Online slots

An online slot looks like a machine with reels, but the result of a spin is decided by a piece of software, the "RNG" (random number generator), which produces a number when the stake is placed and maps it onto a position of each reel. Everything else on the screen (the sound, the near misses, the reels that slow down one after another) is presentation.

The arithmetic of the game sits in a few figures that the studio publishes and the casino displays in the game's own panel, and in this guide we read them one at a time, which is less tedious than it sounds. Our hypothetical player stakes $1 on each of 500 spins, because a figure such as 96 per cent says little until it is turned into dollars.

What the random number generator decides

The result is fixed once the stake is placed, before the reels begin to turn, and the animation that follows shows it without producing it. Two consequences follow: a slot has no memory, so a machine that has not paid for an hour is not "due" (the next spin has the same odds as the first of the evening), and a button that stops the reels early only shortens the display.

The Gambling Commission has forbidden such buttons on games offered in Great Britain ("turbo", "quick spin" and "slam stop" are named in requirement RTS 14E, which it added to its technical standards with the changes published on 2 February 2021), since a faster display means more spins an hour at the same odds. Whether the generator is fair cannot be seen from the screen; laboratories approved by the regulators check it, and the guide to RTP and volatility names some of them.

RTP is an average over millions of spins

The "RTP" (return to player) is the part of all the money staked on a game that comes back to the players across its whole life, as calculated by the studio from the paytable and the weights of the reels. A game at 96 per cent keeps on average four cents of every dollar wagered, which is what a table game calls its house edge, and it is the measure that we use throughout.

Back to the player with 500 spins at $1. The total wagered is $500, so at 96 per cent the expected return is $480, a loss of $20, while at 94 per cent the loss rises to $30. Two points of RTP, a difference that looks like a rounding, make the expected cost of the session half as large again!

The word "expected" carries the weight of that calculation, because over 500 spins the actual result will almost never be the expected one (a single bonus round can turn the loss into a profit, and a cold run can double it). The RTP describes millions of spins and says nothing about the next five hundred.

Volatility and hit frequency

Two games with the same RTP can behave very differently, since one hands back its 96 per cent in a stream of modest payments and the other in a handful of big ones. The first is said to have low "volatility" (the size of the swings around the average) and the second high volatility, and many panels now tell you which kind of game you are opening, sometimes on a scale.

Hacksaw Gaming, for instance, rates its slot "Wanted Dead or a Wild" at 4 out of 5 on its own game page, and some studios go further by publishing the "hit frequency" (the proportion of spins that pay anything at all). Nolimit City gives for "Mental" a hit frequency of 34.19 per cent, free spins on average once in 216 spins, and a maximum payout of 66,666 times the bet with a probability of one in 18 million, which is the kind of panel we would read first.

A hit is not a win, however. Many of the spins counted in a hit frequency return less than the stake (a $1 spin that pays back 40 cents is a hit and a loss). The Gambling Commission forbids British games to celebrate such returns with the sounds and lights that belong to a win (RTS 14F, from the same changes of February 2021).

Paylines, ways and Megaways

A classic video slot pays along "paylines" (fixed paths across the reels, fifteen on the Hacksaw game above), and a winning combination counts only if it sits on one of them, starting from the leftmost reel. A "ways" slot drops the lines and pays matching symbols on neighbouring reels in any row, which on a grid of five reels and three rows makes 243 ways (three multiplied by itself five times).

The "Megaways" mechanic, which Big Time Gaming created and offers to other studios under licence (as its own site states), changes the number of symbols on each reel at every spin, so the count of ways changes too. In our view lines and ways are the least informative figures on a slot; the studio builds the paytable around the RTP that it wants, and the layout only changes how the stake is spread across combinations.

Bonus buys, and the British warning

A "bonus buy" (also called "feature buy-in") lets the player pay a fixed multiple of the stake to enter the bonus round at once, instead of waiting for the scatter symbols to trigger it. The rules state the price and, on many titles, the RTP of the purchase, which can differ from the base game; Nolimit City lists the feature for "Mental".

In Great Britain the regulator has ruled it out. On 17 January 2020 the Gambling Commission said that six operators had been offering "feature buy-in" on slots, in one game for more than three thousand pounds a time, and that the facility did not meet its technical standards (RTS 3A, on the rules shown before a customer gambles, and RTS 14A, on products that encourage chasing losses). The six operators removed it, and every licensee was asked to check its games against the same requirements.

One game, several RTPs

A studio can release the same slot at several return settings, and the casino that installs it chooses one. Hacksaw Gaming's page for "Wanted Dead or a Wild" lists four (96.38, 94.55, 92.33 and 88.42 per cent), so two casinos can offer the same title, with the same graphics and the same name, at returns that are eight points apart.

For our player the gap is measurable: 500 spins of $1 cost an expected $18.10 on the version at 96.38 per cent and an expected $57.90 on the version at 88.42 per cent, which is more than three times as much for what looks like the same game! The installed version can be read only in the game's information panel (or its "help" or "rules" page) opened at the casino where you intend to play, and we would advise opening it at the first spin, not at the hundredth.

What we would read before the first spin

Put together, the panel answers five questions, and we read them in this order: the RTP version installed, the volatility the game declares, how often it hits and how often its bonus arrives, the maximum win and its probability, and whether a bonus buy or a jackpot changes the picture. A progressive slot, for instance, takes part of its return out of the base game to feed the counter (see progressive jackpots).

Two further readings concern bonus money. Free spins are tied to a named slot and a fixed stake, so their value depends on the version that is installed as much as on the terms (our page on free spins does the arithmetic). Any bonus played on slots carries a wagering requirement, whose cost is the edge applied to the total of the bets it requires, the same four cents per dollar that this page began with.

None of this turns a slot into a good bet, since every version keeps something for the house; it only makes the cost visible before the button is pressed. That, in our judgement, is the least a player is owed, and the panel that shows it is one tap away on your screen.

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By the Rand Casinos editorial team · Last updated October 2026 · Online since 2007