How Do Casino Slots Work Online?

Slot machines are very simple to play, which is why they are so popular at land-based and online casinos.

To play, you first select how much you want to wager, and then you spin the reels. Typically, you must match three to five symbols in a row to score a win. Generally, there are a number of different ways to win, known as paylines, and various symbols that will award different prizes.

You’ll be able to see the value of the different symbols for your game of choice by consulting the slot paytable. Digital slots automatically tell you what you’ve won, and sometimes you don’t even know what payout you’ve earned. It’s all part of the excitement. If you have ever wondered how slot machine games work behind the scenes, read on.

How modern slots work

Modern-day slot machines rely on technology to deliver results. They use random number generator software to determine spins. An RNG is a program that randomizes results by sifting through number combinations at rapid speed. The fast-paced nature of RNGs guarantees randomness and ensures that you can’t “time” spins to win more money.

Odds and return-to-player rate also factor into the inner workings of slots. Regarding odds, developers program certain symbol combinations to occur more frequently than others. Symbols that produce the jackpot, for instance, will turn up less frequently than others. A random number generator merely ensures that each symbol occurs randomly — it doesn’t dictate the odds itself.

As for RTP, slot developers program machines to produce an overall payout percentage. RTP is the culmination of all odds and payout frequencies. Real money online slot machines average around 96% RTP, while typical land-based slot machines offer 91% to 96%.

Of course, the technological processes of slot machines aren’t visible to players. Gamblers can’t see the RNG, odds or RTP in action and may have trouble trusting results. Therefore, many licensing authorities require casinos to undergo third-party auditing. The Michigan Gaming Control Board, for example, makes Michigan online casinos and software providers obtain testing from BMM Testlabs. This auditing ensures that the RNG, odds and RTP work as stated.

How mechanical slots worked

Up until the 1970s, all slot machines operated via mechanical reels and other parts. Mechanical slots use a complex set of gears, springs and metal plates to deliver results. These pieces collectively randomize the results, working as if they’re a physical RNG.

Mechanical slot machines also feature a lever. You put quarters or nickels into the machine and pull the lever to set the inner parts in motion. The reels feature a set of symbols that spin in unison with the working parts.

The displayed reel symbols — meaning those visible on the outside of the machine — show winning and losing combinations. For example, you might spin the reels and see a winning combination of three cherries.

Following the advent of video slots in 1976, mechanical slots have gradually phased out over the years. Today, mechanical games are available in limited capacity in land-based and online casinos. You’ll find such machines under the three-reel or “classic” billing at gambling establishments. Downtown Las Vegas casinos (El Cortez, for example) are the most likely to feature classic slot machines.

Why are slots so fun?

Modern slots can feature high-quality graphics, exciting sound effects and fun animations. Many slot machines ring out with triumphant sound effects and may display coins bursting across the screen during payouts.

Most of the newest slot machine games also provide bonus features to enhance the action. These can include expanding wild symbols, random wilds, win multipliers and free spins. These extras pump up payouts and are normally key to games’ biggest wins.

The randomness of slot machines also enhances the entertainment factor. Slots keep you on the edge of your seat due to the unpredictability. Depending upon the game, you could win nothing or 10,000x your bet (or more). Therefore, slot machines offering big-win potential tend to stand out versus even-keel games such as baccarat or blackjack.

The different features of today’s slots

Slot machines encompass a broad range of games with different features. Some slots have basic setups with five reels, few paylines and minimal bonuses. Others may offer exotic gameplay (e.g., Megaways slots) and/or be bursting with features.


Paylines refer to the order in which a set of symbols must fall in order to pay out a winning combination. In most cases, this must be at least three matching symbols from left to right, starting on the first reel.

The simplest fruit machine slots may have a single payline running across the middle of the reels. In a three-reel slot on which each reel displays three symbols, the payline will be a straight line from left to right. If you hit a three of a kind on the middle of the reels, you win.

In real money and free online slots, you’ll find the majority of games have at least 20 paylines and often many more. This is because the paylines in online slot machines do not necessarily run straight across from left to right. In many games, paylines will still run from left to right but via all sorts of paths. A winning payline might zig-zag over the reels or form a V shape. Some games pay in any direction and have over 1,000 ways to win.

You’ll be able to find out the number and shape of the paylines by accessing the paytable menu. Many games have fixed paylines. This means you must bet on all of the paylines. Some, however, allow you to bet on any number of paylines from one up to the maximum bet.

Wild symbols and scatter symbols

As well as the regular symbols you see at an online slot, there are also special symbols. These are wilds and scatters.

  • Wild symbols act exactly like wild cards in some poker games. They take the place of any other symbol. This means that four consecutive symbols followed by a wild symbol count as a five-of-a-kind win rather than four of a kind.
  • Scatter symbols typically pay out higher than average wins for three of a kind or higher. They can often pay a win for just one or two symbols. They usually pay scattered, which is where they get their name. This means they don’t necessarily pay out from left to right and don’t have to start their winning payline combination on the leftmost reel. Often, a set of scatter symbols will launch a feature round.

Slot machine bonus rounds

Feature rounds, or slot bonus rounds, are the best part of online slots in the eyes of many online casino games players.

That’s because
it’s in the feature rounds where you can make the biggest slots profits. Typically, feature rounds will award prizes many times higher than ordinary payline wins during regular slot spins. A feature round may take one of the following forms:

  • Free spins: Receive a free spin, a predetermined number of chances to score payline wins.
  • Pick and win: You have a series of blind choices. Choose from a number of prizes including multipliers, instant wins and other bonuses.
  • Instant win: Immediately receive a cash prize.
  • Re-spin: Your last spin will get a “do-over,” often with additional parks.
  • Mini-game: Play an interactive game to win prizes. This may be skill-based, chance or a mixture of both.

Feature rounds, especially re-spins and free spins, may also have additional incentives. These include winning combinations being held, and extra wilds added to the reels.

Progressive jackpot slots

The biggest slot prizes pay out via progressive jackpots. These slots have this name because the jackpot increases gradually with each wager from players. Often, especially online, several or even dozens of machines are hooked up to the same jackpot.

Each wager adds to the jackpot until some lucky player wins it all. At the time of writing, the largest online slots jackpot win in history was $19.6 million. The largest land-based slot win was in Las Vegas in 2003. An anonymous Los Angeles gambler won over $39.7 million.

What is payback?

Payback refers to the average percentage return you can expect to see on your slot play over the long term.

In Nevada, the law requires slots’ payback percentage to be 75% or higher. Because of the competition in much of Nevada, though, payback percentages are rarely below 90%.

In Arizona, where tribal gambling establishments are the only places to find legal slot games, casinos must maintain a minimum return of 80%.

The Seminole Tribe dominates gambling in Florida, and similar to Arizona, the tribe does not release payback information on any of its gambling machines. For non-tribal gambling establishments in the state, Florida regulations require all gambling machines to pay back a minimum of 85%. Most of these racinos, like Pompano Park, Gulfstream, Casino Miami and Mardi Gras, return 90% or more on average.

You typically find payback gems in small places. Towns like Wendover or Sparks, Nevada, have machines that typically pay back 95% to 96%. Mississippi is a great destination for a high percentage payback, as well. Overall, though, you will find the highest payback slots at online casino sites.

Highest payback online slots

How profitable are slot machines? Ideally, in order to potentially maximize your slots profits, you’ll play online slots with higher return to player. After all, high RTP gives you a stronger chance at winning money. A slot with 96.5%, for example, will pay more over time than one with 95% payback.

The only catch, though, is that you need to find these online slots with high RTP first. Below, you’ll see the current 20 highest-paying US online slots that we’ve come across (RTP may vary by state):

Slot Provider RTP (may vary by state)
Blood Suckers NetEnt 98%
Starmania NextGen 97.87%
Texas Tea IGT 97.35%
White Rabbit Megaways BTG 97.24%
Secret of Atlantis NetEnt 97.1%
Steam Tower NetEnt 97.04%
Blood Suckers II NetEnt 96.94%
Dead or Alive NetEnt 96.82%
Wheel of Fortune: On Tour IGT 96.6%
Divine Fortune NetEnt 96.59%
Pixies of the Forest 2 IGT 96.57%
Donuts BTG 96.54%
Dragon?s Temple IGT 96.53%
Pyramid: Quest for Immortality NetEnt 96.48%
Asgardian Stones NetEnt 96.31%
Butterfly Staxx NetEnt 96.35%
Guns ?N Roses NetEnt 96.3%
Baywatch IGT 96.25%
Cossacks: The Wild Hunt Foxium 96.19%
China Shores Konami 96.1%