Guide

How Do Slot Machines Work? – An In-Depth Guide for Beginners

How do slot machines work? Simply put, these games make use of RNGs to generate random outcomes. This guide explains all you need to know.

Updated · CasinoSafest ledger

Row of slot machines in a Las Vegas casino
On this page 12
  1. The Mechanism Behind Early Slots & Newer Slot Machines Explained
  2. Understanding Slot Machine RTPs
  3. How to Calculate the House Edge
  4. Mechanical Slots vs Online Video Slots
  5. Hopper Fill Slip Explained
  6. Theoretical Hold – What is it & What Makes it Important?
  7. Differences between Class II and Class III Slot Machines
  8. Understanding the Main Slot Symbols & How They Work
  9. Types of Online Slots Available in the US
  10. How to Select the Best Slots
  11. Our Final Thoughts on Casino Slot Machines
  12. FAQs

Disclosure. CasinoSafest is free to use. If you open an account through a link on this page we may be paid a commission by the operator — it never changes which casinos make the list or the score they get. How we rate.

11 online casinos we rate highest right now

Ratings checked against operator terms · last updated

  1. No.01

    El Royale Casino logo

    El Royale Casino

    9.0/ 10 editor score

    Welcome offer230% up to $20,000

    Wagering 40x (b+d)Min deposit $10Code ROYALSHOWCashout InstantLicence Anjouan

    • BitcoinBitcoin
    • DogecoinDogecoin
    • EthereumEthereum
    • LitecoinLitecoin
    • MasterCardMasterCard
    • RippleRipple
    • +8 more
    Good to know
    • Licensed by Anjouan
    • 14 deposit methods on file
    • Taking US players since 2020
    • Cashes out in 6 coins, Bitcoin, Ethereum, Litecoin among them
    • Wagering of 40x (b+d) is above the 35x average on this page
    • No poker room alongside the casino
  2. No.02

    Red Dog Casino logo

    Red Dog Casino

    8.9/ 10 editor score

    Welcome offer225% up to $8,000

    Wagering 40x (b+d)Min deposit $10Code VAGGINGTAILSCashout Bitcoin · 0–1hLicence Anjouan

    • VisaVisa
    • MasterCardMasterCard
    • BitcoinBitcoin
    • LitecoinLitecoin
    • EthereumEthereum
    • TetherTether
    • +9 more
    Good to know
    • Deposits open from $10
    • Taking US players since 2019
    • Licensed by Anjouan
    • Cashes out in 6 coins, Bitcoin, Ethereum, Litecoin among them
    • No poker room alongside the casino
    • Wagering of 40x (b+d) is above the 35x average on this page
  3. No.03

    BetUS Casino logo

    BetUS Casino

    9.3/ 10 editor score

    Welcome offer200% up to $4,000

    Wagering 35xMin deposit $50Code CAS200Cashout Bitcoin · 0–1hLicence Curacao

    • American ExpressAmerican Express
    • MasterCardMasterCard
    • VisaVisa
    • BitcoinBitcoin
    • Bitcoin CashBitcoin Cash
    • LitecoinLitecoin
    • +7 more
    Good to know
    • 13 deposit methods on file
    • Cashes out in 4 coins, Bitcoin, Ethereum, Litecoin among them
    • Taking US players since 1994
    • Welcome match worth up to $4,000
    • Minimum deposit starts at $50, not $10
    • No poker room alongside the casino
  4. No.04

    Las Atlantis logo

    Las Atlantis

    9.0/ 10 editor score

    Welcome offer280% up to $2,500

    Wagering 40x (b+d)Min deposit $30Code UNDERTOWCashout InstantLicence Anjouan

    • BitcoinBitcoin
    • DogecoinDogecoin
    • EthereumEthereum
    • LitecoinLitecoin
    • MasterCardMasterCard
    • RippleRipple
    • +9 more
    Good to know
    • Taking US players since 2020
    • Licensed by Anjouan
    • Cashes out in 6 coins, Bitcoin, Ethereum, Litecoin among them
    • Welcome match worth up to $2,500
    • No poker room alongside the casino
    • Wagering of 40x (b+d) is above the 35x average on this page
  5. No.05

    Slots.Lv Casino logo

    Slots.Lv Casino

    9.4/ 10 editor score

    Welcome offer100% up to $2,000 + 20 free spins

    Wagering 35x (b+d)Min deposit $20Code not neededCashout Bitcoin · 0–1hLicence Curacao

    • VisaVisa
    • BitcoinBitcoin
    • Bitcoin CashBitcoin Cash
    • LitecoinLitecoin
    • MasterCardMasterCard
    • American ExpressAmerican Express
    • +7 more
    Good to know
    • Cashes out in 5 coins, Bitcoin, Ethereum, Litecoin among them
    • Taking US players since 2013
    • Licensed by Curacao
    • 13 deposit methods on file
    • No poker room alongside the casino
    • No bonus code published — the offer is applied at the cashier
  6. No.06

    Cafe Casino logo

    Cafe Casino

    9.0/ 10 editor score

    Welcome offer250% up to $1,500

    Wagering 40x (b+d)Min deposit $20Code not neededCashout Bitcoin · 10–15 minLicence Curacao

    • American ExpressAmerican Express
    • BitcoinBitcoin
    • Bitcoin CashBitcoin Cash
    • EthereumEthereum
    • LitecoinLitecoin
    • MasterCardMasterCard
    • +5 more
    Good to know
    • Cashes out in 5 coins, Bitcoin, Ethereum, Litecoin among them
    • Taking US players since 2016
    • Deposits open from $20
    • Licensed by Curacao
    • No poker room alongside the casino
    • No bonus code published — the offer is applied at the cashier
  7. No.07

    Vegas Aces Casino logo

    Vegas Aces Casino

    8.4/ 10 editor score

    Welcome offer250% up to $2,500

    Wagering 40x (b+d)Min deposit $20Code VAWELCOME1Cashout 3–5 minLicence Comoros Gambling License Authority

    • MasterCardMasterCard
    • VisaVisa
    • American ExpressAmerican Express
    • BitcoinBitcoin
    • Bitcoin CashBitcoin Cash
    • LitecoinLitecoin
    • +1 more
    Good to know
    • Taking US players since 2021
    • Cashes out in 4 coins, Bitcoin, Ethereum, Litecoin among them
    • Deposits open from $20
    • Licensed by Comoros Gambling License Authority
    • No poker room alongside the casino
    • Wagering of 40x (b+d) is above the 35x average on this page
  8. No.08

    MyBookie Casino logo

    MyBookie Casino

    7.9/ 10 editor score

    Welcome offer150% up to $750

    Wagering 40x (b+d)Min deposit $45Code MYB150Cashout Bank wire · 3–5 days

    Good to know
    • Taking US players since 2014
    • Welcome match worth up to $750
    • Wagering of 40x (b+d) is above the 35x average on this page
    • No cryptocurrency cashouts
  9. No.09

    Super Slots Casino logo

    Super Slots Casino

    8.1/ 10 editor score

    Welcome offer250% up to $1,000 + 300 free spins

    Wagering 35x (b+d)Min deposit $20Code SS250Cashout Bank wire · 3–5 days

    Good to know
    • Welcome match worth up to $1,000
    • Deposits open from $20
    • Taking US players since 2020
    • No poker room alongside the casino
    • No cryptocurrency cashouts
  10. No.10

    Wild Casino logo

    Wild Casino

    9.4/ 10 editor score

    Welcome offer250% up to $1,000 + 250 free spins

    Wagering 35x (b+d)Min deposit $20Code WILD250Cashout Bitcoin · 10–15 minLicence Panama Gaming Control Board

    • American ExpressAmerican Express
    • BitcoinBitcoin
    • Bitcoin CashBitcoin Cash
    • DogecoinDogecoin
    • EthereumEthereum
    • LitecoinLitecoin
    • +28 more
    Good to know
    • Welcome match worth up to $1,000
    • 34 deposit methods on file
    • Licensed by Panama Gaming Control Board
    • Cashes out in 16 coins, Bitcoin, Ethereum, Litecoin among them
    • No poker room alongside the casino
  11. No.11

    Bovada Casino logo

    Bovada Casino

    9.7/ 10 editor score

    Welcome offer100% up to $1,000

    Wagering 25x (b+d)Min deposit $20Code CAWELCOME100Cashout InstantLicence Curacao

    • MasterCardMasterCard
    • VisaVisa
    • PayPalPayPal
    • American ExpressAmerican Express
    • BitcoinBitcoin
    • Bitcoin CashBitcoin Cash
    • +15 more
    Good to know
    • Taking US players since 2011
    • Welcome match worth up to $1,000
    • 21 deposit methods on file
    • Cashes out in 5 coins, Bitcoin, Ethereum, Litecoin among them
    • No poker room alongside the casino

21+ only. Scores come from licensing, payment range, game choice and bonus terms — see how we rate. Gambling problem? Call 1-800-GAMBLER.

Our team of professional gambling site reviewers conducts independent research on every brand we feature on casinosafest. We work with some partners on an affiliate basis but this does not affect our listings or ratings.

Are you looking to learn more about slot machines? Well, look no further! Here at casinosafest, we use our experience and knowledge to explain everything that there is to know, from reels and wilds to paylines and bonuses.

How do slot machines work? These games operate using random number generators (RNG). This produces outcomes at a rapid pace and ensures that every spin is completely fair and unpredictable.

Once the player presses a button or pulls the lever, the RNG will determine the result. The symbols displayed on the reels align with the generated outcome which creates the possibility of winning combinations.

The Mechanism Behind Early Slots & Newer Slot Machines Explained

Slot machines are nothing new, with the first having been developed in the United States in 1891. However, as the years have gone by, slot technology has continued to evolve. To kick off our guide on how do slot machines work, let’s take a closer look at the mechanism behind early games and newer games.

Early Slots

Traditional slot machines are operated through a host of gears and levers. Upon inserting a coin, a detector will activate before the brake is released.

At this time, the handle will be available to move. The spinning reels are supported by a metal shaft in the center of the machine, with the reels being stopped using a braking system.

Sensors will then link up to the payout system when a winning combo has been formed. In most cases, mechanical slot machines feature 20+ symbols per reel.

Newer Slots

From the outside, the design of newer video slots resembles the technology found on older machines. However, what makes them different is that they are run by a central computer that makes use of RNGs. This is what determines whether or not a winning combination is formed.

In order to turn and stop the reels, computerized slots use step motors. The computer controls digital pulses which move with precision in order to drive these motors.

Understanding Slot Machine RTPs

Casino floor lined with slot machines
Casino floor lined with slot machines

To figure out how slot machines work, one of the factors you’d do well to consider is exactly what the RTP of a machine means. This term is given to the amount a slot will pay out, on average, over a specific period. In order to assign a game an RTP, millions of simulated spins are done before the game is released.

With slot machines at land-based casinos, the RTP can only be changed by physically altering the software or firmware, whereas those found at online casinos are able to easily change the RTP of the games.

It is important to note that although it is pretty simple for a casino to do this, they can’t just simply choose whichever RTP they prefer. Instead, they must select from the options offered by the developer. For example, NetEnt, a well known slot developer, is one of many providers that offer slots with specific RTPs.

How to Calculate the House Edge

Knowing how the house edge is calculated will bring you one step closer to understanding exactly how slot machines work. Note that because the house edge will vary depending on the slot, you will need to calculate it yourself. In order to do that, you must first know the RTP percentage.

The house edge of a slot can range from anywhere between 1% and 12%. This is higher than the vast majority of other casino games. In order to explain this further, we will take a look at the popular Mega Moolah slot from Microgaming. In the table below, we have outlined how Mega Moolah compares with other game types when it comes to house edge:

Game Developer House Edge RTP 
Mega Moolah Microgaming 2.5% 88.12%
American Blackjack Pragmatic Play 0.45% 99.60%
Golden Wealth Baccarat Evolution Gaming 1.06%-14.36% 93.36%-98.95%
Craps Evolution Gaming 1.4%-16.9% 99.17%

Meanwhile, the RTP of a slot is calculated out of 100%, and the remaining amount needed to reach 100% after the slot has been assigned a specific RTP will be the house edge.

So, for example, if a game has an RTP of 92%, the remaining percentage needed to equal 100% is 8%, making the house edge for that slot 8%.

Mechanical Slots vs Online Video Slots

Whether you play slots in person or at a land-based casino, you’ll have the option to enjoy a number of different types. But what’s the difference between classic 1 payline, 3 -reel slots and a standard video slot? In this part of our guide, we take a closer look at each.

Mechanical Slots

Generally speaking, mechanical slots feature three reels, as well as traditional symbols, such as bars and fruit. The outcome of each spin is determined by a random number generator (RNG), ensuring that all potential combinations are possible. Mechanical slots are also known as classic or traditional slots.

Video Slots

Thousands of these slots are now available online and at land based casinos. Coming with a varying number of reels, paylines, symbols and bonus features, these types of online slots also use a random number generator to determine the outcome of each spin.

However, they are more interactive and dynamic. The number of reels will also vary, with 5 being the most common. Meanwhile, they can have thousands of paylines and bonus rounds for you to enjoy.

These are among the ways in which they differ from mechanical slots. On top of this, when you play online slots like these, you will find a number of different types, including progressive jackpot and Megaways slots.

Hopper Fill Slip Explained

A Hopper Fill Slip is a document that records the replenishment of coins from the coin hopper. The number of coins will decrease upon payouts being made to players.

On the slip, the number of coins placed into the hoppers are indicated. You will also find employee signatures of those involved with transactions, along with the number of the slot machine, its location, and the date.

Theoretical Hold – What is it & What Makes it Important?

Blackjack table with cards and chip stacks
Blackjack table with cards and chip stacks

Theoretical hold worksheets are required by law and therefore provided by individual manufacturers. Worksheets also display information regarding reel strip settings, coins played, number of reels, and payout schedules. Worksheets can be useful for players, as it will give you a better idea of your chances of winning.

Differences between Class II and Class III Slot Machines

In the US, slots are separated into two categories, Class II and Class III. As these have specific requirements that need to be met by law, the logic is therefore different for each of them. When the Indian Gaming Regulatory Act (IGRA) was passed in 1988, it categorised gaming into three classes.

Simply put, Class I refers to games for charity, cultural, and non-profit purposes. Class II referred to bingo, and Class III to games such as roulette, slots, and blackjack. The separation was introduced to restrict Class III gaming on Indian lands while still allowing those that fell under Class I and Class II categories. This was because games that are similar to bingo in nature were seen as less immoral than games under Class III.

As Class II machines weren’t as restricted, Native American casino operators started thinking of unique ways to develop games that technically fell under Class II but resembled Class III Las Vegas style gambling. This is what led to unique slots that differ slightly from your standard slots. Next, we take a look at exactly how this was done and what makes Class II slot games different from what can be found under Class III.

Class II

These slots were designed to follow bingo logic and are seen as a high speed, automated version of bingo. Class II slots display bingo games being run in the background.

When you click on spin, the player will be pooled with others who have also hit spin and each of them will be given a bingo outcome that is predetermined. With this game-type, you are betting against others, rather than the house. As such, there is just a single winner per round.

Class III

Class III slot machines use a random number generator to determine the result. Unlike Class II games, bets are placed against the house, rather than against other players. They often offer larger jackpots than Class II slots. Odds and compliance are the other differences to consider, with the image below being an example of a Class III slot.

Understanding the Main Slot Symbols & How They Work

Regardless of the number of reels, players must form a winning combination of symbols in order to earn a payout. Mechanical slot machines can have 20+ different symbols, compared to potentially hundreds with online slots. As such, there are a huge number of possible combinations.

Each symbol comes with different payouts, with the amount won depending on the number matched on a payline. Symbols are generally in keeping with the theme of the slot. However, you will regularly find cards and gems, while fruit machines are also very common. Below, we will take a closer look at two common symbol types, scatter and wild.

Scatter Symbols

Although there is no trick landing a payout each time you spin, it’s worth keeping an eye out for slots that offer these symbols. This is because scatter symbols are used to trigger bonus rounds, which gives you an even better chance of winning.

The number of scatters required varies between slots. Scatters can land anywhere on the reels, with free slot machine spins and multipliers being the most common rewards. This type of symbol is generally one of the highest paying, while it is represented by a unique graphic as a result.

Wild Symbols

Wilds are colorful symbols that act in a similar manner to the joker in a deck of cards. This is because they can replace other symbols in order to form a winning combination.

An example would be if you have symbols on reels 1 and 3, as well as a wild on reel 3, you will create a winning combo. Wilds also come with rewards in some cases, while acting as the highest paying symbol in others. Depending on the specific slot, it could be that a wild cannot replace scatter symbols.

Types of Online Slots Available in the US

A variety of different slot machines are available in the US today. Each is slightly different, whether it be based on their theme, gameplay or jackpots. The following are the most popular slots.

Pachisuro

Slots that are specific to a particular country are not uncommon. Pachisuro is an example, with this game type originating in Japan. Evolving from pinball games, Pachisuro slots also have an element of Vegas-style slots to them. The reels do not spin. Instead, a small ball moves through pegs in order to form winning combinations.

Fruit Machines

These coin-operated machines generate random symbol combinations on the reels. They are also known as slot machines, with particular combinations resulting in payouts. Slot machine themes vary, whether it be fruit, animals, crystals or fantasy.

Jackpots

If you are looking to win larger amounts, jackpot slot machines are likely to be for you. Progressive jackpot slots are particularly popular. This is because a proportion of each bet placed contributes to the jackpot. Once the jackpot has been one, it will revert to its default amount.

Video Slot Machines

Video slot machines are available at many top online casinos. Developed by many software providers, virtual reels and symbols are used. The number of reels, paylines and bonus features vary, with the first video slot having been released in the 1970’s. These days, most of the best video slots offer free spins and multipliers in-game.

How to Select the Best Slots

Given the increasing number of slot machines that are available today, it is not always easy deciding which is the best for you. Gamblers must consider a number of factors, with each potentially impacting how to win on slot machines.

One quick method of finding the best slot is to check out expert reviews. The best online slots reviews discuss everything from RTP and paylines to bonus rounds and payouts. When you take a look at some of the best online slots for real money, you’ll find that they cater for both beginners and high rollers. Meanwhile, they are also available on desktops, mobile phones, and tablets.

Our Final Thoughts on Casino Slot Machines

Having read our guide, you should now have a better idea about how online casino slot machines work. You will find both Class II and Class III options designed to suit every type of player.

Thanks to quality developers that are always looking for ways to enhance your overall experience, when you decide to check out some of the most popular online gambling sites, you can look forward to a variety of different themes, bonus rounds, and special features.

FAQs

What is a class II slot machine?

Class II slot machines are those that are closest to Las Vegas style slot machines, while still complying with legal regulations surrounding bingo games. Such slots use computers or other technology to determine the outcome of each spin.

How are RTPs calculated?

Understanding the RTP of a slot is an important part of understanding how slot machines work. Basically, the return to player (RTP) of a slot determines the amount that you can expect to earn from bets placed over an extended period. In order to calculate the RTP, divide your total winnings by the amount bet, before multiplying by 100%.

What is the difference between online slots and land-based slots?

There are some key differences between online and land-based slots. While the payout rates and odds are similar, online slots are more convenient. There is also greater variety when it comes to online slots. However, many prefer the atmosphere of playing slot machines at a land-based casino.

Is a Pachisuro the same as a video slot?

A pachisuro combines elements of traditional slot games. Founded in Japan,

What is a scatter symbol and how does it work?

A scatter system will often trigger a bonus round. Coming with a unique graphic, scatters are also commonly the highest paying symbol of a slot. So, when learning how to play slot machines, be sure to look out for both scatters and wilds.

Share this Page
  • Email

Keep reading

Every page

Keep reading

Every page

Keep reading

Every page

Keep reading

Every page

Keep reading

Every page

Keep reading

Every page

Keep reading

Every page

Keep reading

Every page

Keep reading

Every page

Keep reading

Every page

Keep reading

Every page

Keep reading

Every page

Keep reading

Every page

Keep reading

Every page

Keep reading

Every page

Keep reading

Every page

Keep reading

Every page

Keep reading

Every page

Keep reading

Every page

Keep reading

Every page

Before you play

Which sites are legal, and at what age, is decided state by state — not nationally. Check the rules where you live before you put money down, and set your deposit limit first.

Open the state guide Play it safe