols is the fact that they can come in any shape and size.

Depending on the way the game is designed and beyond the fact that the Wild is developed separately for each slot, Wild symbols can also have all kinds of interesting characteristics. For example, you have Sticky Wilds, which will stick to the reel they appear on. The other reels will continue spinning, creating multiple winning combinations.

Furthermore, some Wilds appear as Stacked Wilds, which means that they can fill an entire reel with Wilds. This again provides many new ways in which players can win.

Perhaps the most bizarre Wild is the Walking Wild. This Wild is designed to ‘walk’ across your screen, creating winning combinations along the way. 

Almost all modern slots have Wilds and they have become an important part of the slot machine business.

Scatter Symbols

Scatter symbols serve multiple purposes during basic slot machine gameplay. Depending on the slot, Scatters can be programmed to award special prizes such as free credits or they can trigger special bonus rounds known as free spins.

However, unlike the Wilds, you need at least three Scatter symbols before anything happens. In addition, in most slots, Wild symbols cannot substitute Scatter symbols. Fortunately, this hindrance is compensated by the fact that Scatter symbols do not have to land on an active payline to activate their features. They just need to be visible on the screen.


A multiplier symbol multiplies your winnings by a certain factor. This gives a direct boost to your profit. The height of this boost depends on the multiplier. In many cases, a multiplier symbol is really just a Scatter symbol. Or more specifically, a Scatter symbol that, as an extra feature, also multiplies your winnings.

The moment a Scatter symbol transforms into a multiplier, it can multiply your current winnings by 2 times to 10 times or even more. Most multipliers will not only multiply the winnings on a certain payline, but all the winnings of the current spin.

The Safety and Fairness of Slot Machines

Sometimes players don’t win anything for a while whilst playing slot machines. Some players then start to think that the casino is ripping them off and that the slot machine is not working properly.

With each slot machine, the odds of winning are predetermined, with some games having much better odds of winning than others. It is therefore important that you understand these odds before you decide to start playing a slot machine.

An important rule of thumb that you should always keep in mind to understand how a slot machine works is the following:

The bigger the jackpot or maximum prize you can win, the lower your chances are of winning it. 

Although a slot machine with only smaller prizes may seem a little less attractive, it will probably make you win more often.

Slot machines in the online casino world are developed by software developers who are required to have a license from, for example, the Kahnawake Gaming Commission or the Nevada Gaming Control Board. The same goes for slot machines you see in land-based casinos.

Online casino software developers, such as NetEnt or Microgaming, must be of impeccable behavior and the slots they release are regularly tested by independent testing companies. It would be a huge risk for these casino software developers to manipulate the results and thereby lose their license.

In addition to the casino software developers, online casinos must also be in possession of various permits and licenses. If you choose to play a slot machine that has been developed by a reliable software developer at a reputable online casino, you can rest assured that the slot machine is fair and works the way it should.

How Do Slot Machines Work – Conclusion

Slot machines are without a doubt the most popular games you will encounter in both land-based and online casinos. The game selection is huge and because there are so many different slots these days, it seems that each slot machine works in a different way.

In reality, this is not quite the case. Slots have different features and consist of different setups, but essentially the machines all work in the same way. The outcomes of slot machines are completely random and work on mathematical formulas.

You get the most fun from slot machines when you know exactly how they work, what to look out for and how you can win the biggest prizes. Explaining how slot machines work can be an endlessly long and complex task, but with the knowledge from this guide, you are totally ready for your next slots adventure! Now that you know exactly how slot machines work, why not give them a spin?


  1. Can you make money from slot machines?

    Yes, you can definitely make money from slot machines. Just bear in mind that every slot machine has a built-in house edge causing the casino to have a mathematical advantage over the player.

  2. How do slot machines know when to pay out?

    Slot machines know when to pay out because the Random Number Generator software they use tells them when to do so.

  3. How do slot machines work in Vegas?

    Slot machines in Vegas work the same way as slot machines in other places, including online casinos.

  4. What’s the trick to slot machines?

    There’s no real trick to slot machines. The only word of advice we can give you is to only bet what you can afford to lose on slot machines.

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