Why do implied probabilities add up to more than 100%?

Because bookmakers build in a margin. The amount over 100% is the overround; removing it gives margin-free or 'fair' probabilities.

Is implied probability the real chance?

No. It is the bookmaker's price turned into a percentage. Even after removing the margin it is an estimate, though closing odds are a very strong one.

How to calculate implied probability from odds

By ProbaPredict Data Desk · Last reviewed Fri, 2 Oct 2026

To calculate implied probability, divide 1 by the decimal odds and multiply by 100. Odds of 2.50 imply 40%. For fractional odds a/b, use b ÷ (a + b); for American odds, use 100 ÷ (odds + 100) for plus prices and |odds| ÷ (|odds| + 100) for minus prices. The result still includes the bookmaker's margin.

What is implied probability?

Every price implies a chance. If a bet at 4.00 is fair, it should win one time in four, because over four bets at a stake of 1 you would lose three and get 4 back once, breaking even. So 4.00 implies 25%. Implied probability turns odds into the language of chance, which makes prices in different formats easy to compare.

What are the formulas?

Decimal odds: implied probability = 1 ÷ odds. 2.50 → 40.0%.
Fractional odds a/b: b ÷ (a + b). 5/2 → 2 ÷ 7 = 28.6%. 10/11 → 11 ÷ 21 = 52.4%.
American plus odds: 100 ÷ (odds + 100). +150 → 40.0%.
American minus odds: |odds| ÷ (|odds| + 100). -200 → 66.7%.

Why don't the percentages add up to 100%?

Add the implied probabilities of every outcome in a market and you get more than 100%. That excess is the bookmaker's margin, also called overround or vig. In closing odds across the leagues we cover, the average 1X2 margin is 5.3%, ranging from 4.4% in the Premier League to 6.3% in the Brasileirão Série A.

Average bookmaker margin in closing odds, by league

League	Matches	1X2 margin	Over/Under 2.5 margin
Premier League	1,950	4.4%	4.9%
Bundesliga	1,566	4.8%	5.2%
La Liga	1,969	4.8%	5.2%
Serie A	1,950	4.9%	5.2%
Ligue 1	1,723	5.0%	5.3%
Championship	2,855	5.5%	5.6%
Eredivisie	1,593	5.7%	6.1%
Primeira Liga	1,592	6.2%	6.0%
Brasileirão Série A	2,177	6.3%	not in source

Source: ProbaPredict analysis of 17,375 matches, updated Fri, 2 Oct 2026. Market-average closing odds from football-data.co.uk. Margin = sum of implied probabilities − 100%.

How do you remove the margin?

The proportional method divides each implied probability by the total. It is simple and close enough for most purposes; more complex methods spread the margin unevenly between favourites and outsiders. Our implied probability calculator uses the proportional method and shows the margin and fair odds for up to ten outcomes.

Worked example 1: a 1X2 market

Odds: home 1.80, draw 3.75, away 4.50. Implied: 1 ÷ 1.80 = 55.6%, 1 ÷ 3.75 = 26.7%, 1 ÷ 4.50 = 22.2%. Total 104.4%, so the margin is 4.4%. Margin-free: 55.6 ÷ 1.044 = 53.2%, 26.7 ÷ 1.044 = 25.5%, 22.2 ÷ 1.044 = 21.3%. Fair odds: 1.88, 3.92, 4.70.

Worked example 2: an American-odds total

A US sportsbook lists a soccer total at Over 2.5 -125 and Under 2.5 +105. Implied: 125 ÷ 225 = 55.6% and 100 ÷ 205 = 48.8%. Total 104.3%. Margin-free: 53.3% Over and 46.7% Under. If our model gives Over 2.5 a 55% chance, that's slightly above the sportsbook's margin-free estimate.

How should you use implied probability?

Compare it with your own estimate. If you think an outcome is 45% likely and the price implies 40%, the price is higher than your estimate. Our probabilities, published on every match page and in the market tables, can serve as that estimate; the value bet calculator does the comparison. Be honest about the benchmark: our back-test shows margin-free closing odds are slightly more accurate than our model (see how accurate football predictions are).

Common mistakes with implied probability

Leaving the margin in. Raw implied probabilities overstate every outcome a little.
Using fractional odds directly. 5/2 is not 2/5 = 40%; it is 2/7 = 28.6%.
Treating it as truth. It is a price-based estimate, not a certainty.
Comparing across markets with different margins. Remove the margin from each first.

How do you handle markets with many outcomes?

The method is the same however many outcomes there are: convert every price to implied probability, add them up, and divide each by the total. For a correct score market or a long list of first goalscorer prices, the total is often well above 110%, so removing the margin changes each probability noticeably. Our implied probability calculator takes up to ten outcomes at once and shows the book total, the margin and the fair odds for each line, which is quicker and less error-prone than doing it by hand.

Are there better ways to remove the margin?

The proportional method assumes the margin is spread in proportion to each outcome's implied probability. In practice bookmakers often load more margin onto outsiders, a pattern called the favourite-longshot bias. Alternatives such as the power method or Shin's method try to account for this and give slightly higher fair probabilities to outsiders and slightly lower to favourites.

For main football markets with modest margins, the differences are usually small, often under one percentage point. We use the proportional method throughout, including in our back-test against closing odds, because it is transparent and easy to reproduce. If you compare across methods, state which one you used.

Frequently asked questions

What is implied probability?: The chance of an outcome suggested by its odds. It is calculated as 1 ÷ decimal odds and includes the bookmaker's margin.
Why do implied probabilities add up to more than 100%?: Because bookmakers build in a margin. The amount over 100% is the overround; removing it gives margin-free or 'fair' probabilities.
How do I remove the bookmaker's margin?: The simplest method, the proportional method, divides each implied probability by the total. 50% in a book of 105% becomes 47.6%.
Is implied probability the real chance?: No. It is the bookmaker's price turned into a percentage. Even after removing the margin it is an estimate, though closing odds are a very strong one.

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