How do online slots work?

The basics of online slots

Online slots use random number generation to determine which tiles and symbols are shown on the screen when a player spins the reels. Once you’ve placed a bet, the goal is to try and line up certain combinations of symbols to win. Most online slot game types also have bonus rounds and other special features.

Random number generators

Random number generators (RNG) are the foundation on which all online slots are built on.

Specifically, online slots use pseudo random number generators (PRNGs) which are software algorithms that use mathematical formulas to produce random numbers starting from an initial seed.

The PRNG essentially performs the same task for an online slot game as a dealer shuffling the deck before you play a hand of live casino blackjack: Instead of shuffling the cards to ensure unpredictability, the PRNG produces a random number sequence that determines which tile, icon, or other graphic appears on the screen.

PRNGs are much faster, cheaper to implement, and easier to control and test than ‘true’ RNGs, which rely on unpredictable external phenomena such as electrical interference or changes in atmospheric pressure to produce random numbers.

The randomness of PRNGs means that there is also no 'best time to play online slots'. You chances of winning on a certain game remain the same whenever you play.

Reels & rows

Reels are the vertical columns that spin and display the symbols; rows are the horizontal lines where those symbols land.

The gameplay and the number of potential winning combinations is determined by the size of the grid layout created by the reels and rows.

A common setup is 5 reels and 3 rows, but variations exist, like 6x4 or cascading grids. More reels and rows often mean more paylines or ways to win.

Paylines & paytables

In online slot games, paylines are the paths or patterns where matching symbols must land to score a win. They can run in practically any direction - horizontally, vertically, or across the tiles in unique shapes.

So, how do you know which paylines to aim for? Well, that’s where the paytable comes in.

Paytables are the game’s cheat sheet, showing how much each symbol is worth, how paylines work, and explaining casino bonus features like free spins or multipliers.

When you open an online slot game, the paytables are usually pretty easy to find - check for an ‘info’ button or a small circle with an ‘i’ in it. Click this to bring up information about the game rules, paytables, and more.

Bonus features

Players can unlock bonuses depending on the outcome of the main round. These bonuses are often triggered by specific symbols like scatters.

Common online slot game bonuses include free spins, multipliers, bonus rounds, or mini-games where you can win extra prizes.

Bonuses offer players more chances to win big and introduce plenty of variety into each game.

Some of the most effective bonuses are the ones that tie into the story of themed games, like the Free Falls bonus in Gonzo's Quest.

Some online casinos also offer exclusive slot bonuses.

Jackpots

Jackpot slots are a subcategory of online slot games in which huge payouts can be won during normal play or in bonus rounds.

The two main types of online slot jackpots are fixed and progressive jackpots. Fixed jackpots are a set amount and progressive jackpots grow larger as more people place bets on the game across the casino or network - a bit like how the Euromillions rolls over when it’s not won.

Some games feature multiple jackpot tiers and the biggest jackpots can be truly life-changing amounts of money. One of the biggest online slots jackpot ever won was a whopping £13.2 million that Jon Heywood won from just a 25p stake at Betway Casino.

Special slot mechanics

Online slots developers are constantly finding creative new formats for their games. No longer are we stuck with the same old three-reel fruit machines!

Megaways

Megaways slots use a dynamic reel system where the number of symbols per reel changes on every spin. This creates thousands of different winning combinations, or ‘megaways’. Some games offer over 200,000 different winning combos.

The Megaways format was created in 2015 by Australian developers Big Time Gaming. Their Bonanza Megaways and Fishin’ Frenzy games are probably the best megaways slots. Extra Chilli also deserves an honourable mention.

Infinity reels

Infinity reels slots start with three reels, but more are added with each win and the total number of reels is potentially unlimited. Each new reel added presents bigger win opportunities by boosting multipliers.

The infinity reel mechanic was created by another Australian software developer, ReelPlay. Their El Dorado Infinity Reels, launched in 2019, was the first of its kind and remains one of the most popular options.

Cluster pay

Cluster Pay slots reward players for forming clusters of matching symbols, rather than paylines. Wins trigger cascading effects, for example where symbols are replaced with new ones for more chances to win.

It’s basically the online slots version of Candy Crush and, given that over 150 million people play Candy Crush every month and, given that over 150 million people play Candy Crush every month. It's easy to see why cluster slots are popular.

Hold & win

The hold & win slots mechanic goes way further back than the others mentioned above - it’s been around on slot machines in pubs and land-based casinos for decades.

As the name suggests, hold & win involves locking bonus symbols in place during a re-spin feature.

The best hold & win slots have huge jackpots on offer: Dragon Pearls, for example, has a 5000x prize up for grabs.

RTP & volatility

Return to player (RTP) is a term used to describe the average amount of money wagered that an online slot game returns to a player over time.

It’s usually expressed as a percentage: For example, a 96.4% RTP means the game pays back (on average) $96.40 for every $100 wagered. Short-term results can vary, so this doesn’t mean that you’ll always lose over time - you could hit a jackpot after all.

RTP for online slot games is typically higher than on slot machines at land-based casino venues, but the latter often offer better bonuses.

Volatility measures how often and how much a slot pays out: Low volatility means frequent but smaller wins, while high volatility offers big wins but less often.

In my opinion slots with low volatility, like Starburst, are better suited to beginners and casual players.

You’ll sometimes see people talking about variance in articles about and reviews on online slots - it pretty much means the same as volatility.

Are online slots fair?

Yes, online slots are fair.

Many licensing authorities require new slot games to undergo independent testing by approved test houses before being released to the market.

This testing verifies that the games using RNGs operate correctly and fairly.

Once a game is live, online casino software operators must also monitor its performance to ensure it meets the designed RTP rates.

Many operators now also offer Provably Fair slot games, which use cryptographic techniques to a ‘receipt’ of every outcome produced so that players can transparently scrutinize the mechanics that led to each outcome.

Ryan Davis Site Manager

It’s important to note that, while online slots are generally fair, the house will always maintain an edge. That means, on average, players will lose money over time when playing online slots.

Every online casino we feature on UAE Gambling Guide is properly licensed to offer online gambling services to players. That means you can be sure any slot game on these sites is transparent and fair.

FAQs about how online slots work