The central idea
Bookmaker margin is the excess above 100% created when the reciprocal probabilities of every outcome are added together. It is commonly called overround.
Overround = Σ(1 ÷ decimal odds) − 1
Worked example
At odds of 1.91 on both sides, each price implies 52.36%. Together they total 104.71%, so the displayed overround is 4.71%. That percentage is not the bookmaker’s guaranteed realised profit.
Limits of the calculation
Overround describes the prices offered at one moment. It does not reveal liabilities, customer behaviour or the true probability of either outcome.