Overround & Bookmaker Margin Calculator (AUD)
Enter a market’s prices to get the book percentage, the margin, and the fair odds.
The “overround” (or book percentage) is the sum of the implied probabilities across a market. Anything above 100% is the bookmaker’s margin — the built-in edge that makes the prices add up to more than certainty.
Enter the two or three prices in a market and this calculator returns the book percentage, the margin, and the margin-free “fair” odds for each outcome.
How it works
Each price implies a probability of 1 ÷ odds. Add them up and you get the book: a fair market sums to exactly 100%, and the excess is the margin. Tighter markets (low margin) are better value; a book under 100% is a rare arbitrage.
Stripping the margin out proportionally gives the fair odds — what the price would be with the bookmaker’s edge removed. Comparing the offered price to the fair price is the first step in spotting value.
book % = Σ (1 ÷ odds) × 100; margin = book % − 100%
Worked example
- Implied: 52.6% + 52.6% = 105.3% book
- Bookmaker margin: ~5.3%
- Fair odds (margin removed): $2.00 each
A standard ~5% two-way market. Under 100% would be an arbitrage.
FAQ
- What is the overround?
- The sum of the implied probabilities (1 ÷ odds) across a market, as a percentage. Above 100% is the bookmaker’s margin; the excess over 100% is their built-in edge.
- What is a good margin?
- Lower is better value. Sharp two-way markets can be ~2–5%; multi-runner racing markets are much higher. Comparing margins across bookmakers shows who’s tightest.
- What are “fair odds”?
- The price with the margin removed proportionally — what each outcome would pay in a 100% market. It’s the baseline you compare an offered price against to look for value.
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