How online slots work
An online slot is a random number generator with a paytable attached. Understanding the two takes the mystery, and a few myths, out of the game.
By Ryan Jessop. Last checked .
Key takeaways
- Every spin is decided by a random number generator.
- Reels are a display of a result already drawn.
- Paytables and symbol weighting set the RTP and volatility.
- No pattern, timing or system can predict the next spin.
The random number generator
A slot's software produces a continuous stream of random numbers. When you press spin, the current numbers are taken and mapped to a position on each reel. The outcome exists from that instant; the spinning animation is presentation. The next spin uses fresh numbers with no memory of the last.
Virtual reels and weighting
Each reel is a long virtual strip of symbols. Low-paying symbols appear many times on the strip and high-paying ones few times, which is how a developer controls how often each combination lands. This weighting, together with the paytable, fixes the game's RTP and volatility.
How wins are counted
| System | How a win forms | Reviewed example |
|---|---|---|
| Paylines | Matching symbols on a set line, usually left to right | Big Bass Bonanza, Book of Dead |
| Pay both ways | As paylines, counted from either edge | Starburst |
| Pays anywhere | A set number of matching symbols anywhere on the grid | Gates of Olympus, Sweet Bonanza |
Special symbols
Wilds
Substitute for other symbols. Some expand, as in Starburst, or double wins, as in Fluffy Favourites.
Scatters
Pay or trigger features wherever they land. In Book of Dead one symbol is both wild and scatter.
Multipliers
Multiply a win. Gates of Olympus drops them on any spin.
Features
Each is covered with examples in slot features explained; unfamiliar words are in the slot glossary.
Free spins are the commonest: a set of spins at no cost, usually with an enhancement such as an expanding symbol or growing multiplier. Tumbles remove winning symbols so more can fall. In high volatility games most of the return is paid through features, not the base game.
How fairness is checked
Games offered by UK-licensed casinos must be tested by an approved test house before release, and operators must monitor live performance against the expected return. The rules that apply are in our UK slot rules guide.
Myths the maths rules out
- Hot and cold machines. Past spins do not affect future ones.
- Lucky times of day. The generator does not know the time.
- Stopping the reels. The result is set before they move, and UK rules ban manual stops.
- Stake size changing the odds. A bigger stake scales the wins, not the chances.
- Systems. No staking pattern alters the house edge.
To see these ideas in real games, start with the slot library.
Play safely
Slots are fast, and they are designed so the house wins over time. Decide what the session is worth to you before you open a game, set a deposit limit, and stop when the money is gone. Free, confidential help is available from the National Gambling Helpline on 0808 8020 133. Our safer gambling page lists the tools and services.
Frequently asked questions
Are online slots random?
Yes. Each spin is decided by a random number generator.
Can a slot be due a win?
No. Spins are independent.
Who tests online slots?
Approved independent test houses, before games can be offered by UK-licensed casinos.
Does my stake affect my chances?
No. It scales the size of wins, not their likelihood.