How Slot Machines Work: Mechanics, Randomness, and RTP

When a novice casino player first encounters a slot machine, it seems fairly familiar — after all, it is an iconic device, a symbol of the industry. However, the mechanics of it, as well as the rules it follows, aren’t obvious at first glance, and there’s a lot you should know before you start playing.

We partnered with the owner of Slot Day Michael Slot to uncover the intricate nature of these machines and explain all the important terms associated with casino play.

Basic mechanics of slot machines

Slot machines are deceptively simple: after all, it’s just several reels spinning for a certain period of time and then stopping to show the final result — how difficult can that be? However, there are lots of mathematical nuances involved in determining every single spin.

Let’s start from the basics. There are two main types of slot machines: ones with mechanical reels, and digital ones. The difference between the two seems nonexistent from the player’s standpoint, but mathematically the two are completely different devices.

Mechanical slot machines use physical reels that spin for a certain time and then stop, producing a random outcome, as long as they are designed fairly. Digital devices simulate slot animations, while in fact the outcome is determined by a pseudo-random number generator the moment the user hits the button. Hold that thought — we’ll discuss the nature of randomness in slot machines later.

In order to play, the user has to insert a certain amount of cash or a ticket representing it. The game itself requires only a single input from the player, which is the command to spin the wheel. The rest is determined by chance, which is what differs slots from skill-based games like chess, for example. The rules for slots as well as the number of reels may differ, but in general, the machine states that certain combinations of symbols are counted as wins, and the player’s goal is to get them, despite not having any control over the outcome of each game.

Some games include several other mechanics to incentivize players to keep spinning the reels and engage with the machine. For example, they may allow players to wager not just on the central line of symbols but on others as well.

Another common practice in slot machines is to occasionally offer players “free spins” or other bonuses for landing “scatters” (certain combinations of unaligned symbols). It is worth noting that the spins aren’t exactly free — every machine has a set output requirement, or “return to player”, so the “free” extra spins are in fact pre-calculated offers that don’t affect it. We’ll talk about RTP a bit later though.

Hidden mechanics of slot machines

While the basic features of slot machines are straightforward, there are several mechanics that players can be unfamiliar with, even though they significantly affect their experience with the game.

First of all, the reels on the slot machine can contain a different number of symbols. As noticed by Charles Livingstone in his article for the Australian Gambling Research Centre, “Dolphin Treasure” has four reels with 30 symbols, but a fifth one with 44. Not only that, here’s how many “King” symbols each reel has:

While the actual chance of getting five “King” symbols is very small (1 to 297,000), this placement creates much more “near miss” outcomes than a standard one would produce, resulting in players feeling they need just a few extra spins to achieve the desired outcome. Although it doesn’t make mathematical sense, this is a common bias that certain games exploit to engage players.

Another common tactic casinos rely on is losses disguised as wins. For example, if the user wagers on multiple lines and wins only one of them, losing more money than winning, the game will still celebrate the “victory” with visuals and sound, making losses feel less impactful or even go unnoticed. This tactic, however, is prohibited in certain jurisdictions.

What is RTP?

Despite being one of the most important metrics in a casino game from both a player’s and a legal perspective, RTP is surprisingly tricky for a beginner to grasp. As a result, Michael Slot explains, studies show that lots of players don’t understand RTP correctly, leading to confusion and mistrust <
To put it simply, RTP determines how much money the machine will return to players in the form of winnings. In general, modern devices set their RTPs around 97% and higher. While that sounds good in theory, there are several misunderstandings about the way RTP works, which may mislead players and make them spend money expecting more than what’s actually promised.

When a player sees a slot machine offering a 97% RTP, at first they might think that when they put $100 into it, they will get $97 back eventually. However, RTP is calculated for all players, and not individually.

What that means is that it only truly works on a large scale, explains Michael Slot — for example, the machine will return $970,000 for every $1,000,000 put into it, but that doesn’t mean that it will spread this money equally among players: some will earn more, some will earn less, and some might not win even once.

Not only that, the machine returns 97% of money put into it, which means that if a user puts their winnings back into it, they will get proportionally less money overall — theoretically, and over the course of hundreds of thousands of games.

Think of it this way: the machine promises to return 95% of the money you put in. Let’s imagine that you are a player with an infinite amount of time and money. When you put $1,000 into the slot machine, you are expected to get $950 back eventually. However, if at any point you use the money you won to spin another round, the money you use will follow the same rule — you will only return 95% of it. This way, eventually, you will end up with only a few dollars in your pocket.

But how can you even measure the RTP at all? Well, it usually takes a lot of work. To put it shortly, the game’s designers calculate the RTP themselves by testing and building various mathematical models, and then their calculations are examined by an external committee that also tests the game independently.

How slot machines achieve randomness?

As we’ve mentioned earlier, there are two types of machines: traditional mechanical ones and modern digital ones like Pragmatic Games UK. For simplicity’s sake, we’ll put quantum qualifications for randomness aside — otherwise, we would have to say that almost nothing in our world is truly random at all.

What matters here is the fact that digital devices aren’t capable of producing random outcomes in a general understanding of the word — that’s just not the way computers work. In theory, one can use external sources to create random outputs: for example, Cloudflare video streams a set of lava lamps placed in one of its offices and uses the image data from the video frames to create random numbers for cybersecurity purposes.

However, this is way too complicated and frankly unnecessary for casino purposes. Instead, digital slot machines rely on pseudo-random number generators. From a player’s standpoint, pseudo-randomness is just as unpredictable, so as long as you’re not a hacker with access to every single part of the machine, it doesn’t really affect you directly.

What’s important is that a lot of times pseudo-random machines are compensated — it’s a standard industry practice, says Michael Slot. This means they aren’t just trying to be as random as possible but are instead ensuring that the device consistently reaches the required RTP. What that means is that if the machine has created a lot of loss situations recently, its internal RTP calculator will show that now it needs to produce a bit more wins to compensate for it, and vice versa.

While a player might think that true randomness is more “fair”, in fact, compensated pseudo-random outcomes generally provide a better playing experience, theoretically leading to fewer situations where a player doesn’t get anything at all throughout multiple games.

Another important thing to note here is that none of the casino games are truly “fair”, whether truly random or compensated.

First of all, in most countries, a certain RTP percentage is a strict requirement, meaning that over the course of millions of plays, they give back equal amounts of money to players, as long as their RTPs are the same. Compensated machines just do it more consistently and would require fewer plays to achieve this result.

Secondly, every single casino game is designed in a very specific way to ensure that the RTP is never above a certain percentage. For example, imagine if the slot machines had just one reel with four symbols, and two of them meant an instant win. This would be way more favorable for players, but how soon would the casino go bankrupt? The games themselves aren’t completely “fair”, and they never will be.

Remember that the compensation only really works on a global RTP level, so while it might be less chaotic than “true” randomness, streaks of bad luck can still happen. Also note that because the machine compensates for global RTP, which is calculated over tens and hundreds of thousands of plays, strategizing around it isn’t really viable.

For instance, if you see the player before you lose 10 spins in a row and quit, you might be tempted to play the compensated machine, assuming your chances of winning are higher — but that might not be the case.

Even if RTP was a more personal metric, the machine might have just stabilized its RTP after the previous player to offer an average win rate, or it might have paid too much in the past hour and still not finished the compensation process, resulting in you losing 10 spins in a row as well. However, as mentioned earlier, a player generally cannot predict or expect anything from a slot machine.

Sources:

1. “How electronic gambling machines work” by Charles Livingstone; Australian Gambling Research Centre
2. “Understanding of Return to Player messages: Findings from user testing” by Debbie Collins, Sophie Green, Joanna d’Ardenne, Heather Wardle, Shauna-Kaye; Responsible Gambling Trust.

Disclaimer:

CBD:

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Gambling:

As per the Public Gambling Act of 1867, all Indian states, except Goa, Daman, and Sikkim, prohibit gambling. Land-based casinos are legalized in Goa and Daman under the Goa, Daman and Diu Public Gambling Act 1976. In Sikkim, land-based casinos, online gambling, and e-gaming (games of chance) are legalized under the Sikkim Online Gaming (Regulation) Rules 2009. Only some Indian states have legalized online/regular lotteries, subject to state laws. Refer to the legalities here. Horse racing and betting on horse racing, including online betting, is permitted only in licensed premises in select states. Refer to the 1996 Supreme Court judgment for more information.

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