How online slots work

The random number generator decides the result

Every licensed slot runs a random number generator (RNG). This is a program that produces a constant stream of numbers. When you press spin, the game takes the current number and maps it to a stop position on each reel.

The spinning animation is drawn afterwards. It is decoration, and it plays no part in the outcome. A slot that looks as if it "almost" landed a big win did not come close in any real sense, because the result was fixed the moment you pressed the button.

The same logic applies to every type of slot, whether it is a three-reel classic or a modern video slot with cascading symbols. For more on how the numbers are produced and checked, see RNG and game fairness.

Reels, rows and the grid

The grid is the set of symbols you see. A reel is a vertical column, and a row is a horizontal line across the columns. Many older games use 3 reels and 1 to 3 rows. A common video slot layout is 5 reels and 3 rows, which shows 15 symbols at once.

Each reel is built from a virtual strip, a list of symbols in a fixed order. The strip is usually longer than the part you see, and the game picks where each strip stops. The designer controls how many times each symbol appears on the strip, and this is one of the main ways the game's maths is set.

Some slots break the standard layout. Clusters, grids with 6 or 7 reels, and games where the number of rows changes on each spin all exist. Megaways slots are one well-known example of the last kind.

Paylines versus ways to win

A payline is a fixed path across the grid. If matching symbols land along that path, the game pays. A slot might have 10, 20 or 25 paylines, and you are normally betting on all of them at once. The total bet is the line bet multiplied by the number of active lines.

A ways slot drops fixed lines. Instead, matching symbols on adjacent reels, starting from the leftmost reel, form a win in any position. The number of ways is the product of the symbols shown on each reel. For example, a 5-reel grid with 3 rows on every reel gives 3 × 3 × 3 × 3 × 3 = 243 ways.

Neither format is better for the player in itself. A game can pay out the same long-run share of stakes under either system, because the payout is set by the symbol values and how often they appear.

Symbols and what they do

Most slots use three groups of symbols. Low-paying symbols, often card ranks or simple icons, appear often and pay small amounts. Higher-paying symbols follow the game's theme and appear less often.

Then there are special symbols:

• Wilds substitute for other symbols to complete a win, usually with the exception of scatters and bonus symbols.
• Scatters pay or trigger a feature based on how many land, often in any position instead of along a line.
• Bonus symbols open a separate mini-game or a round of free spins when enough of them appear.

The paytable, which you can open from the game's info or menu button, lists the value of each symbol and explains the special ones. Read it before you play a new game, because rules differ from title to title.

Features: free spins, multipliers and more

Features are rounds or rules layered on top of the base game. Free spins are the most common: a set number of spins played at the bet that triggered them, with no extra charge. Winnings from them may be subject to the casino's own terms if they came from a bonus offer. Our guide to free spins covers that side.

Other features include multipliers that raise a win, symbols that expand to fill a reel, respins, and pick-and-win mini-games. Some games let you pay a set multiple of your bet to jump straight into the bonus round. This is explained in bonus buy slots.

Features change how wins are spread out, not the odds in your favor. A game with a big feature usually pays less often during the base game to make room for it.

Why every spin is independent

A spin has no memory. The RNG does not know what happened on the previous spin, how long you have played or how much you have staked. A long run without a win does not raise the chance of a win next time, and a win does not make another one less likely.

What the game does have is a long-run payout figure, called RTP, and a pattern of how wins are spread, called volatility. Neither lets you predict the next result. Common beliefs about hot and cold games are covered in slot myths.

Slots are designed so that the house keeps a margin over time. Treat the money you play with as the cost of entertainment, set a limit before you start, and stop when you reach it. Help and tools are listed on our responsible gambling page.