RNG and Slots

by James Carter on February 19th 2015.

How They Work

When a slot machine’s reels start spinning and you see the animations and you hear the sounds, your fate has already been decided.

The random number generator (“RNG”) of the slot machine processes hundreds of numbers in a second (works even when you’re not playing) and picks a number between zero and a few billions at the moment you press spin, then assigns the numbers one-by-one to each reel.

Each of the numbers assigned to reels are then divided by the machine’s set value. Once the numbers have been divided, the amounts left after the division decide at what point of each individual reel the spinning stops.

The outcome could be displayed as numbers at the moment the player presses spin; reels, sounds and the waiting time are merely a more entertaining way of displaying the outcome.


In a three-reel slot machine, three numbers are generated by the RNG the moment you press spin, which the computer then assigns to reels one, two and three.

For example, if set value is 128, then each of the numbers are divided by 128. This also means that each reel can get 128 different numbers which all have a symbol (obviously several of the numbers are assigned to same symbols since there aren’t 128 different symbols).

If the number for the first reel was 128,000,030, the calculation would produce a remainder of 30 (when divided by the set value 128). What you will see on the first reel is the symbol equivalent to 30. The same process is then repeated on each of the reels.


Random number generators are required in all slots since the outcomes must be random. The casino always has a built-in advantage (known as house edge or “hold percentage”) which is based on different payouts – randomness stays the same.

Slot machines are required to produce a minimum payback or frequency of wins or both (the limits depend on the jurisdiction). While the RNG always works the same, the EPROM chip inside the machine determines how much players win when they land winning combinations and therefore determines the payback percentage.

So when a casino changes the EPROM chip, randomness remains but payouts change, and therefore payback changes. RNGs never consider the money involved.

Every spin has an equal chance of winning regardless how much players have won, how long it has been since the last win or what the weather is like. RNGs are always equally random.

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