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Understanding the ever-elusive RNG

by Frank Legato


Our reader mail is normally a pretty good gauge of the elements of slot machines that arouse curiosity in our readers. Easily the most-asked question relates to how slot machines reach their results.

More than 20 years after we first published Casino Player, readers are still asking us how the random number generator works. They usually ask this as if the RNG is some kind of little machine that manufacturers add onto a slot to pick the results. They say, “How does the RNG decide on a winner?” Or, “Does the RNG know how much you bet?”

First of all, the RNG is not a device, or a little machine. Second, the RNG does not “decide” or “know” anything; it’s not an entity.

The random number generator is a software program written into the computer game chip of each slot machine. It is properly named, because its function is to do exactly what its name says it does: It generates numbers from a pre-entered set, in a random fashion.

Since the mid-1980s, all slot machines have been essentially stand-alone computers. They are all programmed in a similar fashion. A software engineer will assign each symbol on each reel, or each potential result in a five-card poker game, a number in the program. In the case of video poker, each potential result in a five-card hand of poker is given a single number. In the case of a slot machine, each symbol is given a number, and then some symbols are given additional numbers.

Slot Machine

In a slot, those numbers result in hundreds of potential results on each reel. High-paying symbols are given fewer numbers; low-paying symbols and blanks are given many more numbers. This is how manufacturers can predict, through simulation, what the theoretical payback percentage of a game will be—it all goes back to how many numbers are assigned to each symbol.

Once all the numbers are placed in the program, the laws of probability take over. When the new machine is powered up, the RNG starts generating numbers. It constantly generates numbers, at a high speed—hundreds of numbers per second are generated, in a random sequence, as long as the slot machine is powered up. When a player presses the spin button or pulls the handle to spin the reels, the computer freezes the combination of numbers generated by the RNG at that nanosecond, and displays the corresponding result on the reels.

This all happens in the blink of an eye, and the reels will look like they have spun to certain positions through gravity. On classic reel-spinners, they look and sound the same as reels did when slot machines were mechanical devices, with spring-loaded reels. The handles even feel as if they are still compressing springs as you pull, and releasing those springs to send the reels spinning to a random spot.

In reality, springs in the reels are only there to create that very effect. The reels themselves no longer determine the result; they are only there to display the result the computer tells them to display. What has caused the result is the combination of numbers generated by the RNG at the instant the button was pushed. Mechanical reels perform the same function as a video screen—they are mechanisms to display the computer’s result.

In the old days, one could calculate the odds of a slot machine by counting the number of “stops”—symbols or blanks—and how many times they occurred on each reel. That is no longer a valid calculation, since the 22 physical symbols on each reel may represent hundreds of potential results, because of the duplication in the computer program. Picture a slot reel that is five feet long, with symbols filling the entire reel strip. That is what the computer is duplicating. The RNG is simply generating numbers corresponding to each symbol on that virtual five-foot reel, as if all five feet were spinning to a random result.

In video poker, what you see is what you get. Since there is only one number assigned to each possible outcome, the RNG’s function in video poker is essentially to duplicate a physical game of draw poker, with a 52-card deck (53 with the Joker) shuffled at high speed between each hand. But in slots, there is no way to know what the odds are by looking at it.

It all goes back to the numbers. And to Lady Luck.


This just in… Your choices of slot venues continue to rise. In Illinois, the state Supreme Court just validated the Video Lottery Act, which means there will soon be up to five slot machines in each bar, club or licensed liquor establishment that chooses to participate. That goes along with a bill that is on Governor Pat Quinn’s desk that would add slot machines to state racetracks, and create several more commercial casinos, including one in Chicago. In Ohio, Governor John Kasich just cut a deal with lawmakers that will lead to slots being installed in that state’s racetracks, to go along with new urban casinos in Cincinnati, Cleveland, Toledo and Columbus. Florida is still looking at resort casinos in Miami as a possibility, and Maryland’s new casino will open next year in Anne Arundel County. The hits keep coming…


Quick Tip


Read the screen. Readers often ask us how much of a wager they have to make to get the most out of a slot machine. The answer is simple: Read the screen. Slot machines will always tell you what you need to bet to trigger bonus events, or to qualify for a progressive jackpot. There either will be a paragraph of text spelling it out plainly, or in the classic reel-spinning setup, the instructions will be in the pay schedule: “First Coin” will have certain winning combinations under it; “Second Coin” will activate other combinations, and “Third Coin” or “Max Coin” will usually activate a bonus for the top jackpot.

The point here: It’s all on the face of the slot machine.

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