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How Do Online Slot Machines Work?

What is a slotmachineOur Casino Gorilla is at it again. Today he woke up with a ceaseless urge to discover how online slot machines work. So he woke up early, munched every puddings he had in store as he set sail towards this exploration. Of course, slot machines are considerably different from the way they used to be when the Gorilla was just a toddler. The traditional mechanical slots machine (in brick and mortar, land-based casinos) used springs and tensions to determine the winning reels.

The modern, technologically-based slots use much more sophisticated mechanisms to determine the outcomes of each spin. So as you’d guess, the ‘know-it-all’ Gorilla had a busy day in the office trying to bring you this detailed guide on how online slots operate. As such, the sole purpose of this article is to shed light on this topic and dissolve all the myths and misconceptions that most people believe govern the way online slots work.

What Is a Slot Machine?

For what it’s worth, let’s start by defining what a game of slot denotes.

  • A slot machine, also known variously as a fruit machine, is a casino game with three or more reels which spin when its button is pushed or triggered.
  • Slots, whether traditional or modern, pay off depending on the patterns of symbols displayed on the screen when the spinning stops.

Slots remain the most popular gambling method in casinos and are preferred by many punters because they are less complicated and require no strategy or game plan to be played. The beauty of online slots is that they are computer programmes and can be modelled into various types. However, the most common types of online slots are described succinctly below:

The most common types of online slots:

  • 3-Reel Slots—this is the classic version of slot machines that only have 3 reels and between 1 and 5 pay lines. These types of slots are easy to understand, features fewer symbols, and have clearer payouts.
  • 5-Reel Slots—at present, these types of slots are more prolific with advanced animations and graphics, and often with storylines and video clips interspersed. These have more paylines which can sometimes as many as 50 or 100 pay lines.

How Does It Work?

As aforementioned, the game of slot required no strategy or game-plan to be executed; the results are determined by pure luck. Basically, most slots work in much the same way. Symbols are arranged on vertical spinners (reels), which are spin and stop to reveal a random series of symbols that are arranged on the pay-line.

A player wins by hitting different symbols combinations and patterns. In order to perfectly understand how slots machines work, it is important to understand various terminologies associated with the same. These include:

  • random number generator (RNG)
  • symbols
  • pay lines
  • bonus rounds
  • coin value
  • bet levels
  • progressive jackpots
  • return to player percentage (RTP).

Random Number Generator (RNG)

Random Number GeneratorAlthough the technology behind slot machines have considerably changed over the years, the principle behind how they work remain the same—randomness.

  • The reels spin and stops randomly to form a winning (or losing) pattern/payline.
  • Unlike in traditional machine, where this randomness was determined by cams, gears, kickers, and brakes, the modern machines use a computerised mechanism known as random number generator (RNG).
  • The main purpose of the RNG is to create random sequences in intervals of a millisecond every time a reel spins.

All online slots at casino sites have RNG software that determines the scores they produce; each result is different and isn’t dependent on the previous spin or spins outcomes

It is crucial to note that the RNG doesn’t have a memory and it will therefore not note whether you spent the whole day winning or losing or whether you’ve placed a huge or tiny stake—everything about RNG is random.

To ensure utter randomness, game designers use what in math is referred to as vectors (sometimes called arrays) to represent the reels.

  • While a single slot have a specific number of symbols (normally between 10 and 15), a single reel contain many more symbols.
  • In composing a slot game, developers compose a set of virtual reels (vectors) that contain symbols in codes.
  • The length of reels vary by game that’s why game differ, but all that matters is that the vectors are distributed on the reels in such a way that some are found more than others.
  • Typically, the high-value symbols occur less often than the low-value symbols.

Some players may have an out-and-out understanding of how slots work, but the most important thing to note is that slots outcomes are completely random all because of the RNG.


As mentioned previously, most slot titles have between 10 to 15 standard symbols. Symbols are basically the icons or features that show on the screen to determine whether you won or lost.

In retro and classic slot machines the symbols are:

  1. the featured symbols are commonly represented by fruits symbols such as Grapes, Lemons, Melons, Oranges, and Cherries.
  2. the high-value symbols are normally represented by number (7), a Bell, a Diamond, or the so-called (BAR) symbol.

In modern slots are usually represented by:

  1. playing-cards icons such as Ace (A), King (K), Queen (Q), Jack (J), Ten (10) and Nine (9)
  2. the cards suits including Clubs, Diamonds, Spades, and Hearts
  3. the low and high-value symbols in modern slots vary in design and will appear artistically different from one game to the other. This is because today symbols are designed to fit the theme of the game.

For instance, if you’re playing a slot that is fashioned in underground water, the high-value symbols could be fishes, treasures, ships or something to fit the said theme. Sometimes two matching symbols are enough to give you a winner, while in some cases three or more matching symbols are need. Most slots winning lines are displayed from left to right on the payline, but other may pay from right to left. Others, such as Starburst, winners can be achieved from both either of the two directions.

Symbols in slot games

Here are two examples of  symbols above; Gun’s N Roses and Book of Dead -slot games.

In addition to these regular symbols, there are three other special symbols that are common with slot machines. These include “Wild Symbol,” “Scatter Symbol,” and “Bonus Symbol”:

  • The general use of the wild symbol is to replace other symbols on the reels in order to create a winning combination that wouldn’t otherwise result in a win.
  • Scatter symbol on the other hand normally triggers a bonus such as free spins, sin-spins, or re-spins. If a number of scatter symbols appear anywhere on the reels, you could also hit a win.
  • Bonus symbol is another highly anticipated icon, whose main function is to trigger bonuses such as free spins.

Here are two popular slot machines and examples of their symbols:

GamePayout PercentageSymbolsRead Review
Twin Spin logo Twin Spin 96,6 % Symbols: lucky 7s, fruits, diamonds and BAR symbols. Special Symbols: None
Thunderstruck 2 slot logo Thunderstruck 2 96,6% Lower symbols: traditional "9 to A" playing card icons Higher symbols: Viking long-ship, Valhalla, the Valkyrie, Loki, Odin and Thor


A payline (also known as winning line or betting line) is the grouping of symbols that result in a win in a game of slots.

  • The traditional slot machine models had only one payline with three matching symbols on a horizontal line.
  • In the modern versions, both online and land-based, paylines aren’t just limited to one line, they come in different types depending on the slots games. Some can be horizontal, diagonal, zigzag, or even trapezium.

Most commonly, paylines in slots betting range between 25 to 30, and different paylines leads to different multipliers, free spins, prizes, and bonuses. In fact, some slot titles will give you the freedom to choose the number of paylines you’d want to play. Regardless of the number of paylines or the shapes they take, it’s always crucial to remember that a payline must cross the same symbols for it to be a winner.

  • If it’s a 3-reel slot, the payline must cross 3 similar symbols
  • if its 5-reel slot, the payline must cross 5 similar symbols

Coin Value and Bet Level

Coin Value and Bet LevelCoin value and Bet Level are two features in a slot game that allows you a wider range of different stake amounts, so you aren’t limited to just the coins multiplied by the paylines. Coin Value is based on the actual coins, for instance,—£0.01, £0.02, £0.03, £0.04, £0.05, £0.10, £0.20, £0.50, £1.00, and £2.00. The range of Coin Value depend on the slot, and although most hit £2.00, some are limited to just £0.05.

Essentially, the Coin Value represents how much you want to spend on each payline in the slots.

  • For example, if you are playing a 30 payline slot and you choose a Coin Value of £0.02 your total stake will be £0.60 (£0.02 x 30).

On the other hand, Bet Level exists purely to give you more freedom in controlling and personalising your stake.

  • Instead of having the normal payline values of £0.01 to £2.00, you can get a little more creative to get even better or higher stake values.
  • For example, if you set a Coin Value of £0.03 and Bet Level of 3 you can get a value of £0.09 which is not possible with the normal coin value alone.
  • Multiply the resulting value (£0.09) with a payline of, say 25; you get an even better bet stake.

Progressive Jackpots

In the world of slots, there are progressive jackpots. Basically, these are just the games of slots with only bigger pay-out values. Well, regular slots are in fact referred to as non-progressive jackpots in that their values are fixed. Their values are always the same no matter how many times they are played or won. As you’d guess, regular slots have substantially lower pay-out values compared to progressive jackpots.

In contrast, progressive jackpots are not fixed at all, and the amount will continue to increase as more players join in and play.

  • It takes only a tiny percentage of each bet placed to sort of fuel the entire jackpot making it bigger.
  • Unlike regular video slots, these games are normally worth thousands or millions of pounds.
  • When a progressive jackpot is won, the prize will disappear and defaults back to its standard value.

However, in terms of paylines, symbols, and other slot features, nothing separates regular slots and progressive jackpots.

Here are three popular jackpot games that you should definitely try:

GamePayout PercentageFurther InformationRead Review
Mega Moolah logo Mega Moolah 94,5 % There are four different levels of progressive jackpot to play for with Mega Moolah: The mini, minor, major and mega jackpots. The mega jackpot gives the best chance to win a life-changing sum of money.
Arabian Nights logo Arabian Nights 95,6 % if you are lucky enough to see five Arabian Nights symbols together on an active pay-line, you have just won a jackpot which regularly extends into the millions. Even one or two Arabian Nights symbols will give you a chance to win a larger than usual amount.
Hall of Gods logo Hall of Gods 95,5 % There are three different levels of progressive jackpot to play for with Hall of Gods: the mini, midi and mega jackpots. The mega jackpot can be expected to deliver several millions.

Return to Player (RTP)

Return to player, or as commonly shortened to RTP %, is the phrase used by casino sites to describe the long term theoretical expected payback percentage from all wagers on a particular game of slot. In a nutshell, the RTP tells how high (or low) a slot’s payout percentage is.

Return to Player (RTP) By use of a simple mathematical example:
  • a certain slot has a RTP of 96.1%
  • you stake a bet of £1000
  • if all things remain equal, you’d expect to get a return of £960.10 in wins from your £1000 stake.

Every slot has its own RTP percentage, and the higher the percentage, the more often you can win from a particular slot over a longer time period. What’s important here is that, while the RTP is important, it’s just a theoretical statistical calculation that shouldn’t entirely be taken literally. You need to keep in mind that there are always instances for random wins, some very huge, like hitting a jackpot like Mega Moolah. Importantly, the RTP will only tell you the average estimation for the slot machine in question.

Final Thoughts

Slots machines, whether online or land-based, remain the most popular attraction in the world of gambling. The appeal of these games lies in the fact that anyone can win at any time. All it takes is for you to place a bet, click the spin button, and wait for your luck. However, as the Casino Gorilla has revealed, behind the scenes, slots games are anything but simple. The randomness in results is thanks to complex, computerised, and innovative feature that is RNG:

  • In this case, players can always rest assured that the casino doesn’t have a hand in determining the result of the slot machines.
  • No strategy (other than responsible gambling) is applicable in winning the game of slot.
  • Maybe check the games RTP value to determine your chance, but other than that only count on your luck.
  • Even so, you can hit a mega jackpot on a low-RTP game, just the same way you can dwell on a losing streak on a high-RTP slot.

Author: Leo Wilson
My name is Leo and I am a contributing author at Casino Gorilla. Over the years, I have developed a love for online gaming. I have spent many hours sitting in land casinos and traveling to different venues. The first time I gambled online, it was love at first spin and I haven’t looked back since.
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