A slot machine picks its result with a random number generator the instant you press spin, and the reels you see are only a display of that result. Over a very large number of spins the machine returns a fixed percentage of the money wagered, called return to player or RTP. Regulators set minimums: Nevada requires at least 75% and New Jersey requires at least 83%, so the casino keeps at most 25% or 17% of every dollar in the long run.
This guide explains how the random number generator works, what RTP, hit frequency and volatility mean, and why no spin is “due”. It includes a small slot machine we built and solved exactly, so you can see where the percentage comes from. The regulatory figures come from the regulations themselves, and the calculations are our own.
How does a slot machine work?
A slot machine uses a random number generator, or RNG, to choose the outcome. The RNG runs constantly, producing numbers many times a second. When you press spin, the machine takes the current number and maps it to a set of reel positions, and the reels then display that outcome.
New Jersey’s slot machine approval standards state the principle: the RNG selects the outcome, and after that selection the equipment cannot make a variable secondary decision that changes the result shown. The standards also require the reel display to fairly represent the symbols the RNG selected.
Most modern machines do not have one stop per physical symbol. They use a longer list of virtual reel positions, and many positions can show the same symbol. That is how a game can show a rare jackpot symbol as a single position on the visible reel while giving it a different, much smaller chance in the underlying list.
Three consequences follow from that design:
- Every spin is independent. The machine has no memory of earlier spins.
- The display does not decide the result. The outcome is fixed before the reels stop.
- The chances are set by the game design, not by what the reels look like. A symbol that looks close to the payline is not “nearly” the chance it appears to be.
What do RTP, hit frequency and volatility mean?
These three numbers describe a slot’s maths, and each answers a different question.
- RTP (return to player): the share of the money wagered that the game pays back over a very long run. A 90% RTP game pays back $90 per $100 wagered on average, and the casino keeps $10.
- Hit frequency: how often a spin pays anything at all. A game can hit often with small wins or rarely with large ones.
- Volatility (variance): how widely results swing around the average. High volatility means long dry spells and occasional big wins. Low volatility means frequent small wins.
Two games with the same RTP can feel completely different, because the RTP says nothing about how the return is spread across wins.
What are the legal minimum RTPs?
Regulators set a floor on the theoretical return, and the floors differ widely. Nevada’s Regulation 14.040 requires gaming devices to pay out a mathematically demonstrable percentage of amounts wagered that must not be less than 75 percent, according to the Nevada Gaming Control Board. It also requires that all possible game outcomes be available on each play. New Jersey’s standard is higher: each game must have a theoretical RTP equal to or greater than 83 percent.
The table shows what those floors mean in money. We calculate the average loss on $1,000 of total wagers, assuming the game sits exactly at the minimum.
| RTP | Casino keeps | Average loss per $1,000 wagered |
|---|---|---|
| 75% (Nevada minimum) | 25% | $250 |
| 83% (New Jersey minimum) | 17% | $170 |
| 90.4% (our example game below) | 9.6% | $96 |
| 95% | 5% | $50 |
The regulations set the floor, not the actual figure on a particular machine. The actual RTP of a specific game is set by its design and may be published in its rules or by the regulator, depending on where you play.
How does a slot’s RTP come from its design? A worked example
We built a small three-reel slot to show exactly how the percentage arises. Each reel has 20 equally likely stops: 1 seven, 3 BARs, 4 bells, 4 cherries and 8 blanks. A play costs 1 coin and pays as follows:
| Result | Pays (coins) | Chance | Share of RTP |
|---|---|---|---|
| Three sevens | 500 | 1 in 8,000 | 6.25% |
| Three BARs | 100 | 1 in 296 | 33.75% |
| Three bells | 25 | 1 in 125 | 20.00% |
| Three cherries | 10 | 1 in 125 | 8.00% |
| Cherries on reels 1 and 2 only | 2 | 1 in 31 | 6.40% |
| Cherry on reel 1 only | 1 | 1 in 6.25 | 16.00% |
| Total | 21.15% of spins pay | 90.40% |
Adding the last column gives an RTP of 90.4%, so the casino keeps 9.6 coins per 100 wagered. The game pays something on 21.15% of spins, or about once in 4.7 spins, and most of those wins are 1 coin, which merely returns the stake.
Look at where the return sits. Only 6.25% of the RTP comes from the top prize, which arrives once in 8,000 spins on average. A player who sees a three-sevens win once in a few thousand spins is seeing the part of the return that is built from rare events, and a player who plays only a few hundred spins is likely to see none of it.
How big are the swings?
The swings are far larger than the average loss. In our toy game each spin has an average loss of 0.096 coins, but the standard deviation of a single spin is about 8.4 coins, because a few large prizes dominate the variation.
Over 1,000 spins, the average loss is 96 coins. The standard deviation of the total is 8.38 times the square root of 1,000, or about 265 coins. That means roughly two sessions in three finish between a loss of 361 coins and a gain of 169 coins, and the rest fall outside that range. A session can finish well ahead even though the machine keeps 9.6% in the long run.
This is why a short winning session says almost nothing about the game’s RTP, and why a long losing run does not mean the machine is “cold”.
Is there such a thing as a hot or due machine?
No. Each spin is selected independently by the RNG, so the last result has no effect on the next. A machine that has not paid a large prize for a long time is not more likely to pay one next, and a machine that just paid a jackpot is not less likely to pay one.
What can feel like a pattern is a feature of volatility. In a game where most of the return sits in rare large prizes, long losing runs are an expected feature, not a sign of anything wrong.
Near misses work the same way. The standards in New Jersey allow alternate spin animations only if they do not change how often the top symbols appear compared with the real reel layout, and only if the display is not misleading. A reel that stops one position above the jackpot symbol has not come “close” in any meaningful sense, because the outcome had already been selected before the reels moved. Our guide on how games hold your attention explains why near misses feel so compelling.
What are common slot machine mistakes?
Most mistakes come from misunderstanding how outcomes are chosen. The usual ones:
- Believing a machine is due. The RNG has no memory.
- Moving machines to “find a loose one”. The return is a long-run average, not a signal about the next spin.
- Treating a near miss as progress. The result was fixed at the moment you pressed spin.
- Ignoring the pace. More spins per hour means a larger total wager, and the average loss scales with it.
- Playing to get back a loss. The edge applies to every spin equally.
What does playing slots cost per hour?
The cost per hour is the amount you wager per hour times the house edge. If you play 600 spins an hour at $1 each, you wager $600 an hour. At the 75% Nevada minimum, the average loss is $150 an hour. At the 83% New Jersey minimum it is $102. At our example game’s 90.4% it is about $58. The 600 spins an hour is our own assumption, and your real pace may be faster or slower.
Pace is under your control. Choose a limit before you start, play with a fixed amount, and stop when it is gone. Our guide to setting a gambling budget and time limit shows how, and the Safer play toolkit lists tools and support. If gambling is causing harm, visit our get help page.
Slot machines are legal in some places and not in others, and the rules differ by state and country. Check is gambling legal where you live before you play.
Where should you go next?
To see how a fixed edge works on a different game, read roulette odds and payouts. For a game where decisions change the edge, see how to play blackjack. And for the idea behind all of these numbers, read probability and odds in plain language.