A modern slot decides every result with a random number generator (RNG), a piece of software that picks the outcome the instant you press spin. The reels you watch are only a display of that result, and the paytable decides what it is worth, set so the game keeps a small share of everything staked over time.
That is the whole machine in two sentences. The rest of this guide takes each part in turn, so you know what is happening when the reels stop and, just as usefully, what is not happening.
The four parts of every slot
Strip away the theme, the music and the animation, and every slot has the same four working parts.
| Part | What it does |
|---|---|
| Random number generator | Produces the numbers that decide where each reel lands |
| Reel strips (or a symbol map) | Translate those numbers into symbols on the screen |
| Paytable | Says which combinations pay and how much, as a multiple of your stake |
| Stake | The amount you bet per spin, which every prize is scaled from |
The house edge is not a separate part. It is a property of the other four working together: the mix of symbols on the reels, combined with the prices in the paytable, gives a long-run return a little under 100%.
Step one: the random number generator
When you press spin, the game asks its RNG for a fresh set of numbers. In an online slot this usually happens on the operator’s or supplier’s server, not on your phone or laptop. The numbers are produced by an algorithm that is tested to make sure each result is unpredictable and unrelated to the one before it.
The important consequence is independence. The game has no memory of the last spin, the last hour or the last player. A big win does not make the next spin less likely to pay, and a long run of losses does not make one more likely. Our guide to random number generators goes into how this works and how it is checked.
Step two: mapping numbers to symbols
The RNG produces numbers, not cherries. Each reel has a strip, a list of symbol positions, and the number chosen for that reel points to a position on it. The symbols at and around that position are what you see when the reel stops.
Many games use what is called a virtual reel: the strip the maths works from can be much longer than the handful of symbols you see, and some symbols appear on it more often than others. This idea goes back to a 1984 patent by Inge Telnaes, and it is why a slot can offer a top prize far rarer than a simple count of the visible symbols would suggest. The history of the slot covers how the machine got from mechanical reels to this point.
Newer layouts work the same way underneath, even when they look very different. A Megaways game picks how many symbols land on each reel as part of the result. A cluster pays grid fills each cell from the RNG. The display changes; the principle does not.
Step three: the reels are a show
Once the result is fixed, the game animates the reels so that they land on it. The spinning, the slowdown and the anticipation as the last reel turns are presentation. Nothing you do during the animation changes the outcome.
In Great Britain, the game design rules that apply to online slots from 31 October 2021 shape that presentation. Each spin must take at least 2.5 seconds, slam stops and turbo modes are banned, and the game must not celebrate a return that is equal to or less than your stake as if it were a win.
Step four: the paytable sets the value
The paytable lists every winning combination and what it pays. Prizes are usually shown as multiples of the stake, or as cash amounts that change when you change your stake.
It also explains the special symbols. A wild usually stands in for other symbols to complete a win. A scatter usually pays wherever it lands and often triggers a feature such as free spins. How wins are counted, whether along paylines or across ways to win, is set out there too, and our guide to paylines vs ways to win explains the difference.
Step five: the stake scales everything
Your stake is the total you bet per spin. In nearly every slot, changing it does not change the probability of any outcome. It only changes the size of the prizes, because they are paid as multiples of what you bet. A combination that pays 10 times stake pays £2 on a 20p spin and £10 on a £1 spin.
Online slot stakes in Great Britain are capped at £5 per spin for players aged 25 and over, and £2 per spin for 18 to 24 year olds. The stake limits page has the detail, and slot stakes explained covers coins, lines and bet levels.
Where the house edge comes from
Put the reel strips and the paytable together and you can calculate, exactly, what the game returns on average for every pound staked. That figure is the return to player, or RTP. The remainder is the house edge.
A worked example
Imagine a deliberately tiny, illustrative slot with only three possible results per spin:
| Result | Chance | Pays (× stake) | Contribution to return |
|---|---|---|---|
| Big win | 1 in 100 (0.01) | 20 | 0.01 × 20 = 0.20 |
| Small win | 38 in 100 (0.38) | 2 | 0.38 × 2 = 0.76 |
| No win | 61 in 100 (0.61) | 0 | 0 |
| Total | 1.00 | 0.96 |
Add up the contributions and the game returns 0.96 of every stake on average: an RTP of 96%, which is a house edge of 4%. Stake £1 a spin for 1,000 spins and the theoretical return is £960, an expected loss of £40.
Real slots have thousands of outcomes rather than three, but the arithmetic is identical. RTP explained goes further, and the house edge in numbers shows what it means per hour of play.
What a slot does not do
It helps to be clear about what is not going on, because a lot of slot folklore rests on these ideas.
- It does not get “due”. Each spin is independent, so a game that has not paid for a while is no closer to paying.
- It does not react to you. A certified game does not tighten after a win or loosen to keep you playing.
- It does not run hot and cold. Streaks happen in random results; they do not predict anything.
- It is not beatable by timing. You cannot press spin at a better moment, because you have no way of knowing what number the RNG will produce.
Each of these is taken apart in our guide to slot myths.
What this means for you
Knowing how a slot works will not change your results, and nothing will: the house edge applies to everyone. What it does is let you see the game for what it is, entertainment with a price, and judge that price. Set a budget before you start, use the limits your site offers, and treat money spent as spent.
The short version
- A random number generator decides each result the moment you press spin.
- The reels are an animation of that result, not the cause of it.
- The paytable sets what each combination pays, as a multiple of your stake.
- Changing your stake changes prize sizes, not your chances.
- The mix of symbols and prices gives an RTP below 100%; the gap is the house edge.
- Every spin is independent, so slots are never “due”.
Questions people ask
Do the reels on a slot actually spin to decide the result?
No. On any modern online or video slot the result is chosen by a random number generator the moment you press spin. The reel animation is a way of showing you that result, not the thing that produces it.
Can a slot know how much I have won or lost?
A properly certified slot does not adjust its outcomes to your balance or history. Each spin is drawn from the same set of probabilities, which is what independent testing is there to confirm.
Does a higher stake give better odds on a slot?
In nearly all games the probability of each outcome is the same at every stake; wins are simply paid as a multiple of what you bet. Read the game's rules screen in case a particular game says otherwise.
Why do slots pay back less than you put in?
Because the payouts are set so that, over a very large number of spins, the game returns a little less than it takes. That gap is the house edge, and it is how the game pays for itself.
Next guide: RTP Explained: What Return to Player Means in Slots