When you stake a spin, the game draws numbers from a random number generator and maps them onto positions on each reel strip. The symbols you see are a presentation of a result that has already been decided. Payouts come from the paytable, and how often each symbol appears on its strip decides how often each payout occurs. Features are triggered by the same draw.
On this page
- Order of events
- Numbers are drawn first, then scaled to reel positionsSource 1
- What sets the frequency of a win
- How many times a symbol appears on each reel stripSource 2
- Rule in Great Britain
- A customer must commit to each game cycle individuallySource 3
- Minimum spin time there
- 2.5 seconds from one game cycle to the nextSource 4
From stake to result
The visible sequence in an online slot is a stake, spinning reels, then a result. The underlying sequence is different.
- You commit a stake to a game cycle.
- The game requests numbers from a random number generator.
- Those numbers are scaled to positions on each reel strip.
- The game reads the symbols at those positions and checks them against the paytable.
- The reels animate to show the positions that were already chosen.
The technical standard for this in Great Britain requires that scaling "maintains the qualities as detailed", so the mapped positions are as evenly spread and unpredictable as the raw numbers1. How that generator is built and tested is covered in why past spins do not affect future ones.
The animation is a presentation layer. Stopping the reels early, if a game allows it, does not change anything, and in Great Britain the standards state plainly that a system "must not permit a customer to reduce the time until the result is presented"4.
Reel strips and symbol weighting
Each reel is a list of positions, and each position holds a symbol. That list is the reel strip. A symbol that appears many times on a strip is landed often; one that appears twice is landed rarely. This is what people mean by weighting.
Because the strips are usually longer than the three or five rows on screen, the on-screen grid is a window onto a much longer list. Counting symbols you can see tells you very little about the real frequencies.
Worked example: a published design
The Wizard of Odds publishes the complete maths for a teaching slot called Five-Line Australian Reels, stating that "This design is for educational purposes only"2. It is useful here precisely because every number is public.
The design uses 64 stops on each of three reels2. Symbol counts differ by reel: red sevens appear 13 times on reel one, 12 times on reel two and just twice on reel three2.
That single asymmetry does most of the work. A combination needing a red seven on the third reel is far rarer than one needing it on the first, even though the symbol looks equally common on screen.
The paytable then attaches a value to each combination, up to 2,500 coins for three flags on a one-coin bet2. Multiplying each combination probability by its payout and adding them together gives the total return, which the published summary puts at about 96.84% on single-coin play2.
That is the whole mechanism: frequency from the strips, value from the paytable, return from the two multiplied together.
The paytable is the contract
Everything a slot will ever pay is listed in its paytable and rules. Nothing else pays.
In Great Britain, an explanation of the rules "must be easily available to the customer before they commit to gamble", and information enabling a customer to judge their chances of winning must be available before they commit5.
That disclosure is where you find the return figure, the feature rules and any restriction on eligibility. The meaning of the return figure itself is covered in our RTP guide.
How features are triggered
Features are not bolted on after the result. They are outcomes of the same draw.
A typical trigger is a count of a particular symbol landing anywhere on the grid, which is why scatter symbols do not need to line up. Other games trigger on a symbol landing in a specific position, or on a separate random draw made alongside the reel result.
Published game pages show how varied this is. Hacksaw Gaming describes Wanted Dead or a Wild as a "5-reel, 5-row slot" with "15 Paylines", whose DuelReels feature is activated by landing VS symbols that expand into wild reels, alongside three separate bonus rounds including one where "All Wild symbols that land are sticky and can be part of any number of winning combinations again and again"6.
The important structural point is that the money paid by a feature is not extra. It comes out of the same total return. A game that pays a great deal inside its bonus round pays correspondingly less outside it, which is the link between features and the win pattern described in our volatility guide.
Design rules that shape how a slot plays
Where a market regulates game design, those rules change the product itself rather than the maths. Great Britain is the clearest published example.
| Rule | Requirement |
|---|---|
| Individual commitment | "The gambling system must require a customer to commit to each game cycle individually"3 |
| Minimum spin time | "It must be a minimum of 2.5 seconds from the time a game is started until the next game cycle can be commenced"4 |
| No turbo | The system "must not permit a customer to reduce the time until the result is presented"4 |
| No parallel games | The system "must not offer functionality which facilitates playing multiple games at the same time"4 |
| No celebrating a loss | The system "must not celebrate a return which is less than or equal to the total stake gambled"4 |
| Net position on screen | "All gaming sessions must clearly display a customer's net position" since the session started7 |
That last pair deserves attention. A spin that stakes £1 and returns 40p is a loss, and a game presenting it with the sound and animation of a win is describing it inaccurately. Watching the net position figure rather than the individual spin is the reliable way to read a session.
A session is not the long run
The published return is an average across an enormous number of plays. The Gambling Commission states that it is "an average achieved over a significant number of plays, not each time a game is played"8.
Two things follow.
First, a few hundred spins can finish far above or far below the published figure, and that is expected behaviour rather than evidence of anything.
Second, because every spin is independently drawn, the result of one spin carries no information about the next. Longer play does not correct a bad run; it simply increases total turnover, and expected cost rises with turnover.
Reading a slot before you play it
- Open the rules screen. Check the return figure in use, the feature rules and the maximum win.
- Note the volatility rating if the studio publishes one. Each studio sets its own scale, so treat it as that studio's own description.
- Look at where the return sits. A game with most of its return inside a rare feature behaves very differently from one paying steadily.
- Ignore the reel animation. It is drawn after the result is chosen.
You can compare published figures across titles in the KiraBet slots section and by studio in the provider directory.
- Random number generator (RNG)
- Hardware or software that generates the random numbers used to decide game outcomes. In regulated markets, RNG output is tested to show that it is statistically random and unpredictable.
- Paylines
- Fixed patterns across a slot's reels along which matching symbols must land to form a win. The number of paylines, and whether they can be changed, is set by the game's rules.
- Scatter
- A slot symbol that pays or triggers a feature wherever it lands, rather than needing to appear on a payline. The game's rules state how many scatters are needed and what they trigger.
- Wild
- A slot symbol that substitutes for other symbols to help complete a winning combination. The game's rules list any symbols it cannot replace.
- Hit frequency
- How often a game produces a win of any size, expressed as a percentage of plays or as one win in a given number of plays. It is separate from RTP, and a counted win can be smaller than the stake.
Video slot 
Video slot 
Video slot Gambling involves financial risk, and every slot is designed to return less than is staked over time.