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How to calculate the probability of winning a bet

If you do not have an almanack with the results of matches, mistakes are inevitable. But you can estimate the odds better than bookmakers.

Probability theory in betting helps professionals beat bookies. Correct estimation of the outcome increases the chances of profit at a distance. In this article, we will look at probability theory in sports betting and discuss how to calculate bookmaker's margin and determine the value of a bet.

Probability theory in sports betting


Probability theory is a branch of mathematics that studies the regularities of random events and values, as well as their properties and operations. It allows you to estimate the probability of an event compared to another.

There are three approaches to defining probability:

  • A priori. Also called the Bayesian method.
  • Empirical.
  • Subjective.
  • Bayesian method.

In the a priori approach, the probability of each event is determined in advance. For example, when a perfectly symmetrical coin is tossed, there is a 50% chance that heads or tails will happen.

According to the Bayesian method, each flip will result in heads or tails. In practice, this is unlikely. Heads or tails may happen several times in a row, but if you flip a coin more than once, the ratio evens out. This uneven distribution is called dispersion.

Empirical Approach

By the empirical method, the probability of an event is determined by the formula:

P = N / X, where N is the number of possible outcomes, X is the number of all possible outcomes.

Subjective probability

Determined based on an observer's experience, public opinion or analysis of a particular situation. When estimating the probability take into account as many factors as possible.

How to calculate the bookmaker's margin and the probability of the outcome

The odds of the outcome reflects the probability of the event, according to the bookmakers, taking into account the margin.

The margin is the bookmaker's commission, which allows you to go in the plus for the distance.

The probability can be calculated by the formula:P = 1 / K, where K is the bookmaker's odds.

Find the bookmaker's margin by the formula:M = (S - 1) x 100%, where S is the sum of the probabilities.

How to determine the value of the bet

The odds show the bookmakers' opinion, so some outcomes are undervalued. Such bets are called a walk-away bet.

A bet is a valuation bet if K x P > 1, where K is the odds, P is your estimate of probability.