| Updated: September 26, 2025All gambling games are designed to give the casino an advantage – known as the house edge – and slots are no exception. Yet these constantly buzzing machines are unlike any other casino game. Card and dice players can easily calculate their odds and know exactly what they are up against; this is not the case for reel spinners.
The mathematics behind slots is rather complex because of the incredibly high number of possible outcomes and combinations. On top of this, casinos prefer to keep all the parameters that go into the calculation of their slot odds a secret.
Despite these obstacles, a serious reel spinner always seeks to understand how the house edge works and how to calculate it. This gives them insight into which machines to play and which ones to avoid. This article explains how the house obtains an edge in slots, how to calculate it, and how to spot the games that can give you a positive expected value.
The House Edge
Figuring Out the House Edge in Slots
There is No Way To Reduce the House Edge in SlotsFiguring Out the House Edge in Slots
Also known as “hold,” the house edge of slot machines is set by programmers and mathematicians working for the games’ manufacturers. Casinos receive only a few optional paytables that the slot can use as a reference.
Suppose, for example, that Bellagio is looking to purchase the Game of Thrones slot from manufacturer Aristocrat. The manufacturer offers Game of Thrones machines with several theoretical returns, such as 90%, 92%, and 94%. Bellagio chooses the machines with an average return of 94%, and this becomes the percentage players collectively receive back.
From the manufacturer’s point of view, the process unfolds as follows: A mathematician first analyzes the slot and creates its paytable by building the so-called PARS sheet, a term that stands for “paytable and reel strip.”
This document is then sent to the programmers, who use it as the blueprint for writing the game’s code. The sheet provides vital information about symbol hit frequencies and payouts for winning combinations.
The house edge of slots varies from as little as 0.5% to as high as 25%. Slots that yield a casino advantage of under 4% are considered loose, while those with an edge over 8% are deemed tight. Ultimately, the house edge is about striking a balance.
If it is too low, around 0%, the casino will not generate sufficient long-term profits and will quickly remove the machine from its floor. If it is too high, players will lose money at a faster pace and will soon lose interest in the slot.
The casino hold for slots can be calculated with a computer, but you would need to run through all possible symbol combinations. One must know how many symbols appear on each reel and how many spaces lie between them. To complicate matters further, modern slots use virtual rather than mechanical reels, allowing a huge number of virtual symbols per reel.
Worst of all, players have no way of knowing how many symbols are mapped to these virtual reels. Casinos keep PARS-sheet information strictly confidential and would never reveal it to the average player.
There is No Way To Reduce the House Edge in Slots
What’s most peculiar about all this secrecy is that the information is nearly useless from the customer’s standpoint. A common misconception is that slot games are programmed to go through hot and cold cycles over certain periods. In reality, each stop on the reels has the same chance of landing as any other. The odds are determined in advance and remain the same on every spin.
Even if a player understands the reel stripping of a given slot, that knowledge gives them no real advantage. They cannot use it to devise a strategy capable of overturning the house edge built into the game. No such strategy exists for random games composed of independent trials like slots.
Consider a coin flip: most people know there are two possible outcomes – heads or tails – and both have equal chances when a fair coin is tossed. However, this knowledge alone does not allow anyone to predict which result will occur on the next flip.
The good news is that you can still estimate the house edge you are facing on a particular slot without access to the game’s PARS sheet. All you need is the theoretical return to player percentage, or RTP.
Many reputable online operators willingly share this information. Online and land-based casinos operating in the UK, in particular, are even required to make this percentage available under their licensing terms.
The RTP tells you what percentage of all money wagered on a slot is returned to players; the balance represents the casino’s advantage. Therefore, all you have to do is subtract the RTP from 100%.
Let’s use Microgaming’s Game of Thrones slot (the 243-ways version) as an example. The RTP of this game is listed at 95.10%. Accordingly, the house edge it yields is 100 – 95.10 = 4.90. This means the casino collects nearly $0.05 from every dollar players wager on this slot in the long run.
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Casino Myths and Tricks
Can Casinos Manipulate the Odds to Increase Their Edge? Casino Tricks to Boost Slot ProfitsCan Casinos Manipulate the Odds to Increase Their Edge?
A recurring complaint among losing players is that casinos routinely manipulate the odds and payout percentages to squeeze a higher edge from their slot machines. The common misconception is that this happens covertly at the flick of a switch, even while someone is playing the machine.
Another misunderstanding is that floor personnel are instructed to “tighten” the machines during busy weekends and “loosen” them for the rest of the week.
Contrary to these assumptions, gambling operators cannot reduce payout percentages merely by pressing buttons. To do so, the machine’s existing EPROM chip must be replaced with a new one.
The swap does not occur in secret. A high-level casino employee must first complete extensive paperwork and submit it to the local gambling regulator for approval. If permission is granted, the casino must then purchase a new approved chip, which is, of course, expensive.
More importantly, the chip change cannot be performed while someone is playing the machine. In Nevada, for example, locally regulated casinos are prohibited from changing slot chips unless the machine has been inactive for at least four minutes.
The machine must also display a message on the screen to inform players that it is under maintenance. Chip replacement is a time-consuming process, especially with popular slots, because the casino must wait until activity slows. The bottom line is that changing a slot’s payback percentage rarely makes financial sense.
Online casinos face similar hurdles when it comes to altering odds. The theoretical return percentages of online slots are set by the games’ software developers. When suppliers such as NetEnt and Playtech create a slot, they assign a fixed return percentage that applies to all gambling operators using their software.
Gambling sites seldom have a say in slot payback percentages, although in some cases they may choose from several predefined options. Software supplier RealTime Gaming (RTG), for example, offers operators more flexibility with RTP options ranging from 91% to 97.5%. Nevertheless, online casinos running this software must select a specific percentage and stick with it – they cannot alter the odds back and forth on a daily basis.
Casino Tricks to Boost Slot Profits
Like most businesses, casinos seek ways to boost the profits they earn from their games. In the slot arena, this goal was advanced with the introduction of multi-coin games featuring multiple paylines, which quickly overshadowed classic three-reel slots in popularity.
Many video slots generate a higher casino hold because their payout percentages are lower, around 95% or 96%. This reduction in return causes players to burn through their bankrolls more quickly. A $100 bankroll that would last an hour on classic slots might now last only 30 minutes on modern multi-credit games.
The arrival of the virtual reel completely revolutionized slot play by enabling casino operators to offer astronomical jackpots. To offset potential losses on machines with bigger jackpots, casinos had little choice but to reduce the odds of winning by making those huge prizes much harder to hit.
No longer limited by physical space, game designers can now map a greater number of symbols and blank spaces onto each reel, collectively known as stops. The game’s Random Number Generator determines which symbols appear on the screen at the end of each animated spin.
Each stop on the virtual reels corresponds to specific random numbers, but the numbers, symbols, and blanks are not weighted evenly. Most random numbers are linked to blanks – non-paying positions on the reels.
The opposite is true of winning symbols, which are tied to far fewer random numbers. The result is that slots using virtual reels ultimately give you longer odds of winning than machines that rely on physical reels.
These odds remain hidden from the player, who has no way of knowing the total number of stops or how many random numbers are assigned to each. In many jurisdictions, including the state of Nevada, casino operators are not bound by law to display the winning odds on their slot machines. Some regulators even argue this would diminish the element of surprise inherent in slots.
The way machines react to low-paying combinations can also be misleading. Modern video slots require players to place bets on the paylines to activate them. This increases the number of credits played per spin.
As a result, losing spins are often disguised as winning ones, encouraging patrons to keep playing. How is this possible? On some multi-line wagers, payouts are smaller than the total amount the player has bet.
Suppose, for instance, you are playing Betsoft’s Fruit Zen slot, a five-reel game with 10 adjustable paylines and bets of up to 10 credits per active line.
You bet 1 coin of $0.10 on each of the 10 paylines, risking $1 per spin. A combination of three raspberries hits on line 2 and awards you 5 credits.
You are $0.50 down on this spin, yet the machine still reacts as if you have made a profit. Your “win” is accompanied by exciting sound effects and flashy animations. This approach is intended to keep reel spinners at the machines for longer periods.
As we have already explained, the longer you play, the more spins you complete – and the more the house edge begins to assert itself.
Slots and Positive Expected Value
All casino games, including slots, are rooted in mathematics, allowing players to estimate how much a given bet is worth. On the casino floor, you have two choices: play a game with a positive expected value or one with a negative expected value. Expected value, or EV, represents the average amount a specific bet is worth. Let’s examine how slots stack up in terms of EV.
Defining Positive Expected Value What Slots Offer Positive Expectation to the Player and When Slots with Mystery JackpotsDefining Positive Expected Value
Expected value is easier to illustrate with a coin-flip example. If you flip a fair coin, the chances of landing tails are the same as those for heads. In the short term, the coin may produce more heads than tails, but if you flip it thousands of times, the outcomes will average out so that each occurs roughly 50% of the time.
Now, suppose you wager a dollar per flip and win a dollar each time your prediction is correct. You are paid at true odds, so your expected value is zero. After 100,000 flips, there is a good chance you will show little to no net profit.
Gambling seldom works this way because games rarely pay at true odds and one party – normally the house – holds the advantage. The house maintains a consistent edge over players, although a few games can yield positive expectation for the player.
Such is the case with blackjack and some video poker variations, where you can earn a net profit over the long haul through advantage play and strategy. Long-term expected value is calculated by multiplying your edge by your average bet. If you want your hourly expected value, you must also factor in the average number of wagers you make per hour.
Assume you find a full-pay version of Double Double Bonus Poker where a full house pays 10 credits and a flush pays 6 credits on the first betting level. Such a machine offers a long-term return of 1.000670 for every dollar wagered, or 100.067%.
It yields a positive expectation with optimal strategy because the player enjoys an edge of 0.067% over the casino. The long-term expected value of a $25 bet, for example, equals 0.00067 x 25, or 0.01675. In other words, you will win a little over a cent for every $25 you stake in the long run – not much, but at least the game provides positive value.
What about slots? The vast majority of machines offer return percentages below 100%. Let’s see how Playtech’s Goblin’s Cave compares with the full-pay video poker example above. This is one of the highest payers in Playtech’s library, with an RTP of 99.32%.
Here the player disadvantage is -0.0068, or 0.68%. Consequently, the expected value of a $25 bet is 25 x -0.0068, or -0.17, causing you to lose $0.17 on this wager over the long run. Unfortunately, most slots, with very few exceptions, are negative-expectation games.
What Slots Offer Positive Expectation to the Player and When
Some progressive jackpot slots become advantageous for players once the top prize exceeds a certain value. The challenge is determining exactly when this theoretical edge appears, as game developers are reluctant to reveal the details needed to calculate the odds.
This problem does not exist with video poker and other card games, where you can easily compute the odds by knowing the number of cards in play and the hand that triggers the progressive jackpot.
A progressive slot offers positive value once its break-even point is surpassed. Break-even points for progressive blackjack and video poker are readily available online, but the same cannot be said for slots because their developers prefer to keep this information confidential.
Slots with Mystery Jackpots
An exception involves slots with mystery jackpots, where the top prize is awarded once the meter reaches a specific value. You can determine when such a slot gives you an advantage by using the following formula: t = m x (h + r) / (h + 2 x r).
- t represents the target point
- m stands for maximum jackpot
- c corresponds to the percentage contribution toward the jackpot
- h represents the house edge
- r is the rate at which the jackpot meter rises
Naturally, you must know the value of each variable to use the formula. Determining the house edge is straightforward when you play online slots; we have already explained how to calculate it when you know the game’s theoretical return percentage.
Uncovering the jackpot contribution is harder because few suppliers share this information. Here is an example with a fictitious slot so you can see the formula in action. Assume the mystery jackpot drops at about $1,500 and the slot has a house edge of 7% (an RTP of 93%).
The jackpot contribution from each bet is 3%. Inserting these variables into the formula gives us t = 1,500 x (0.07 + 0.03) / (0.07 + 0.03 x 2). The result shows that the slot starts offering positive expectation when the jackpot meter climbs to roughly $1,153.
Slots with mystery progressive jackpots can be a smart bet because they allow players to identify positive-EV situations. Of course, players must still do their homework by tracking progressive jackpots at various casinos and gathering the data needed for the calculations.
If the house edge and jackpot contribution are unknown, you can make an educated guess about these variables. When playing at land-based casinos, state gaming reports can serve as guidelines because they contain information about machines’ average payouts for each coin denomination.