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How Long Does It Take for a Slot Machine to Hit Its Long-Term Payback?

14 March 2011

By John Robison

Hi John,

I always read about the various payout percentages on machines, some as high as 99%, others as low as 85%. I've been to Mohegan Sun (terrible results in numerous trips, little action) and many of the AC casinos. I don't seem to find machines that show the percentage. Also, over what time period does a machine hit that percentage: a day, a week, a month?

I also find the Random Numerator Generator confusing. Supposedly it grinds out continuously, even when not in use. So if someone hits it big, supposedly the very next spin has the exact same chance to yield yet another big hit. How then does that fact play into the percentage a machine pays out? If each spin is controlled by the RNG, and the machine is programmed to only pay out x amount, it seems contradictory, unless of course that ultimate payout of say 85% is viewed over a very extended period of time.

I'd love to hear some reasoning on this.


Dear Tom,

Time on a slot machine is measured in spins, not ticks of a clock. A slot machine's actual payout percentage gets closer and closer to it's long-term payback percentage as it gets more play. A machine doesn't hit its long-term payback percentage after a certain number of months, but we can be pretty sure that it is very close after a few million plays.

As for the Random Number Generator (RNG), consider this analogy to how a slot machine works. Let's say a dollar slot machine has 32,768 possible combinations. That's 32 stops on each virtual reel, a not uncommon situation on slot machines.

Now let's say that we have 32,768 index cards, one for each possible combination. Written on each card is a combinaiton of symbols and the amount of money that is won when that combination lands on the payline. Most card have $0 written on them, because most combinations pay nothing. Many cards have small amounts, like $1, $2, $5 and $10. Just a very few cards have the jackpot amount written on them.

When you play a spin on this slot machine, we need to pick an index card to see how much you won. We get a number from the RNG, which teels us the number of the index card that will tell us the result of this spin. The reels spin to the combination on the card and you win the amount written on the card.

If we want to raise the long-term payback on this machine, we change the combinations on some cards to higher-paying combinations, perhaps changing some of the $0 cards to have winning combinations.

In summary, the long-term payback is controlled by the numbers written on each card. The RNG is just the mechanism that tells us which card to pick to find the result of a spin. The RNG has nothing to do with a machine's long-term payback.

Best of luck in and out of the casinos,

Send your slot and video poker questions to John Robison, Slot Expert, at Because of the volume of mail I receive, I regret that I can't reply to every question.

John Robison
John Robison is an expert on slot machines and how to play them. John is a slot and video poker columnist and has written for many of gaming’s leading publications. He holds a master's degree in computer science from the prestigious Stevens Institute of Technology.

You may hear John give his slot and video poker tips live on The Good Times Show, hosted by Rudi Schiffer and Mike Schiffer, which is broadcast from Memphis on KXIQ 1180AM Friday afternoon from from 2PM to 5PM Central Time. John is on the show from 4:30 to 5. You can listen to archives of the show on the web anytime.

Books by John Robison:

The Slot Expert's Guide to Playing Slots