What is a bookmaker's margin?

It is the amount by which a market's implied probabilities add up to more than 100%. If home, draw and away imply 47.6%, 29.4% and 27.8%, the book is 104.8% and the margin is 4.8%. It is how the bookmaker builds in profit.

How do you calculate implied probability from odds?

Divide 1 by the decimal odds and multiply by 100. Odds of 2.10 imply 47.6%; odds of 3.40 imply 29.4%.

What are fair or margin-free probabilities?

They are the implied probabilities with the margin removed. The proportional method divides each implied probability by the total book, so they add up to exactly 100%.

What is a 2-way market?

A market with two outcomes, such as Over/Under 2.5 goals or both teams to score yes/no. With both sides at 1.91 the book is 104.7% and each fair probability is 50%.

Is the proportional method exact?

No method is exact. Bookmakers often add more margin to outsiders, which the proportional method spreads evenly. It is simple, transparent and close enough for most football markets.

Implied Probability Calculator: Odds to % and Bookmaker Margin

Odds of 2.10, 3.40, 3.60 imply probabilities of 47.6%, 29.4%, 27.8%. They add up to 104.8%, so the bookmaker's margin is 4.8%; without it the fair probabilities are 45.4%, 28.1%, 26.5%.

Implied and margin-free (proportional) probabilities
Outcome	Odds	Implied	Fair %	Fair odds
Home	2.10	47.62%	45.43%	2.20
Draw	3.40	29.41%	28.06%	3.56
Away	3.60	27.78%	26.50%	3.77

How to use the calculator

Enter the odds for every outcome in one market. It starts with three rows for home, draw and away, the standard football match result market. Use the 2-way preset for markets with two outcomes, such as Over/Under goals, or add rows (up to ten) for markets like correct score bands or an outright. Odds can be decimal, fractional or American in any mix. Results update as you type and the link in your address bar keeps the numbers, so you can share them.

The formulas

Implied probability of each outcome = 1 ÷ decimal odds.
Total book = the sum of all implied probabilities. A fair market adds up to exactly 100%.
Margin (overround) = total book − 100%.
Fair probability (proportional method) = implied probability ÷ total book.
Fair odds = 1 ÷ fair probability.

Worked example 1: a match result market

A bookmaker prices a home win at 2.10, the draw at 3.40 and the away win at 3.60. The implied probabilities are 47.6%, 29.4% and 27.8%, which add up to 104.8%. The extra 4.8% is the margin. Dividing each figure by 1.048 removes it: the fair probabilities become 45.4%, 28.1% and 26.5%, and the fair odds are 2.20, 3.56 and 3.77. Those fair figures are what you would compare with a model such as our home win predictions.

Worked example 2: Over/Under 2.5 goals

Both Over and Under 2.5 goals are offered at 1.91 (10/11, or -110). Each implies 52.36%, the book is 104.7% and the margin is 4.7%. Removing it gives 50% each, which means fair odds of 2.00 on both sides. If our Over 2.5 goals prediction for the match is 56%, our fair odds are 1.79, shorter than the bookmaker's margin-free 2.00.

Why the margin matters

The margin is the price of betting. A market with a 3% margin is cheaper for the bettor than one with 8%, even when the favourite's price looks similar. Margins tend to be lowest on top-flight match results and highest on niche markets and accumulators, where every leg's margin multiplies. Comparing margins across bookmakers is a quick way to see who is charging more.

Common mistakes

Leaving an outcome out. If you enter only home and away, the book looks far below 100% and the numbers mean nothing.
Using odds from different bookmakers in one market. The margin then reflects nobody's book.
Treating fair probabilities as the truth. They are the bookmaker's view without the margin, not a guaranteed chance.

Convert single prices with the odds converter, compare a price with our model in the value bet calculator, or read how we compare our numbers with bookmaker closing prices on the accuracy page.

Implied Probability Calculator