Slot Odds, Winning Probabilities and House Edge
If you've tried online gambling, you probably know that the odds generally favour the house. Whatever game you choose, the casino has an advantage. That margin varies by game, and slot machines are often among those with less favourable odds. Still, not every spin is a loss. Slots differ in probability, house edge and potential payouts, so some may return more than others over time.
Slot Machines: An Introduction
Slot machines have been around for more than a century, though they became especially prominent in the 80s and 90s. Their simple format helps explain why they remain popular in both land-based and online casinos.
There's little to learn before playing. You do not need to master complicated rules, bet types or casino etiquette: press the spin button and see whether a winning combination appears on the reels. By comparison, Craps, Blackjack and Roulette generally require some knowledge to play, or at least to play well. A button or lever starts the spin, and the machine provides the result. Slots can also offer relatively low-cost entertainment when played responsibly. Smaller wagers are available, which is why some are called ‘Penny slots’.
You can play slot games online or visit a land-based casino, though this guide later explains why the odds of winning are lower at the latter. In either setting, casino slots are regulated to support fair, random outcomes. Regulation does not remove the house advantage, but there are several things to consider when making more informed choices.
How Do Slot Machines Work?
A spin takes less than a second, and its outcome is selected when you press the button. Every slot game uses a random number generator (RNG), a software algorithm that assigns a result to each spin. It cycles through as many as millions of possible outcomes before stopping on one at the microsecond the spin command is given.
After a losing streak or two, some players claim that slot machine random number generators in the gaming industry are rigged. They are not. Independent third-party testing companies examine these systems to check for fair, random results. Rigging would also put slot developers at substantial risk of being discovered quickly.
Slot Odds Explained
Slots are among the easiest, if not the easiest, casino games to play online. Their accessibility is a major reason for their popularity: select a game, spin the reels and hope for a payout. Understanding how likely a payout is, however, is much more complicated than starting the game.
The RNG cycles through the many possible reel outcomes to produce a fair and entirely random result.
Reel symbols, their payout values and any bonus features all affect what a spin can win. Available combinations, hit probability, payouts and RTP together shape the odds of a slot game.
Slot Probability and RTP
Probability describes how likely an event is to happen. It is expressed as the ratio of favourable outcomes to the total number of possible outcomes. The same concept is used to estimate the chance of rainstorms or by car insurers assessing the likelihood of a car accident.
Online slots use software and algorithms to assign different probabilities to symbols landing in winning reel combinations. The share of money wagered that a slot returns over a long period is known as its Return to Player, or RTP.
Many online casinos and/or software providers publish a game's RTP, though some do not. RTP is never 100%, as that would leave the casino without a statistical advantage. The difference between the stated RTP and 100% is the online slot's house edge. From the player's perspective, the same relationship is sometimes described as the odds of winning.
For example:
Suppose an online slot has an RTP of 95% over an extended period. It would return 95% of the money wagered and retain the remaining 5% as profit, making 5% the house edge. Put simply, a $100 wager could return $95.
This is a simplified illustration, since the RNG determines when and how payouts occur. RTP does not calculate a separate return for your own money, so getting exactly 95$ back from your $100 is unlikely. One player might turn $100 into $300, while another could lose their entire $100.
Understanding Slot Volatility
Volatility, also called variance, is another factor that affects how a slot distributes payouts across its RTP. It describes both the size of wins and how often they occur.
- Low Variance: Low-volatility slots usually hit more often, so wins tend to show up regularly. Those payouts are generally smaller than wins from higher-volatility games. A low-variance slot might pay every couple of spins, although the payout may still be less than the cost of the spins that led to it.
- High Variance: High-volatility slots work the other way around. Wins are less frequent, but can be larger when they land. These games often feature bigger jackpots or valuable bonus rounds that trigger only occasionally.
- Medium Variance: Medium-variance slots fall between the other two categories. They tend to pay more often than high-variance games, with larger average wins than low-variance slots.
Players often have distinct preferences when it comes to volatility. High rollers with larger budgets may choose high-variance slots, since their bankroll can handle a run of losing spins while they wait for a substantial win. Players with smaller bankrolls may favour low-variance games, which can keep their balance going for longer.
Online Slots vs. Brick-and-Mortar Slot Machines
Where you play can affect the slot odds available. Online casinos and land-based venues differ in a few important ways.
Playing Slots Online
Convenience is the main appeal of playing online. You can play at home, on the bus to work, or anywhere else with an internet connection. You also avoid waiting for a seat or making your way through a busy casino floor to find the game you want.
Online casinos also have a much wider choice of games and stakes. Thousands of current slot titles are available, from progressive jackpots and classic slots to Megaways games. Many can be tested in free demo mode without risking real money.
Online play is also described as offering better odds, since online slots have an average RTP of 96%. As the next section explains, this average is considerably higher than the stated average for land-based casinos.
Playing Slots at Brick-and-Mortar Casinos
Online play is convenient, but land-based casinos can offer a fuller social and sensory experience. You can dress up for a night at a luxurious venue, take in the sights and sounds, dine at a restaurant, or stop at the bar for a quick drink. Some casino properties also have nightclubs.
The main disadvantage of brick-and-mortar venues is that their machines have much lower average RTPs. That does not necessarily mean casinos intend to treat players unfairly; their operating costs are higher than those of online casinos. They have to cover staff, larger energy bills, rent and other overheads, so their games generally offer less favourable odds.
Land-based casinos may have slots with payout percentages as low as 80%. Some machines still offer a higher RTP. For instance, Wheel of Fortune is a three-reel slot with one payline and an RTP of 96.08%, higher than that of most land-based slots discussed here.
The Best Slots Odds
Almost 1,500 new online slots are released each year, on top of the thousands already available, so finding games with the best odds can take time. Casino Ideal has picked out several high-RTP slots available at many top online casinos. The titles below return a greater share of the money they take than many other slots. They are among the closest things to winning slots, but statistical results still mean players lose over time. Here’s what each one offers:
- Book of 99 (Relax Gaming) - 99%
Relax Gaming’s 'Book of 99' video slot is often considered one of the better options for slot odds at reputable online casinos. In this Slots Odds, Winning Probabilities and House Edge guide, Evan Carrington and Casino Ideal note that the 99 in the title refers to the game’s RTP of 99%. In theory, this represents a return of 99% of money wagered over a long period. It does not guarantee a winning session or mean every spin will pay.
Book of 99 follows the 'Book' slot format. Along with its high payout percentage, it features familiar free spins with an expanding symbol. There are no extra bonus rounds, but its wild collection feature can create payouts and may also unlock the free spins bonus.
- Blood Suckers (NetEnt) - 98%
High payout percentages aren’t exclusive to new video slots. NetEnt released Blood Suckers in 2009, and its payout percentage remains higher than that of most online slots. That return has been a major part of the game’s success since its release.
Blood Suckers has five reels, three rows and 25 fixed paylines. Features include wild symbols, a picking game and free spins. The game has low volatility and often pays out every other spin. Paired with its high payout percentage, that low variance may suit players looking for strong odds with a smaller bankroll.
- Jokerizer (Yggdrasil) - 98%
Yggdrasil’s Jokerizer is another option for players seeking a high-RTP slot. Its stated RTP is 98%, placing its odds among the strongest in this selection. The house edge is 2%, similar to the figure associated with blackjack.
At first glance, Jokerizer is a relatively simple slot with 5 reels, 3 rows and 10 fixed paylines. Its features are Mystery Win, which can award up to 6,000x the stake, and wild symbols. Its medium-to-high variance may make it less suitable for a smaller bankroll.
- Zeus Lightning: Power Reels (Red Tiger Gaming) - 97.73%
Red Tiger Gaming has released high-RTP online slots over the years. One of its strongest examples is Zeus Lightning: Power Reels, an Ancient Greek-themed game with seven reels, 6 rows and 30 paylines. Its stated RTP is 97.73% of the money it takes, making it one of the options highlighted for better online slot odds—not a guarantee of a win.
Features include Expanded Lightning, which enlarges one symbol type until it fills the reels. The free spins feature can also award spins with up to three wild symbols appearing on the reels.
- Blood Suckers Megaways (Red Tiger Gaming) - 97.66%
Blood Suckers Megaways is another high-RTP title from Red Tiger Gaming. It’s the sequel to Blood Suckers, the earlier slot known for its strong return to player percentage. This game’s payout percentage is slightly lower at 97.66%, but its maximum win is higher, reaching 20,521x the stake.
The minimum bet is 0.10 and the maximum is 4.00, both relatively low compared with some other slot machines. That range may work for casual players. Bonus content includes base-game modifiers such as random wilds, super wilds and a win multiplier, as well as free spins that can be triggered or purchased through the buy bonus feature.
Can You Improve the Slot Odds?
You can’t change a slot machine’s underlying odds: its RNG produces random results in line with the slot’s RTP. You can, however, consider the many slot-playing tips available and use those that support informed, responsible decisions.
Choosing a high-RTP game may improve your long-term mathematical position, while players often avoid progressive jackpots because their large prizes generally come with a lower RTP. Managing your bankroll can also help you play longer by limiting excessive wagers and controlling session length. None of these choices guarantees a win.
Casino bonuses can offer another way to play with less of your own money, subject to their terms. When you sign up at casino sites, you may come across no deposit bonuses, welcome bonuses and free spins. A win during bonus play can add funds to your bankroll, but wagering conditions still apply.
You may also come across betting systems such as the Martingale, although these are generally better suited to 50/50 casino games such as Roulette and Baccarat. Modified systems may be promoted as ways to extend a winning streak or reduce losses, but no system can guarantee profits or beat the game’s underlying odds.
