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How are Slot Machines Programmed

How are Slot Machines Programmed

Table of Contents

Introduction to Slots Odds 

According to the Merriam-Webster dictionary, an algorithm is defined as follows: ‘A procedure for solving a mathematical problem in a finite number of steps that frequently involves repetition of an operation.’ In slots parlance, algorithms are important insofar as they are programmed mathematical codes to randomise the outcomes of slot machine spins. They do this by way of Random Number Generators (RNGs).

Slots players want to understand how slot machines are programmed. The answer to this question lies in the complex mathematical codes, and programming that governs the results of every slot spin. This broad topic routinely covers many parallel areas such as casino hold percentages, random number generators, slot odds, and the like. We begin our discussion with a myth. That being that slots odds are wholly determined by random number generators (RNGs). Truth be told, random number generators do not set slot machine odds. This practice stopped over a decade ago.

Sophisticated casino software and powerful gaming technologies have evolved at breakneck speed. With increasing competition, and shrinking profits to contend with, casino operators have adopted innovative new technologies to reduce costs, increase efficiency, and boost profitability. Coupled with that are new regulatory frameworks which can mandate compliance from operators. In other words, there are upper limits and lower limits on payback percentages for slot machine games.

Slots operators who want to retain their licences must comply with statutory requirements. In answer to the question how are slot machines programmed, one cannot ignore the importance of regulatory constraints. In essence, it is lawmakers who can determine the upper and lower limits (payout percentages) of slot machine games. Sometimes, lawmakers choose not to set limits. The actual mechanics of it is a different story; this includes, software, hardware, and the expertise required to put it all together.

According to the rules of the UKGC, the MGA, the NJDGE, and numerous other regulatory authorities, casinos must submit statistical reports to gaming commissions and oversight authorities. Various watchdog organisations such as TST (Technical Systems Testing), eCOGRA Safe & Fair, iTech Labs, and others are responsible for validating the performance, credibility, and transparency of random number generators, and the computer codes that generates slot machine outcomes.

How Many Winning Combinations Are There with Slots?

Think of a Money Wheel Game, or even a Roulette Wheel for a moment. Every time the casino host spins the wheel, any listed outcome is possible. With slot machine games, the wheels are reels, but the number of winning combinations is immense. For starters, there are multiple reels spinning at any given time. Classic slots typically feature 3 reels, while video slots feature 5, 6, or 7+ reels. On each reel, there are multiple symbols. In the early days, classic 3 reel slots featured 10 symbols on every reel, with 1,000 possible combinations.

When blank spaces are included on the reels, this has a multiplier effect on the possible combinations that can result. When bigger reels are used, it’s possible for them to hold 20 symbols and 20 spaces on each reel. On a standard 3 reel slot with 40 symbols + spaces, you can expect 40 x 40 x 40 = 64,000 possible outcomes. That is still minuscule compared to what is possible with today’s video slot machine games. If you consider a 5 reel slot with 40 symbols + spaces, that’s 40 x 40 x 40 x 40 x 40 = 102,400,000 possible combinations.

The formula for determining the odds of winning on a slot machine game is as follows:

The Number of Winning Combinations divided by the Number of Possible Combinations.

Naturally, the highest number of possible winning combinations will result in the largest percentage payoff for that specific slot machine game. If you only need a Joker to form on the reels to generate a payout, and there are 100 possible Joker combinations, then you simply divide 100 by the number of possible combinations in the slot machine game to get the percentage payoff.

The mathematics gets incredibly complicated, given that certain combinations have a much higher weighting than others. If we take a basic 3 reel slot, the number of winning combinations with 20 symbols on each reel is 20 x 20 x 20 = 8000. Then, we would have to add all the possible winning combinations and divide that figure by 8000 to get the odds of winning on a slot machine game.

Getting Technical with Slot Games

We know that slots usually have between 3 reels and 5 reels. The classic slots tend to have fewer reels and fewer paylines than the video slots and jackpot slots. As we get into the nuts and bolts, it is clear that rare winning combinations are associated with bigger payouts. Nowadays, slots usually have up to 50 stops per reel, sometimes more. The more stops, the easier it is for big jackpots to be offered. Think of a 5 reel slot with 10 stops vs one with 50 stops. 10 x 10 x 10 x 10 x 10 =10,000 outcomes vs 50 x 50 x 50 x 50 x 50 = 312,500,000 outcomes.

Recall symbol weightings in slots games? Modern-day computing software can easily adjust payout probabilities through weightings at the click of a button. If there is a high payout symbol that can come up once every 80 spins, then the chances of hitting that symbol on a 5 reel slot are 80 x 80 x 80 x 80 x 80 = 327,680,000. The high-frequency symbols land often, so payouts are much smaller. These payout combinations are designed to entice players, with small winnings.


We answered one of the most commonly asked questions – how are slot machines programmed?  Random Number Generators (RNGs) are sophisticated programs designed to randomise game outcomes. Nowadays, cutting-edge technology and software programs make it easy for casinos to instantly adjust slot machine payback percentages, and weightings of symbols.

While many slots players follow old wives tales, it’s important to separate fact from the fiction. There are no loose slots, or tight slots. There are RTPs which govern the long-term return to player. Progressive jackpot slots, fancy video slots, and slots with bonus games (pick’em bonuses, free spins) tend to have lower RTPs, with higher potential payouts.

When choosing slots, consider volatility. High volatility slots have lower RTPs and bigger payouts, while low volatility slots have higher RTPs but smaller payouts. Your appetite for risk determines which slots games you will play, how much you will bet, and what you can expect to win back over time.