How Slots Work
An insider's guide to the inner workings of our favorite—and
most frustrating—games
by Bill Coleman
You’ve probably heard and read different explanations
of what goes on inside slot machines. Some of the issues are
quite complicated, and writers may oversimplify them; at the
same time, there are a multitude of myths out there that have
confused and misled players.
It never hurts to know the facts. So without getting too heavy
into the mathematics or technical details, here’s a basic
primer on how slot machines actually work.
Before we start, one note: The information in this article
does not apply to every machine in the country that looks
like a slot or video poker machine. While most slot and video
poker machines in the US work the way this article describes,
machines in some Indian casinos and some states controlled
by the state Lottery work differently.
This information is accurate for machines in Nevada, Atlantic
City, Mississippi and Indian casinos in many states. Most
states that have “Class III” gaming compacts have
machines that conform to the same standard. California and
Arizona are examples. States that have Class II gaming compacts,
such as Florida and North Carolina, have machines that work
differently. All regulated slot machines in the country are
random and fair; they just use different methods to choose
winning or losing results.
It’s also worth mentioning that Indian gaming and its
regulations are relatively new, and conditions change fairly
frequently. Today there are 28 states with Native American
casinos. More appear to be on the way.
A brief history of slot machines
In the beginning, machines were strictly mechanical. Gears
whirled when the handle was pulled, and the reels spun and
came to a halt. Each symbol on the reel had the same chance
of appearing as any other. This is the way slots operated
until the late 1980’s when computers began deciding where
the reels would stop. For the past 15 years or so, the spinning
reels on a slot machine have been just for show. The actual
result is selected by a computer program called a Random Number
Generator, or RNG for short.
With mechanical slots, each symbol had an equal chance of
appearing in the payline. There were only so many symbols
(including blanks) that could fit on a reel. The majority
of reels had 20 or 22 symbols, or “stops.” The term
“stop” was derived from the fact that the reel had
to stop in one of 20 or 22 positions to show that symbol clearly
on the payline. If there were 20 stops and 3 reels with 1
jackpot symbol on each reel, the chances of lining up three
symbols for a jackpot were 1 in 8,000.
With these machines, the smaller the chance of hitting a top
jackpot, the higher the jackpot could be. If a jackpot hit
every 8,000 spins on average, on a 3-reel machine that accepted
up to 5 coins, then the biggest payoff had to be less than
40,000 coins (5 coins X 8,000 spins). Otherwise, the machine
wouldn’t make money over time. If a machine was designed
to give smaller, more frequent payouts, that meant fewer coins
were available for the top jackpot.
Then, in the 1970’s and 1980’s, game designers started
to add more reels. With more reels there was less chance of
hitting the big jackpot, so they were able to make the top
prize larger. If a game had 20 stops and 5 reels, the chance
of hitting the big one dropped to 1 in 3,200,000. Adding more
reels made the games themselves much larger. Casinos would
often place these huge machines, which contained 5, 7 and
even 8 reels, right inside their doors. Players loved taking
a shot at a prize that was often the size of a state lottery
jackpot. But the machines were expensive, took up a lot of
space, and were awkward to operate.
Then came the arrival of computer technology, the slot industry’s
watershed event. An invention called the “virtual reel”
freed up the number of possible outcomes from the physical
limitations of the reels. The number of symbols you could
fit on a reel was no longer a concern.
A game could now be programmed so that the odds of hitting
a jackpot were, for instance, five million to one. If the
Random Number Generator selected numbers for each reel that
corresponded to the jackpot symbols, the reels would line
up with those symbols and you’d win. Though the games
basically looked the same as before, with three reels that
spun and stopped, virtual reels allowed for gigantic, multi-million
dollar jackpots. (The newer “Australian-style” video
slots with 5 video reels are the latest advance in this type
of machine. Everything from here on applies equally well to
video as well as reel slots.)
Virtual Reels
Essentially, the virtual reel creates an imaginary reel inside
the computer’s brain. Because it’s imaginary, it
can have as many stops as the programmer wishes it to have.
The machine you played last night with the million-dollar
top jackpot might contain 3 imaginary reels, each with 147
virtual stops. Let’s say one of those 147 stops contains
a jackpot symbol, and 20 stops are cherries. The physical
reel in the machine might only have two cherry symbols. But
anytime the RNG picks a number that corresponds to a cherry,
the physical reels will stop on one of the two cherries. And
it will always stop on the symbol that is “mapped”
to the number selected, never on the “other” one.
Each physical stop is assigned to a virtual stop.
If a machine has 147 virtual stops on each reel then there
are 3,176,523 different combinations. The RNG will pick 3
numbers. In this case, each number is between 1 and 147. The
first number maps to the position of the first physical reel,
the second to the middle reel and the last to the third reel.
Just imagine the RNG spinning three imaginary reels, each
of which stops either on a blank space or a symbol. The computer
then determines the physical stops that each virtual stop
maps to, directs the machine to spin the reels, and has the
reels stop in those positions. (The result of your spin is
determined well in advance of the reels actually stopping.)
Finally, the computer checks to see if the combination pays
anything and, if so, how much.
Payback percentage (theoretical return) of reel slots
We’ve all seen neon signs like “This bank pays 99%”
over a bank of machines. Although you might assume it’s
a wild exaggeration, these numbers are in fact truthful. But
that’s not to say they aren’t misleading. You see,
slot machines are programmed to return a certain percent of
the money wagered over an infinite number of plays. The key
phrase is “over an infinite number of plays.” In
reality, after a few million pulls the returns will be close
to the theoretical return (but not exactly that amount).
The more you play, the more this
payback percentage matters. If you play slot machines only
a few times a year, you’re not going to see these long-term
results. But if you play several times a week, you will. Over
time, your bankroll will go further when you play a higher
return machine.
Here’s another important point. In the short run, the
machine’s hit frequency will probably mean more to your
play than its payback percentage. High hit frequency machines
generally give back lots of small pays (often less than your
bet). This allows you to keep playing for a longer time. Low
hit frequency machines usually pay off less often, but in
larger amounts—so unless you still manage to hit a lot
of small jackpots, or one larger jackpot, your bankroll won’t
last as long.
To truly understand how a machine works, you first need to
understand what the payback percentage is and how it’s
calculated. In order to illustrate this, let’s return
to the days of mechanical slot machines—when each symbol
on each reel had exactly the same probability of appearing
on the payline as any other symbol.
Let’s say our old-school mechanical machine has only
one reel, with ten stops. Five of those stops are blank. One
stop has the jackpot symbol, a pineapple. One has an orange,
one has a plum, one has a watermelon and one has a cherry.
These symbols, and how much they pay if they appear on the
payline, are as follows:
| Stop |
Symbol |
Pays |
|
1
|
Pineapple
|
8
|
|
2
|
|
|
|
3
|
Orange
|
4
|
|
4
|
|
|
|
5
|
Plum
|
4
|
|
6
|
|
|
|
7
|
Watermelon
|
2
|
|
8
|
|
|
|
9
|
Cherry
|
1
|
|
10
|
|
|
If you play 10 times and get each stop once, there will be
19 coins returned. This is the machine’s “cycle.”
This is a term used to describe every possible combination
of symbols on the reels. It does NOT mean that the machine
is programmed to display each combination of symbols once,
and then start over. It’s a mathematical fiction that
enables us to calculate the payback percentage.
Let’s say it takes two coins to play. To play a cycle,
you would pull the handle 10 times. At 2 coins per, you would
wager 20 coins. As we saw earlier, the cycle pays 19 coins.
So 19 / 20 = 95%. Our machine has a theoretical return of
95%. All slot machines can be analyzed exactly the same way.
With virtual reels and millions of possible combinations on
the imaginary reels, it takes a computer to make the calculation—but
it’s done exactly the same way.
How can you be sure that machines pay back exactly what they’re
supposed to? Well, in reality, they don’t. Ever. Over
many millions of pulls they will return very close to the
payback amount, but it’s a theoretical return, not an
actual one.
Now we’ll make our machine virtual. Its computer has
an imaginary reel with 100 stops. If we wanted it to pay back
98%, we’d calculate that a cycle (100 spins at $2 per)
would cost 200 coins. 98% of 200 is 196. So we’ll make
sure that the cycle returns 196 coins. We could give the top
jackpot a large share of this, leaving fewer coins for smaller
pays, or we could do the opposite. It depends on how we want
our machine to play. If we add more reels and more stops,
we have the kind of machine most players enjoy at their local
casino.
Bonus Rounds
By now, you may be wondering how bonus rounds fit in. Just
think of the bonus round activator as another winning combination.
The bonus round has a fixed number of possible outcomes and
a known probability of each one happening. Some bonus rounds
are predetermined; others depend on the choices the player
makes.
Let’s examine a predetermined bonus round. Say it has
two possible outcomes: you’re either going to win five
coins, or 25 coins. The RNG is programmed to pick a number
between 1 and 10. If it picks 7, then 25 coins are won; any
other number results in the 5 coin win. When the bonus cycle
is complete, the total is 70 coins. Since there are 10 possible
outcomes, the average is 70 / 10 or 7 coins.
Adding a bonus to a traditional machine might make you think
you stand to win more money, but in reality it makes no difference.
Other pays are reduced, or occur less often, to make up for
the cost of the bonus. Bonus rounds add fun and excitement
to slots, but they don’t make them pay out more. The
theoretical return is set when the machine is first programmed,
and you’ll get the programmed payback, more or less.
If you’re lucky, you’ll make a profit; if you are
unlucky, your credits will disappear fast. But hopefully you’ll
have fun, at least.
Conclusion
Now that you know how slot machines work, you can see how
silly it is to think that a machines is “due,” or
“hot” or “cold.” You can hit a jackpot
right after being paid for one. Every spin is a separate event;
each is completely random. The chances of winning in the short
term is left strictly to chance. Some people can play a machine
that returns 85% and go home with a fistful of money, while
others go broke playing a 99% game. It’s called gambling.
So go out and have fun, and don’t bother trying to explain
to the player next to you how the machines really work. They’d
never believe you.
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