A slot machine picks each outcome with a random number generator at the instant you press spin, and the paytable decides how much that outcome pays. The reels are only a display. The casino's edge comes from how the paytable is built, which is why the machine loses to the house on average no matter how or when you play it.
The random number generator
Inside the machine, a program produces numbers continuously, thousands of times a second. Pressing spin takes whichever number is current and maps it to a set of reel positions. Because the numbers run whether or not anyone is playing, the timing of your press, the size of your bet and what happened on the last spin do not feed back into the next outcome.
The paytable and RTP
The paytable lists what each symbol combination pays. Add up every outcome weighted by how often it occurs and you get the game's RTP (return to player): the long-run share of money wagered that comes back. Casino slots commonly sit below 100%, so the machine keeps the difference. An RTP of 90% means about $10 kept per $100 wagered, as an average over a very large number of spins.
Volatility
RTP tells you the average cost. Volatility tells you how that average arrives. A low-volatility game returns small amounts often, so a session drifts down slowly. A high-volatility game returns nothing on most spins and pays in large pieces now and then, so a session often ends well under the average and occasionally far over it. See slot bankroll and variance for how to size a loss limit around that.
Why "hot" and "due" are myths
If every spin is an independent draw, a machine cannot be due. A losing streak is not a debt the machine owes you, and a machine that just paid is not drained. Changing machines, raising the bet after a loss or waiting for someone else to leave changes nothing about the odds of the spin you are about to make.
The machines that do carry state
A small group of machines do keep something between players: a must-hit-by progressive meter that has climbed toward its ceiling, coins banked toward a feature, a bonus that builds with play. The individual spin is still random. What changes is how much the spin is worth, because the prize or the feature is closer than at a reset. That is the whole basis of advantage play, covered in can you beat slot machines? and, for meters, must-hit-by progressives explained.
What this does not mean
Understanding the machine does not make an ordinary slot beatable, and even a favorable spot is an average over many sessions that any one session can lose. Only play with money you can afford to lose and set the limit before you sit down. If gambling stops being fun, call 1-800-GAMBLER.
Where to look next
- Can you beat slot machines? What works, what doesn't, and why.
- Calculators for the expected return and break-even of specific games.
- All 360+ machine guides.
Frequently asked questions
How do slot machines work?
A random number generator (RNG) picks the outcome the instant you press spin, and the reels display that result. The payout rules in the paytable and the game's return (RTP) are set by the manufacturer and the casino, so the long-run cost is fixed while any single spin is random.
What does RTP mean on a slot machine?
RTP, return to player, is the long-run share of money wagered that a game pays back. A 90% game keeps about $10 of every $100 wagered over a very large number of spins. It says nothing about what your next spin or your next session will do.
What is slot machine volatility?
Volatility, also called variance, describes how the return arrives. A low-volatility game pays small amounts often. A high-volatility game pays rarely and in larger pieces. Two games with the same RTP can feel very different to play.
Can a slot machine be hot or cold?
No. On a standard machine each spin is independent, so a long dry streak does not make a payout more likely and a recent jackpot does not make one less likely. The exception is a machine that carries visible state between players, such as a progressive meter near its must-hit amount, which changes what a spin is worth but not the odds of any single spin.
Are slot machines in a casino rigged?
They are regulated, and in Nevada the rules for payback and testing are set by the state. The casino's edge is built into the paytable and is the reason the house wins on average. It is not a mechanism that reacts to you personally. You must be 21 or older to play.