How Do Slot Machines Work?

Online slots are undeniably one of the most popular games at licensed online casinos. The success of these games is due in part to the constant innovation and competition between providers, who strive to offer visually stunning and entertaining games.  

Have you ever wondered how a slot machine functions beyond its appearance? You can find out as you continue reading. 

How the Outcome of Each Spin of the Reels Is Determined

Playing a slot machine

All online slots, regardless of type, theme, and offers, have two very different parts: the interface (the display) and the internal software. 

We can consider the interface of an online game the external part of the game that players interact with. It typically includes a theme that comprises a combination of graphic design and sound effects that create a unique atmosphere and feel for the game. The theme is often chosen based on the genre of the game, such as adventure, fantasy, or even associated with a particular film or character.  

The graphics of a game’s interface is crucial in capturing players’ attention and creating a sense of immersion. Providers compete to create visually stunning interfaces that offer an engaging and entertaining experience.  

You should know that the sound effects used in the interface are also important, as they help to create a realistic and immersive experience. Overall, the interface of an online game is a crucial aspect of its design, as it plays a significant role in attracting and retaining players. 

The slot machine’s internal software, on the other hand, is responsible for everything from the placement of symbols on the reels to the activation of bonuses and the awarding of prizes. This software is known as a Random Number Generator (RNG). It generates combinations that determine the outcome of each spin, ensuring that the game is fair and unbiased. The RNG is rigorously tested and audited to ensure compliance with industry standards, so players can be confident in the integrity of the game. 

The RNG (Random Number Generator) and game interface in online slots operate independently from one another. Understanding how they work is rather essential if you want to know the game’s dynamics.  

While the interface becomes active only when someone plays the slot machine, the RNG software continuously generates random combinations regardless of whether a game session is running or not. The independence between the two ensures that online slots are 100% random, unalterable, and unpredictable.  

Any combination can occur at any time, without being influenced by the number of runs or any external factors. To better comprehend the randomness of online slot machines, it is helpful to think of them as memoryless machines. They don’t recall previous spins or have a predisposition towards future ones, allowing all possible combinations to be equally available for each spin. 

Progressive Jackpot

Progressive Jackpot

There is a growing curiosity about the future of progressive jackpot slots in light of the claim that there are no identifiable patterns between runs. Many people are wondering how this will impact the outcome of these types of slots and whether it will affect the ability of players to win the big jackpot prize. 

Well, even though they are generally referred to as “progressive”, these games still work with an RNG. So in reality the winning combination is generated thousands of times per minute. However, statistically, there is a small probability that it coincides with the moment a player presses “Spin.” 

They have traditionally been referred to as progressive jackpots since the days of physical machines based on mechanical principles of randomness. These were the type of machines where players were mostly uncertain whether the next player would win the prize. 

Presently, a more appropriate term would be “accumulated jackpot slot machines.” This is because the jackpot progressively increases by accumulating a small percentage of each player’s spins. 

Despite popular belief, experts argue that the accumulation of plays and cash in online slot machines does not necessarily indicate that a progressive jackpot is about to be won. The chance of winning the biggest jackpots remains the same, regardless of how much they have accumulated. Therefore, there is no truth to the notion that a progressive jackpot is more likely to be won when it has reached a large sum. 

Credits, Paytables and RTP

Calculations on a blackboard

A portion of every player’s bet contributes to the grand prize for a progressive slot. This is exactly what is explained in the so-called “slot credits.” This includes information about the following: 

  • Common and special symbols: This represents the total number of symbols to form combinations, values, the reels where they are available and (in the case of special symbols) where/how they activate the bonus. 
  • Special features: These are the game’s extras with which you can get bonuses. Some of these features include stacked wilds, disappearing winning combinations in cascade mode etc. Some types are present in slots produced by different suppliers, while others represent real peculiarities of some of them. 
  • Bonus Games: These are additional gameplay features in slot games that offer players a chance to win extra rewards or prizes beyond the main game. 
  • Paytable: These are charts or tables that provide information about the payout structure and winning combinations. They typically show the various symbols or card combinations that can result in a win, along with the corresponding payout amount for each possible winning combination. 
  •  RTP: The “Return To Player” option usually appears on the last screen, next to the “Malfunction Alerts” and “Credits”. However, some casinos post it outside the game, alongside informational text with game instructions, the range of bets they accept, the maximum prize etc. 

When you launch a slot and activate the game interface, the first thing you usually see is the developer’s name. This is followed by a short introduction to put the game into context. As players dive into the game, they’ll be acquainted with a wealth of information about its unique and exciting features.  

At this point you can start playing right away by clicking on the “Spin” button. You can also utilize the ‘Info’ or ‘Pay Table’ option at the bottom or top of the screen. Here are the graphical instructions of the game, which you can take advantage of. 

Activation of Bonuses in Slot Machines

Welcome Bonuses in Casinos

Different online slot bonuses represent some of the most interesting features. These features include all game components that promote winning combinations in the base game, ‌creating more ways to win. 

These are some of the most frequent features you can look forward to: 

  • Free Spins Rounds:  These are triggered when a specific combination is completed, usually with Scatter symbols (or other symbols specific to this feature). Free spins can appear on the same screen as the base game or on a separate screen if you enter bonus stages which also lead to higher prizes. In numerous slot games, players can retrigger free spins. This feature allows players to trigger more free spins if they land the winning combination again, adding to the number of spins they have already won. 
  • Bonus Games: Engage in an immersive gaming experience and increase your chances of winning big with bonus games! These provide players with the opportunity to win additional prizes or rewards beyond what is normally offered in the main game. Bonus games often have unique gameplay mechanics and can include interactive elements, mini-games, or other challenges that add to the excitement and entertainment of the overall gaming experience. 

Verification and Audit of RNGs


The theme and the type of interface are the most common aspects to highlight the differences and classify slot games, but ‌they all work with the same system. Interestingly, the quantity of reels in a slot game is merely a reminder to the traditional brick-and-mortar slot machines of the past. The days of mechanical gears are long gone and with them the tricks of players who tried to predict ‌prizes. 

Independent agencies like eCOGRA and GLI are responsible for verifying and periodically monitoring the Random Number Generators (RNGs). This operation is usually carried out in collaboration with casino operators, suppliers, and licensing institutions. Typically, it ensures that the RNGs used in casinos are reliable and unbiased, providing a fair and transparent gaming experience to players. 

Probability Check

Besides the reliability benefit that RNGs offer to online slots, the fact that they work through software also allows for greater statistical control of payout frequencies. 

When it comes to slots, the term “RTP” or “return to player” is often defined as a calculation derived from the analysis of multiple play sessions. However, the truth is that the RTP program is incorporated into the slot game from the very moment it is designed. This means that the volatility of each slot is already taken into account by the program, as it has an impact not only on the odds of winning in the base game, but also on any bonus features that may enhance the chances of winning a prize on each spin. 

When the final version of a new slot is ready, the RTP is recalculated. This time, however, from the average of prizes achieved over several play sessions long enough to trigger all features and bonus games. 

This explains why most of the high volatility slot machines come with fixed payouts and jackpots, thus making up for their lower average RTP than the rest of the games. The same happens with the value that the developers place on slot symbols with lower volatility (higher frequency of winning on few spins), so that they drop fewer prizes, thus allowing the casino to maintain the house edge. 


Even if you see the reels of online slots spinning (not counting some exceptions like Bubble Craze and Apollo Rising that do without them), the symbols of the combinations do not depend on how and how much they spin. The RNG sends combinations randomly and continuously to the interface, even when the interface is down. That’s why with online slots everything is simply a matter of luck. 

Tobi Opeyemi