How do online slots calculate odds?

If you are unfamiliar with online casinos and playing online slot games, you may think it is all about pushing a button and waiting for the outcome when the reels stop. Unfortunately, there is no way you can influence the outcome of the reel spins. However, you have a certain edge if you understand the odds of winning the biggest prizes, how winning combinations are paid out, and the payout percentage.

The truth is the odds of collecting a payout are directly related to the number of reels and the number of symbols each reel has. The symbols on the pay lines that come to rest are sometimes called stops.

Here, we are going to discuss how odds are calculated in online casino slots:

The number of possible combinations is both very easy and tough to calculate – it depends on the number of slots (reels) on the machine and the number of symbols. For example, for a free slot machine with six symbols, the total combination of numbers are 6 * 6 * 6 = 216.

For the same three-slot online game with 20 symbols, the number of combinations is 20 * 20 * 20 = 8000. Thus, if we assume that a given slot offers a colossal payout whenever a cherry symbol appears, the chances of you winning the prize are 1/20 * 1/20 * 1/20, which equals a probability of 1 to 4000.

Using this simple method, you can calculate the odds of any symbol combination. For calculation, you need to know how many times the winning symbol appears on each reel.

Some online slots have a higher number of reels and symbols, and the calculation of odds is quite complex. For example, with online slots having five reels and 32 symbols, the total combination comes out to be more than 3 million.

Let us now see how to exactly are odds calculated for a simple three-slots machine. Assume each slot has six symbols – Joker, apple, orange, banana, melon, and lemon. The payout is as follows:

1. All three jokers, you earn 30 coins

2. Three of the same kind, you earn ten coins

3. Any two jokers, you get four coins

4. Any one joker, you get one coin (your investment)

Number of winning combination for all of the above situations

1. Three is only one way in which all three jokers can come

2. Three of a kind – so on the first slot you can get any of the five fruit and the other 2 slots get the same as in first, so 5 such combinations are there

3. Two jokers can appear in slots 1 and 2, slots 1 and 3, and slots 2 and 3. The third slot will have fruit. So total combinations are 1 × 1 × 5 + 1 × 5 × 1 + 5 × 1 × 1 = 15.

4. Joker anywhere, so the other two slots can have any fruit. So total combinations are  1 × 5 × 5 + 5 × 1 × 5 + 5 × 5 × 1 = 75.

The total combinations of winning are 96.

Winning CombinationNo of CombosWinPayoff for 1 coin% payoff
3 jokers1303013.953%
any 3 fruit5105023.256%
any 2 jokers1546027.907%
any 1 joker7517534.884%
Total96 215 
% of winning combos44.44%Payout99.537% 

Calculating the payoff percentage:

(1 × 30 + 5 × 50 + 15 × 4 + 75 × 1)/(6 × 6 × 6)

= 215/216

≈ 0.99537

So the payoff percentage in this is 99.54 which is good for a slot machine. If everything had equal weight then odds would have been 44.44 percent and no one will play with the odds of winning this low.

Control Over Payout Percentages By Regulatory Bodies

Regulations keep track of payout percentages, so players can be assured that online casino operators do not engage in unfair practices. In addition, online casinos with a good reputation often have their games audited by third-party organizations, such as eCOGRA. Regular audits are also conducted, so you can easily calculate the casino’s edge over you and your fellow players by checking the payout percentages for the online slots you are interested in.


It is possible to calculate the odds of online slots and the payout percentage of other online slot machines. All you need to know are two factors – the number of reels and symbols. With more symbols, the calculation may get tough and, in some cases, not feasible.

You cannot use math to create a formula that opens the door to frequent high payouts or progressive jackpot wins. However, knowing the mathematical concept of a specific slot puts you in a better position to profit from your bets in each spin.