How slot machines work
aka, How a specific payback is achieved
“Michael Bluejay's comprehensive explanation of how slot machines work [is], in my opinion, the best one out there.” —Gaming the Odds
NOTES:
(1) This page covers normal slot machines (aka "Class
III"). Many Native American casinos instead use "Class
II" slots based on bingo or the lottery because local laws don't
allow regular slots. Class II machines look pretty much the
same on the outside as regular slots, and you still get a random
result, the machine just arrives at that random result a bit
differently from what's described below.
(2) This page covers traditional slot machines.
The new skillbased slots are covered on a separate
page.
No popups, no download, no registration, no B.S., just the game. One click and you're in.
Slots are random
Before you see how slots work, you simply have to understand that the outcome of each spin is random. This is a pretty easy concept, but many people just refuse to believe it. If you're not convinced that slots are random, then see my article on how slot machines are random first, then come back here. Don't worry, I'll wait.
Picking the symbols
On a slot machine, a random number generator (RNG) picks a random number for each reel, which each number matching a stop on its reel. Then the machine directs the reels to stop on the spots selected by the RNG.
Note that by the time the reels are spinning, the game is already over. The RNG has already selected the stops, and the reels spin sort of as a courtesy to the player. Slot machines don't even need visible reels—you could just put your money in and the machine could tell you whether you how much (if any) you won. Wrap your head around that one for a minute. The presence of the visible reels makes no difference in the game—they're just there to show you what the computer already picked.
How the stops are selected
A typical nonprogressive video slot has dozens of stops
per reel. An electromechanical slot
uses an (invisible) "virtual reel" of 64 to 256 stops, which are
mapped to the 22 stops on the physical reel. The physical reel
isn't big enough to hold all the stops that are needed, so it's the
big one that's used in the computer program. (example
source)
If you saw a worker open up an electromechanical slot machine you might see a reel like the one on the right, if it were unfolded. There are various symbols spread across 22 stops. Yes, the blanks count as stops. You might think that since there are 11 blanks you have a 50% chance of hitting one, and since there's only one jackpot symbol you have a 1in22 chance of getting it. But it doesn't work that way, because we're not really working with a 22stop reel. We're really working with an invisible reel of like 128 or so stops, controlled by the computer. The computer will pick a number from 1128, each of which is mapped to a specific symbol. Here's a hypothetical map for the reel shown at right:


of symbols 





















Say the computer picks #53. That's a blank, and it tells the reel to stop on a blank. If it picks #75, then it tells the reel to stop on a cherry. If it picks #127, then the reel tops on the jackpot symbol.
Most of the numbers are for the lowerpaying symbols, so that's what's more likely to get chosen. That's what we mean when we say the reel is weighted. Some symbols are more likely to be chosen than others, even if they appear the same number of times on the physical reel.
So you don't really have a 1 in 22 chance of hitting the jackpot symbol on this reel. Your odds are actually 2 in 128, or 1 in 64.
And of course, the most likely symbol is a blank. On our sample machine, you have a 73 in 128 chance (57%) of drawing one of those.
Speaking of blanks, when the computer picks a blank, it actually picks a specific blank. Same for the other symbols that appear on the reel multiple times, like cherries and certain bars. The table above was simplified to make things easier to understand, but now that we've come this far, let's now look at how every single position on the reel might be weighted.

Symbol 
Number 
of Chances 
























































































The fourth column (Number of Chances) shows the weighting. We've got a 2 in 128 chance of landing on the first stop (a cherry), and an 8 in 127 chance of hitting stop #5, the Red 7. Notice how the blanks surrounding the Jackpot symbol, #20 and #22, are heavily weighted. They're more likely to be selected, resulting in the "nearmiss" effect. You think you just almost got the jackpot symbol, but it's really an illusion. You weren't close at all. It's like the blanks above and below the jackpot have little magnets on them.
So far we've talked about only one reel, though most slots have three or five, and each reel is actually weighted differently. As you go from reel to reel the weighting gets heavier, so you're more likely to hit higher paying symbols early on. By the third reel the higherpaying symbols are even less likely. This results in another kind of nearmiss effect: How many times have you gotten JACKPOT, then another JACKPOT, and then... a blank? After the first two hits you're holding your breath for the third reel, but in reality your odds are poorer for getting that third jackpot symbol than they were for getting either of the first two symbols. However, for the rest of this discussion, we're going to assume that each reel is in fact identical in order to make the math easier.
Hitting the jackpot
So now that we know the weighting of the reels, we can answer
that elusive question: What are the odds of hitting the jackpot?
Here's the answer. Assuming we have three identical reels as listed
above, then the odds of getting the jackpot symbol on any reel is
2/128. The probability of hitting the jackpot on all three reels is
2/128 x 2/128 x 2/128 = 1 in 262,144. (If you played fast at 800
spins for 8 hours a day, you'd hit the jackpot on average once every
41 days.) This in fact is the odds of hitting the jackpot on
Red White & Blue. (See more on jackpot
odds.)
Calculating the payback
Now that we know the weighting of the reels, we can calculate the payback for this machine, which the percentage of money the machine would pay back over an infinite number of spins. Of course you can't play for an infinite amount of time, but the point is, the longer you play, the closer your return will come to what the payback suggests.
Our slot has the following paytable.






















To find the payback of the machine, we multiply the probability of each winning hit times the payout for that hit, then add them all up, as shown in the following table. I included a "How Calculated" column if you're interested in seeing how I derived the probabilities. The numbers I use there came from the first table, above ("Total no. of symbols" column).




How calculated 




2/128 x 2/128 x 2/128 




8/128 x 8/128 x 8/128 




11/128 x 11/128 x 11/128 




13/128 x 13/128 x 13/128 




16/128 x 16/128 x 16/128 




(16+13+11)/128 x (16+13+11)/128 x (16+13+11)/128 




5/128 x 5/128 x 5/128 




((5/128)x(5/128)x(1285)/128)x3 (prob. 1st reel x prob. 2nd reel x prob. NOT 3rd reel; then multiply all by 3, to account for the 2 cherries appearing in any of 3 different positions  1,2 or 2,3 or 1,3) 




(5/128x(1285)/128x(1285)/128)*3 prob. 1st reel x prob. NOT 2nd reel x prob. NOT 3rd reel; then multiply all by 3, to account for our single cherry appearing on any one of the three reels 


Total 

So this is a 96.3% machine, meaning that if you played it forever, you'd get back 96.3¢ for every $1 you put into it. Of course you can't play it forever, and in the shortterm anything can happen, but the longer you player, closer your return will come to 96.3%—meaning you will have lost 3.7% of all the money you bet.
Of interest is that the small payouts account for most of the payback. The single cherry alone provides nearly a third of all the money you get back from the machine. Same for "any bar / any bar / any bar". The jackpot itself comprises less than 1% of the total payback.
Note that some figures are not exact due to rounding.
The RNG is constantly picking numbers
The RNG is always working, even when you're not playing, picking
hundreds of random numbers per second.
The reason the machine constantly picks numbers is so that no one can discern any pattern in the numberpicking process and therefore predict a winner. It's extremely unlikely that anyone could do so even if the RNG didn't keep picking random numbers all the time, because the number of random numbers in a complete cycle is astronomical, but having the RNG pick numbers all the time removes even the fantastically remote possibility that anyone could predict the outcome.
Par sheets
Slot makers create a "Par sheet" for each slot which lists the reel symbols and the paytable. From this the payback can be calculated, and a programmer can write the computer code for the slot. This data is similar to the tables I provided above for my fictional slot. I have a separate page about par sheets, along with several actual examples.
Near misses
Earlier we saw how the symbols on electromechanical slots are weighted. There are only 11 blanks on the physical reel, but chances the RNG will pick a blank is much higher than 1 in 11. In fact, it will favor the blanks immediately above and below the jackpot symbol. Hitting these blanks gives players the illusion that they almost landed the jackpot symbol, because the jackpot symbol is physically close to the payline. But it's not mathematically close. In reality, the player wasn't close to landing the jackpot symbol on the payline at all.
As you might expect, research shows that the nearmiss effect
keeps players playing longer.
The Wizard of Odds cites an unnamed source who said that Nevada
regulations say that a stop on a reel can't be weighted more than
six times more than either stop next to it.
Video slots show the actual reels rather than virtual reels. As such, the kind of nearmiss described above won't artificially appear on video slots. (In theory, there might be some video slots that use virtual reels, but I haven't seen any evidence of this.) However, video slots use another method to make a nearmiss effect: they put fewer jackpot symbols on the 4th and 5th reels vs. the 1st, 2nd, and 3rd reels. When players line up the symbol on the first three reels they'll feel they were close to getting a 4th and maybe 5th symbol, but the reality is that it's much harder to get those righthand symbols.
In 1988, it was discovered that certain machines were using a
different, illegal kind of nearmiss technology. The
slot would first choose the stops randomly, and if it was a losing
combination, rather than showing the actual combination selected, it
would choose another combination to display, which was more likely
to show jackpot symbols just above or below the payline.
Do I have free will in the bonus round?
In a bonus round where you can pick from multiple boxes which reveal a prize, players often wonder, "Does it matter which box I pick? Are the various prizes truly scattered among the boxes, or am I gonna get (say) 10 credits no matter which box I picked?"
For the answer to that we turn to the authority on these kinds of questions, the Wizard of Odds, who says: "Based on seeing par sheets and speaking to industry insiders I can confidently say that if the alternative choices are shown at the conclusion of a bonus round then the game is honest about them. In other words the prizes were randomly determined and what you see at the end is truly how they were hidden. However in games where the alternative choices are not shown the odds are likely similar to a prize wheel, with lower probabilities for the higher wins." (source)
Play slots online
I suggest you play something other than slots because the slot odds are so bad. You could also play online with fake money, because then it doesn't matter if you lose. A good casino for freeplay is Bovada, since it requires no download and no registration. (If you see a registration box, you can close it and continue without registering.) You can play with real money too, though I hope you won't (or at least won't bet more than you can comfortably afford to lose).
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All my slot machine articles
 Slot machine basics. How much it costs to play, how much you can win, expected loss, why they're a bad bet, why they're popular, how you can limit your losses, speed of play
 How to play slot machines
 Slot returns. How much they pay back.
 The Randomness Principle. Slots don't continually get looser and tighter as they're played. They don't have to.
 How they work. Explains the randomness principle, and runs through the math to show how a game returns a particular payback percentage. There's a companion page on Par sheets.
 Slot Machine Myths
 Slot Machine B.S. Wrong info that's published elsewhere.
 Strategies. Tips for increasing your chances of winning, and saving money.
 Slot Jackpots. Odds of hitting the jackpot, progressive jackpots, and other jackpot topics.
 SkillBased Slots. The scoop on the new games in which your results aren't entirely determined by chance.
 Slot Machine malfunctions. How and why slot machines screw up, causing players to think they've won the jackpot when they really haven't.
 Slot Machine Simulator. I programmed an exact replica of the Blazing 7s slot (oddswise). Click it to play thousands of spins in one second and see how you do.
 List of good Bovada slots. I spent a full day surveying Bovada's voluminous offerings and extracted only the few with nice, modern graphics and mobilecompatibility.