How to convert odds into probability

Football odds are simply another way of writing a probability. The probability behind decimal odds is found with a single division:

probability = 1 ÷ odds

Odds and probability move in opposite directions: lower odds mean a higher probability, higher odds a lower one. If an outcome's odds double, its probability halves. That is why harbigol shows probabilities on the site, not odds.

Why do the three outcomes add up to more than 100%?

Convert the home, draw and away odds of a match into probabilities and add them up: you will usually get more than 100%, for example 105% or 110%. The excess is the bookmaker's margin – its built-in profit. To find the true probabilities, the excess has to be removed.

Removing the margin

The simplest method divides each probability by the total. If the raw probabilities are 47.6% · 29.4% · 27.8% (104.8% in total), dividing each by that total gives margin-free probabilities of about 45.4% · 28.1% · 26.5%, which add up to exactly 100%. harbigol does this separately for many bookmakers worldwide and averages the results to capture the market consensus.

What is a margin-free probability good for?

A margin-free probability shows the chance the market would put on an outcome if the bookmaker took no profit. It is the fairest way to compare different matches, or the same match measured at different times.

A probability is an estimate; it does not decide a match. An outcome with a 70% probability still fails roughly three times in ten.