How slot machines work: RNGs, weighted reels & why no slot is due

A plain guide to how a slot picks its result, how reel weighting sets the odds, & what Britain’s Gambling Commission requires of online games.

What happens when you press spin

A random number generator, or RNG, is the part of a slot that produces numbers with no predictable pattern. In its analysis of IGT’s three-reel Red White & Blue, Wizard of Odds says that once a bet is placed & spin is pressed, the machine selects three random numbers, one for each reel.

Those numbers come from an RNG “constantly drawing random numbers at a rate of thousands per second”. According to the same analysis, the numbers chosen at the moment play starts decide the final outcome: “the outcome is predestined the moment you press the spin button”.

The spinning reels are a display of a result already decided. PlaySmart, which covers OLG’s casinos & OLG.ca, puts it the same way: the RNG “constantly generates numbers that correspond to game symbols”, & the symbols are “locked in place the moment the spin button is tapped”.

Reels, stops & virtual reels

A stop is a position where a reel can come to rest. In Red White & Blue, Wizard of Odds says, “each random number has 64 equally likely outcomes”, & each number is matched to a stop through a lookup table, a chart that turns a number into a symbol on the reel.

The analysis gives an example: random numbers 26, 8 & 43 would produce Blue 7, Blank, Red 7. Several numbers can point to the same picture; stops 60 to 62 on reel 1 all lead to the same 1 bar symbol.

On the physical reel, “There are exactly 22 groups of like symbols on each reel”, which the analysis describes as the standard number of stops on an electro-mechanical three-reel machine, known as a stepper slot. Each reel’s 64 numbers are therefore directed to those 22 physical positions.

Symbol weighting & the real odds

Weighting means how many of the 64 numbers are given to each symbol. Wizard of Odds says the totals for Red White & Blue came from an information card on a machine in the Netherlands, where reel details are sometimes posted, & that its lookup table is an estimate reverse-engineered from those totals.

On reel 1, just one of the 64 positions maps to the red 7, the most valuable symbol. Blanks take 32 of the 64 positions on each of the three reels.

Multiplying the positions gives 64 x 64 x 64, or 262,144 possible results. The top award, red 7, white 7, blue 7, is one combination, a probability of 0.000004, or 1 in 262,144.

Weighting also explains other prizes. White 7s sit on 6, 1 & 7 positions across the three reels, so three white 7s can land in 6 x 1 x 7 = 42 ways. Of all 262,144 results, 216,672 pay nothing.

Return to player, or RTP, is the share of money staked that a game pays back on average over time. Wizard of Odds calculates 86.58% for this game at the one-coin level, meaning “for every dollar bet at the one coin level, the player will get back 86.58 cents, on average”, rising to 87.47% at three coins.

Paylines & ways to win

PlaySmart says paylines, sometimes called “betting lines” or “winning lines”, are the ways symbols can line up for a win. Lines can run straight, diagonally or in zigzags, & winning combinations “can vary from one to thousands”.

Many games let players choose how many lines to bet on, though PlaySmart notes some have fixed paylines, set in advance for every spin. It says picking more, fewer or different paylines “won’t increase your odds of winning”, while more lines mean “paying more for each spin”.

PlaySmart also warns about false wins, where frequent small payouts do not cover a bet spread across several lines. Its advice is to “Keep an eye on whether your total credits have gone up or down after each spin.”

Some games drop fixed lines altogether. According to Evolution, its studio Big Time Gaming, which it calls the “Creators of Megaways”, launched Bonanza in 2016 with reels whose symbol heights change on every spin, “fitting two to seven symbols per reel”.

Evolution says that “With up to 117,649 win ways, the system replaced fixed paylines”, & that Megaways is now “licensed to hundreds of developers worldwide”.

Near misses are losses

A near miss is a losing result where a top symbol lands just above or below the payline. In the Red White & Blue table, Wizard of Odds notes that “only stop 45 on reel 1 is mapped to the red 7 symbol”, while “the blanks above and below it have five positions each”.

That gives ten of reel 1’s 64 positions to the blanks directly beside the red 7, against one for the 7 itself. The analysis says this “causes the often seen near miss effect, where the reel stops directly above or below the highest paying symbol.”

A near miss pays only for whatever sits on the line. In Great Britain, the Gambling Commission’s technical standard RTS 7, last updated on 7 May 2024, bans designs that may mislead about the likelihood of results, “including substituting losing events with near-miss losing events”.

Its guidance adds: “Games may not falsely display near-miss results”, meaning a game may not swap one losing outcome for a different losing outcome.

Why a slot is never due

Because each result comes from the RNG at the moment play starts, a long run of losses does not make a win more likely. PlaySmart says: “Winning at slots is purely random. Machines are never overdue for a jackpot”.

British rules forbid games that balance themselves out. RTS 7 says “Adaptive behaviour (that is, a compensated game) is not permitted”, defined as “automatic or manual interventions that change the probabilities of game outcomes occurring during play”. Bonus features with different rules are still allowed if they are triggered by random events.

The standard also says random numbers “are to be used in the order in which they are received” & “may not be discarded due to adaptive behaviour”. Numbers may be discarded only if they fall outside the range a game needs, which should be logged & investigated.

Under RTS 7, a game’s rules, payouts & outcome probabilities may not be changed while it is available for gambling, except as its rules provide, & any such change must be brought to customers’ attention.

Testing & certification

RTS 7 requires that random number generation & game results be “acceptably random”, meaning it can be shown to a high degree of confidence, for example through accepted statistical tests, that outcomes are random. It adds that the mapping of random numbers to results “should be in accordance with prevailing probabilities, pay tables, etc.”

According to the Commission’s testing strategy, it “maintains and publishes a list of approved test houses authorised to perform third party testing”. A test house is an independent lab that checks games against the standards.

RNG testing includes “review of RNG documentation”, “review of source code to verify the implementation of RNG is in accordance with the RNG documentation” & “statistical testing of raw output of RNG and scaled or shuffled decks data”. Test houses also research the RNG for publicly known weaknesses.

Game testing checks the “maths, artwork or rules as displayed to players, and theoretical RTP”, along with actual RTP through simulation & manual testing. The strategy says “All new games and RNGs can only be released once the testing has been completed and the report provided to the Commission.”

Minor updates that do not affect fairness can be released without external retesting, but the Commission says these processes are subject to an annual audit by an approved test house.

British slot rules, limits & staying in control

The Commission’s stake limit guidance says online slots in Great Britain have a maximum stake per game cycle of £5 for customers aged 25 or over & £2 for those aged 18 to 24. A game cycle runs from starting a game until all money staked has been lost or all winnings paid out.

“The £5 limit for all adults went live 9 April 2025 and the £2 limit for adults aged 18 to 24 went live 21 May 2025.” The guidance says the limits apply to online slots only & “do not apply to other casino games, such as roulette or blackjack”.

Under RTS 14, last updated on 12 January 2026, there must be at least 2.5 seconds between starting a slot game & starting the next game cycle, & the player must release & press the start button again each time.

The same standard says “Features such as turbo, quick spin and slam stop are not permitted”, & “The gambling system must not celebrate a return which is less than or equal to the total stake gambled.” It also says products “must not actively encourage customers to chase their losses”.

On staying in control, PlaySmart says setting a spend limit is key to playing responsibly, & that online players can “set up a deposit limit to help stay on budget”.

Questions

Are slot machines random?

The result comes from a random number generator at the moment play starts, & the reels only display it. In Great Britain, the Gambling Commission’s RTS 7 requires results to be “acceptably random”, meaning it can be shown to a high degree of confidence that outcomes are random, for example through statistical analysis using generally accepted tests.

Can a slot be due to pay out?

No. PlaySmart says “Machines are never overdue for a jackpot”, & British rules ban compensated games that change the probabilities of outcomes during play.

Do more paylines improve my chances?

PlaySmart says picking more, fewer or different paylines won’t increase the odds of winning, though more lines mean paying more for each spin. Small wins may not cover a bet spread across several lines.

What is the maximum stake on online slots in Great Britain?

According to the Gambling Commission, the limit is £5 per game cycle for those aged 25 or over, live since 9 April 2025, & £2 for those aged 18 to 24, live since 21 May 2025. The limits do not apply to games such as roulette or blackjack.

What are Megaways slots?

Evolution says Big Time Gaming launched the Megaways mechanic with Bonanza in 2016, with two to seven symbols per reel changing each spin. It offers up to 117,649 ways to win in place of fixed paylines.

Sources

• gamblingcommission.gov.uk
• gamblingcommission.gov.uk
• gamblingcommission.gov.uk
• gamblingcommission.gov.uk
• wizardofodds.com
• playsmart.ca
• evolution.com

Sources checked 30 October 2026. Information only. 18+.