In comparison to most sports, scoring in soccer is a less frequent event, which makes short tournaments and knock-out games highly unpredictable and league performance the leading indicator of the true strength of a team.
When it comes to betting, it is the average number of goals scored per match that both bookmakers and bettors look at when assessing the odds for the Over/Under markets.
Here's a simple, step by step guide on how to calculate the probabilities of expected number of goals for a match and how to convert them into odds, so that you know what odds to bet your money on and what odds to avoid.
Step 1 – Find the average number of goals per game
The fact that Philippines United Football League has an average of as high as 5.75 goals per game and Spain’s Tercera Division an average of as low as 1.71 (according to soccervista) highlights how broad the range of average number of goals per game can be depending on a league. Therefore, the best place to start is by finding the average number of goals per game for the two sides of the match you are interested in betting on.
Step 2 – Calculate the probabilities
The random nature of goal scoring is the reason that allows statistics to follow a Poisson distribution as a simple predictive model for Over/Under betting. Let's see what that means in practice.
In his book "Taking Chances", John Haigh drafted a table that calculates the probabilities of a team scoring 0, 1, 2, 3 and 4 or more goals based on their average number of goals per game.
In a match where the home side has an average expectancy of 1.2 goals and the away side 0.8, it is easy to determine the probabilities for the Under 2.5 goals by using the above table.
First we need to determine the correct scores that will lead to a winning Under 2.5 bet (0-0, 1-0, 0-1. 1-1), find the respective probabilities for each team from the above table and then calculate the probabilities of each score by multiplying.
Since we know the probabilities of each potential score that can lead to Under 2.5, we add them to get the probability of the match ending in Under 2.5 Goals and, therefore, a winning Under 2.5 bet. In the above example, that would be:
13.5 + 10.8 + 10.8 + 12.96 = 48.06 percent
Step 3 – Determine what odds are worth betting on
Now that we have established the statistical probability of the match to end in less than 2.5 goals, the only thing that is left is to determine is what odds you should accept.
To convert probability into decimal odds, use the following simple formula:
Decimal odds = 100/ probability
In the above example, the probability of 48.06 corresponds to: 100/48.06 = 2.08
What that means is that you should only put your money on the Under 2.5 goals, if you find a bookmaker offering odds higher than 2.08.