Bookmaker Margin Calculator
Enter the decimal odds for every outcome of a market and see the bookmaker's built-in margin (the overround), the payout percentage, and what the fair odds would be with the margin stripped out. Works for two-way markets such as tennis and three-way markets such as football 1X2.
Use decimal odds. For a two-way market (tennis, over/under, handicaps), leave the third field blank. Converting from fractional or American odds? Use our Odds Converter first.
Implied Probability & Fair Odds
| Outcome | Odds | Implied Probability | Fair Odds |
|---|---|---|---|
| Outcome 1 | 2.40 | 41.67% | 2.55 |
| Outcome 2 | 3.30 | 30.30% | 3.51 |
| Outcome 3 | 2.90 | 34.48% | 3.09 |
Fair odds show what each price would be if the bookmaker's margin were removed and the implied probabilities summed to exactly 100%.
Your Results
The lower the margin, the better the value. Margin = the sum of implied probabilities minus 100%.
How the Bookmaker Margin Is Calculated
Every decimal price converts to an implied probability: 1 ÷ odds. A fair market's implied probabilities would add up to exactly 100%. Bookmakers price every outcome slightly shorter than fair, so the sum lands above 100% — the excess is the margin, also called the overround or vigorish.
Take a football match priced at 2.40 for the home win, 3.30 for the draw and 2.90 for the away win. The implied probabilities are 1 ÷ 2.40 = 41.67%, 1 ÷ 3.30 = 30.30% and 1 ÷ 2.90 = 34.48%. They sum to 106.45%, so the margin is 6.45% and the payout percentage is 100 ÷ 1.0645 = 93.94%. On average, for every £100 staked across this market in proportion to the odds, the bookmaker expects to pay back £93.94.
Margins on Two-Way Markets
Two-way markets make margins easy to compare at a glance. When both sides of a coin-flip market are priced the same, the payout percentage is simply the odds divided by two:
| Both Sides Priced At | Margin | Payout |
|---|---|---|
| 1.98 / 1.98 | 1.01% | 99.0% |
| 1.95 / 1.95 | 2.56% | 97.5% |
| 1.91 / 1.91 | 4.71% | 95.5% |
| 1.87 / 1.87 | 6.95% | 93.5% |
| 1.83 / 1.83 | 9.29% | 91.5% |
The difference between 1.98 and 1.83 on the same market looks small on the betting slip but compounds into a ninefold difference in margin. Comparing prices across bookmakers before placing a bet is the single cheapest way to improve long-term results.
Frequently Asked Questions
What is a bookmaker margin?
The margin (or overround) is the bookmaker's built-in edge: the amount by which the implied probabilities of all outcomes exceed 100%. A market whose implied probabilities sum to 106.45% carries a 6.45% margin — that surplus is how the bookmaker profits regardless of the result.
What counts as a low margin?
On major football and tennis markets, anything under 3% is genuinely low, 4–6% is typical, and above 7% is expensive. Margins are usually higher on lower-league football, outright markets and niche sports, where bookmakers carry more pricing risk.
How do I convert decimal odds to implied probability?
Divide 1 by the decimal odds: 2.50 implies 1 ÷ 2.50 = 40%. To convert between decimal, fractional and American formats first, use the Odds Converter.
Why do margins hit accumulators so hard?
Margins multiply with every leg. Five legs each priced at a 95.5% payout give a combined payout of just 79.4% — an effective margin of 20.6% on the whole bet. Work out combined odds with the Accumulator Calculator and keep an eye on how many legs you add.
Knowing the margin helps you find better prices, but no price makes gambling a reliable way to make money. 18+. Please bet responsibly — our responsible gambling resources are always available.