How slots work – the mechanics behind winning

How slots work? The mechanics behind slots

Online games, whether they are used for gambling purposes or not, form part of a huge industry worldwide. As technology continues to evolve and the player experience improves, the design and development of online casino games also change over time.

Slot games are a perfect example. What started out as a virtual equivalent of the 3×3 slot machine has become a world unto its own, with high standards for fairness, safety and quality.

Even the simplest online slot is actually a combination of sophisticated elements that work together to ensure complete randomness and unpredictability, which is, at the end of the day, the cornerstone of all gambling games.

In this guide, we will be looking at how slot games work – the software that powers them and important terms associated with them – and what happens when a win takes place.

To the player, this is simply a matter of the right symbols lining up the right way but in truth, a whole lot of effort has gone into making sure that result is completely fair. Our guide will explain exactly what goes on behind the scenes when you spin a slot.

Table of contents:

The mechanics behind slots

There are hundreds, possibly thousands, of unscrupulous websites and individuals offering people ‘slot strategies’, often against payment. These hucksters claim that through some special knowledge, they have figured out how to ‘beat the system’ and outwit slot games, resulting in nice juicy wins.

Smart players don’t fall for this ploy because smart players know that slot games are completely, 100% random. Some games like blackjack and poker require the player to know the rules and exhibit some level of skill in order to win.

This is why experienced poker players do better than novices. Not so with slots. You can play for years on end, across hundreds of different slots, and still have the exact same chance of winning as the person who has only just discovered slots and decides to give them a whirl.

The defining feature of a slot game is its randomness. To be clear, card games like poker also involve a high degree of randomness, achieved when the dealer shuffles the cards so there is no way to predict what cards will be dealt. However, because of the way slots are built, using a technology called a Random Number Generator, there is even more randomness introduced to the result of the spin.

Astute players will check out a slot’s RTP or Return to Player value before they decide to spin. This number, given as a percentage, gives the player a broad idea of the slot’s behaviour and how much of their bankroll they can expect to lose when playing that specific title.

Volatility is another element to consider when looking at how a slot awards wins. This factor will not tell us how much a slot pays out (that’s down to RTP) but how and when. Variance or volatility describes the size and frequency of wins, which can in turn help players decide if that is the game for them.

Random Number Generator

Often referred to as RNG, a Random Number Generator is the software that powers a game and guarantees a fair and random outcome. RNGs have many different uses, not limited to online slots. Video games, statistics, science and even the selection of jurors rely on RNG technology for completely unpredictable outcomes.

In slot games, the role of RNG is to come up with a sequence of unique symbols each time the reels are spun. By ensuring that no two spins are alike, the RNG introduces the element of risk in a fair way. After all, if we knew what the next spin was going to look like, that would affect how much or how little one would wager and there would be no point in gambling at all.

Random Number Generator

To appreciate what an RNG does, let’s take a look at some practical examples. Our imaginary slot for this exercise is pretty basic having only 3 reels and each one of those has 10 symbols on it. That’s three sets of ten symbols in total:

10 x 10 x 10

Altogether, this means there are 1,000 possible combinations of symbols. When the number of symbols goes up, say to 20 symbols per reel, we are looking at:

20 x 20 x 20

This means 32,000 possible combinations. Now, if we take a look at modern slot games, many of which favour the 5-reel structure, we could be looking at a number of combinations that stretch into the billions.

RNG software ensures that all those potential combinations have the exact same chance of appearing on every spin, no matter what result the previous spins have landed.

The RNG part of a slot game undergoes very close scrutiny before the game is made available to the public. Independent auditors put the slot through a rigorous testing protocol, which will help determine if the game is completely random. If a game fails these tests, the developer would need to go back to the drawing board and fix the RNG software before putting it forward for another audit.

Most casinos, and certainly the reputable ones, will only host games that have had their RNG tested and confirmed, but there are some shady brands out there that are less stringent in their practices. Players who want a fair go should avoid these at all costs – dodgy RNG software rarely ends up benefitting the player.

Return to Player

When reading about slots, including in JohnSlots slot reviews, you’ll often come across the term RTP. This stands for Return to Player and is displayed as a percentage. This indicates how much of their bankroll a player is likely to win back over time. The crucial part of this explanation is the last one – over time.

The RTP percentage is the result of a theoretical calculation that is carried out on the result of many many spins. It does not mean that a player is guaranteed to win back that percentage amount of their wagers, only that over the long-term, this is roughly the amount they can expect to gain back.

Another way to look at RTP is as a description of house edge. In a game with 96% RTP, the house edge is 4%. This means that the house (i.e. the casino) will likely receive 4% of player wagers through this particular game.

Return to Player value

Games with a high RTP mean a lower house edge and vice versa. As most players will tell you, the house will always win. There are no games with 100% RTP out there precisely because this would mean no profit for the casino.

The definition of high RTP slots changes depending on who you ask. At JohnSlots, we think that anything about 97% would fall under this category. We have written more about high RTP slots in another article and some examples we mention there are:

Given the range of RTP values in slot games today, how do they compare to other casino titles? The house edge can and does vary between casinos and between games and their different variants.

Take roulette, for instance. On a single zero wheel, the house edge is somewhere around 2.7% while on a double zero wheel, that goes up to 5.26%. That’s the same game but with a slight difference between the wheels. Video poker gives the house an edge that ranges between 0.5% all the way up to 5%.

All this is to say that the house or casino will always have a built-in advantage. That’s how they run a business after all. But when it comes to selecting a game based on house edge, how slot machines compare will depend on their RTP. Other factors come into play as well. If you have no idea how to player poker, you’re a whole lot more likely to end up with nothing at the end of the day, while slots do not depend on skill at all.


There’s yet another aspect of a slot machine that will determine what kind of experience a player has and that’s volatility. This is also called variance and it describes the estimated size of player wins and how frequently they occur.

The best volatility slots

Confused? We were too at first. Here is how you should think about variance. Let’s say that one particular slot has 96% RTP. This means that, over time and on average, a player who wagers £100 will win back £96. Remember, this is a broad generalisation and as we have explained above, RTP doesn’t really work that way. But for the sake of argument, let’s go with this example.

  • Low volatility slots are characterised as awarding small, frequent wins. So our imaginary player might bet five times and receive a win, and then bet another 6 times and receive a small win and so on.

Those small wins supplement the player’s bankroll, which means they will have enough cash to play with over a longer time before they either choose to stop or run out of cash. The projected £96 in winnings would arrive in small increments that allow continued play.

Low volatility slots are the perfect choice for new players who prioritise the opportunity to play for longer over raking in the big wins. Games like Starburst are a fan favourite and it’s no wonder many casinos include spins on this slot in their welcome packages.

  • High volatility slots: However, let’s say our example slot is highly volatile. This means that the player can go a long time without a win but when that win happens, it will be significant. There is the risk that a win does not take place at all – there’s the RNG in action for you – and without a win, the player’s budget does not allow for further bets.

In our 96% RTP slot example, having a high volatility means that the player could go for a long stretch with no wins at all, then win £96 or even more on a single spin.

High variance slots introduce high levels of risk and reward, which will most appeal to players who enjoy that element in their gameplay (and have the bankroll to pull it off). Popular high volatility titles include The Wishmaster by NetEnt, 300 Shields by NextGen and Raging Rhino by WMS.

  • Medium volatility slots: Perhaps the biggest number of games fall under the medium variance slots category. These balance payout size and frequency to the point that the majority of players enjoy. Games like Mustang Gold by Pragmatic Play and Wild-o-Tron 3000 by NetEnt are just two examples of the huge range of slots that are considered medium volatility.

Slot volatility can be difficult to find. Most developers do not publicly state their slots’ variance levels though trusted sites like JohnSlots are given access to this information. Our slot reviews include slot variance as we believe this can really help players.

However, there are other ways to figure this out for yourself. The RTP can give you a hint in some cases – having a low payout rate can sometimes indicate that a slot is a high volatility – but this is not an exact science.

For a more hands-on approach, make use of the demo or free play feature many slot developers offer. This allows players to study a slot’s behaviour, how often it pays out and by how much, without risking any of their own cash. Demo play also gives you the chance to see bonus features in action and can help to decide if you want to play that slot for real. We highly recommend slots free play for these many reasons.


Slot games are fun and exciting, which is why so many millions of people around the world enjoy playing them! Slot mechanics may not seem like a terribly interesting element of these games but they influence every aspect, from random results on a spin to the likelihood of landing a large win.

Understanding how these elements affect slot functionality can help players make the right decisions on which games to play. RTP and volatility mean very little in isolation but putting that information together will give a better idea of what the player can expect.

If you want to know more about slots and how to play them, read our beginner’s guide.