How Slot Machines Work: Reels, RNGs and Payouts

A slot machine picks its symbols with a random number generator, and the pay table then turns those symbols into a payout. Nothing about the spin is decided by the reels you watch spinning: the result is chosen first, the reels are an animation that lands on it, and every spin is independent of the one before. The machine makes money because its prizes are set to return less than the total wagered, not because it schedules wins and losses. This guide uses Cashout Arcade's two slot games, Sun Temple (5 reels) and Slots (3 reels), to show the real numbers behind each of those statements.

Short answer

1. The random number generator draws each symbol independently with a fixed weight.
2. The pay table pays for certain combinations. Prizes are scaled so the total returns 97%.
3. Hit frequency (how often you win anything) and volatility (how uneven the wins are) are set by the weights and the pay table, and are separate from the return.
4. Machines are never due. The draw has no memory.

Reels, symbols and paylines

Sun Temple has 5 reels and 3 rows, which makes a 15-cell grid. It has three pay lines, and they are the three rows. A line pays when 3, 4 or 5 matching symbols appear in a row starting from the leftmost reel; two of the gem symbols (Turquoise and Carnelian) also pay for a pair. The bet covers all three lines at once.

The symbols, from most common to rarest, are the Lapis Gem, Turquoise Gem, Carnelian Gem, Golden Scarab, Ankh, Sun Amulet and Pharaoh Mask. Two special symbols sit outside that ladder:

• The Sun Disc is a wild that appears only on reels 2, 3 and 4. It replaces every symbol except the Temple Door, and carries a multiplier of 2x (60% of the time), 3x (30%) or 5x (10%). Multipliers on a winning line add up and multiply that line's pay.
• The Temple Door is a scatter. Three or more anywhere on the grid start a bonus.

The pay table is on the Odds page: for example, three Lapis Gems pay 0.5118x the bet, five Pharaoh Masks pay 153.5608x. Those numbers are the pay for a line, and a spin can have up to three lines at once.

The random number generator

When you press spin, the server (not your phone) draws a result for every one of the 15 cells. Each cell is an independent weighted draw, made with the same secure random source described in the RNG guide. Then it works out which lines pay, applies any wild multipliers, and sends the finished result to the app, which animates the reels onto it.

Two consequences follow from that design:

• The reel animation is decoration. The symbols the reels slow down past, or the way they stop, cannot change the result because it was fixed before the spin began. Skipping or speeding up the animation changes how long you wait, not what you get.
• Cells are independent. The symbol in one cell does not influence its neighbours. That is why the three rows of Sun Temple behave as three separate chances.

The UK Gambling Commission's technical standard for random outcomes (RTS 7) expects exactly this behaviour from a regulated slot: outcomes that follow their published rules and probabilities, and games that do not adapt their odds to a player's run of luck.

Virtual reel weights: why it is not 1-in-N per symbol

A common belief is that a reel with 20 symbols gives each symbol a 1-in-20 chance. Real slots are not built that way, and neither is Sun Temple.

Electronic slots store each reel as a list of weighted positions. A historic design, described in the prior art cited by US patent 5,380,008, keeps more "virtual" positions in memory than the reel has physical stops, and several virtual positions can map to one stop. Rare symbols get few virtual positions, common ones get many, so the picture of the reel tells you very little about the odds.

Sun Temple's weights are simple. Out of 1,000 on each cell:

Now see what the weights do to a headline combination. A Pharaoh Mask is 2% of every cell, which sounds reachable. But five in a row from the left needs a mask (or a wild standing in) on all five reels of one line: roughly 0.02 x 0.065 x 0.065 x 0.065 x 0.02, or about 1 in 9.1 million per line and 1 in 3.0 million per spin across the three lines. Compare the same calculation for the most common symbol:

(Computed from the weights above with Sun Discs counted as matching on reels 2 to 4; the chance of three scatters, by the same method, is 1 in 106.7, which agrees with the 1 in 107 on the odds page.) The pay for five of a kind grows far faster than the chance falls. That is the pay table at work: the machine pays 153.56x for an event that happens about once in 3 million spins, and 5.12x for one that happens about once in 120.

Hit frequency, RTP and volatility

Three separate numbers describe how a slot behaves, and the RTP guide and volatility guide cover each in detail. Here is Sun Temple's published picture, from the Odds page:

Any win comes up 35.98% of the time (the hit frequency). A win bigger than the bet comes up 10.46% of the time, so most "wins" return less than you put in. The overall return is exactly 97%. The biggest result in the table is 12,044x, still under the game-wide cap of 50,000x per round.

The mismatch between 35.98% and 10.46% is why a slot can feel generous (lots of flashing, lots of small wins) while the balance quietly drains. Hit frequency counts any payout; only the payouts above the bet add to your balance.

A simpler machine, with the maths shown: Slots

Cashout Arcade's 3-reel Slots game has few enough combinations to show everything. At medium risk, each reel draws one of six symbols with these weights: Cherry 32, Lemon 26, Bell 18, Bar 12, Diamond 8, Seven 4 (out of 100). Three of a kind pays by symbol. Exactly two cherries anywhere pays 1.9539x. Here is how the 97% is built:

The small, frequent win (two cherries) delivers 42% of the return; the jackpot-looking Seven row delivers about 4%. Machines are tuned by moving weight between rows of this table. The "high" risk setting shifts weight toward the top: Three Sevens appears 1 in 37,037 spins and pays 2,964x, while a win of any kind is still around 30% of spins. The return stays 97%.

Bonus rounds and hold-and-spin

Sun Temple's feature is its bonus. Three or more Temple Doors, which happens about once in every 107 spins, start one of two bonuses chosen at random, half each:

• Temple Link is a hold-and-spin round. The doors lock in place as golden scarabs. Every empty cell then lands a scarab 8% of the time per respin, each new scarab resets the respins to 3, and the bonus ends when respins run out or all 15 cells are full. Scarabs carry cash values, and three jackpot values: Mini (20.4747x), Major (102.3738x) and Grand (1,023.7388x).
• Pharaoh's Offers shows you an amount and lets you take it or pass to see the next, up to four times; the last must be taken. Each offer is the one before multiplied by 0.3x, 0.6x, 1x, 1.6x or 2.84x with chances of 28%, 24%, 20%, 18% and 10%. Those chances average exactly 1x, so an offer is worth the same whether you take it or pass. Taking the first offer, passing every time, or anything in between gives the same average bonus.

Together the lines pay about 48.6% of the return and the two bonuses pay the rest, according to the odds page. The jackpot scarabs in Temple Link and the longest chains of rising offers in Pharaoh's Offers are what reach the top of the table.

Do machines get "due"?

No. A machine that has not paid for 200 spins is exactly as likely to pay on spin 201 as a machine that just paid. The weights are fixed, the draw has no memory, and nothing in the engine records "spins since last win". A cold machine is not charging up and a hot one is not about to stop. The streak maths, including how long a drought can run before it is even surprising, is in the gambler's fallacy guide.

The same reasoning removes other myths. Betting more does not change your odds per coin. Switching machines does not reset anything. Playing at a different time of day does nothing, because the game does not look at the clock or at your balance when it draws.

Free play and real money

Slots in free-to-play apps and real-money slots use the same ideas: weighted draws, pay tables, a built-in return. The differences are what you can lose and what is checked. In a regulated real-money market, the odds and the random generator are tested against standards like RTS 7. Free-to-play slots usually are not regulated as gambling, so whether the game publishes its return is up to the publisher. Cashout Arcade publishes every game's return, payout limit and rules on the Odds page, and the coins have no cash value. If you are choosing a free slot app, our guide to free slots lists what to check.

Related

• Sun Temple game page
• Slots game page
• House edge explained