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Is the RNG on a Slot Only Get Hit Once Per Spin?

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Written by Joshua

I have received the question on multiple occasions if an Random Number Generator (RNG)-driven slot machine is hit once per spin, or is it hit more than that (given multiple reels, etc.). So I asked an expert in the field whom I occasionally chat about these topics, and got a pretty interesting answer:

STATUS: Some slots will hit an RNG once; some will hit once per reel. How this is managed can depend on the approach that the slot is designed.

As yesterday’s post has begun to highlight another aspect of this topic that I learned from this conversation, a slot can take a “reels first” approach to its math or a “prize first approach.”

Reels First is more steeped in standard slot tradition and how mechanical reels were designed, which carried into how virtual reels play out on screen for many slot developers. In that scenario, each reel outcome is based on a reference of the RNG to determine where it will stop.

Once all five reels stops’ are determined the game can figure out what the pays for that spin will be, based on what symbols are present for that combination of reel stops. This is where you may see “pays evaluated” in the pay table – it’s evaluating what is owed based on the pay table and the combination of symbols landed.

On the opposite site, Prize First determines whether the spin is a loss or a win (and how much if it is a win), therefore determining what the prize will be. It uses the RNG to determine the prize. Then the machine will display a combination of reels that lines up with that prize.

Since the prize selection is what’s determining the outcome, only one RNG call is needed, since the reels are effectively “for entertainment purposes only.”

This may sound familiar if you are hearing about things like Bingo machines or Historic Horse Racing games, where the wins/prizes are determined outside the game and the game only shows the outcome. But in a Vegas-style slot designed in this manner, it still relies on that RNG inside the slot to determine the outcome, but it works in reverse order to that “reels first” approach.

So, it ultimately comes down to the game’s design. In the end the math is designed to pay back a certain amount over time, have a certain hit frequency, and so on. Just how it accomplishes it from a design perspective may differ.

Michael Shackleford, also known as The Wizard of Odds, was interviewed as part of an Australian special a number of years back, and he talks about how the RNG worked to select a reel space for each reel. Hear his explanation starting at 18:28 on this video:

About the author


My name is Joshua, and I’m a slot enthusiast who works in tech as a marketer by day, and dabbles in casinos periodically during off-times. Know Your Slots will reflect my interests in understanding the various ways you can play slots, travel, casino promotions and how you can get the most out of your casino visits.

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