Proofset AcademyModule 2 · Sports bettingLesson 2.4Free · 25 min

The bookmaker's margin and fair odds

Add up the implied probabilities of the two players in a match: you always go over 100 %. That surplus is the bookmaker's margin. It is hidden in every price. Knowing how to measure it and remove it means recovering what the market really thinks.

Part 1 · Animation

Over 100 %: the hidden surplus

In a tennis match there are only two outcomes: A wins or B wins. Their true chances therefore add up to exactly 100 %. Yet with odds of 1,80 and 2,00, the implied probabilities come to 55,6 % + 50,0 % = 105,6 %. The 5,6 extra points are the margin (also called the overround).

Why a margin? The operator sets its odds slightly below what the chances would justify, on all outcomes at once. Over a large number of bets, whoever wins each match, it therefore pays out less than the true chances would give. That is what makes its business work.

Part 2 · Your turn

The margin calculator

Set the two prices for a match. The table calculates the margin, each player's probability without the margin and the fair odds.

Player A's odds1,80
Player B's odds2,00
Player APlayer BTotal
Odds—
Implied probability (1 ÷ odds)
Margintotal − 100 %
Probability without margin100 %
Fair odds (1 ÷ probability without margin)—

Part 3

Fair odds, step by step

Let us go back to the match priced at 1,80 against 2,00 and do the calculation by hand.

1. The implied probabilities

1 ÷ 1,80 = 55,6 % for A. 1 ÷ 2,00 = 50,0 % for B. Total: 105,6 %.

2. The margin

105,6 % − 100 % = 5,6 points.

3. Remove the margin

Divide each probability by the total: 55,6 ÷ 105,6 = 52,6 % for A, 50,0 ÷ 105,6 = 47,4 % for B.

4. Fair odds

1 ÷ 0,526 = 1,90 for A, 1 ÷ 0,474 = 2,11 for B. The operator shows 1,80 where the fair odds would be 1,90.

This method, which divides each probability by the total, is the simplest and the most widely used. There are others, which split the margin differently between favourite and underdog. The Academy uses the simple method.

Part 4 · Proofset measurement

The average margin, measured

6,4 %

Average margin on the odds Proofset recorded for 5 617 matches on the ATP and WTA tours in 2025-2026 (de-duplicated database, report of 13/10/2026).

We do not know when each price was recorded (opening or closing). The margin varies by operator, match and timing.

How to read it: on a typical match, the two implied probabilities add up to about 106 %. For a bet to make sense, you therefore need a true probability higher than the market's, and higher still by enough to cover the margin (lesson 2.5).

Part 5 · Your turn

The same match, three operators

Three fictitious operators offer the same match. Which one gives the best odds on player A? Click to compare.

Click on an operator.

Key point: for the same true chance, a lower margin means odds closer to fair odds. Comparing odds between licensed operators is a simple habit, and it changes nothing about the player's chances.

Why it matters for a bet

  • The odds offered are always lower than the fair odds. The margin is there on every bet, even when you cannot see it.
  • The market probability is the probability without the margin. That is what you should compare with your own estimate, not the raw implied probability.
  • A bet only makes sense if the true probability exceeds the implied probability of the odds, margin included. Lesson 2.5 makes this the central rule.
Part 6

Test yourself

7 questions. You need 6 correct answers to pass the lesson.

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