How to calculate margin in an AFL head-to-head market

Take both sides from the same bookmaker

For a two-outcome head-to-head market, convert each decimal price to an implied fraction by dividing one by the price. Add the two fractions and subtract one. A positive result is the quoted market's overround, often called its margin. Use prices from the same bookmaker, match and capture time.

Taking the highest club A price from one book and the highest club B price from another answers a different question: what the best cross-book pair looks like. It does not describe either bookmaker's individual margin.

What the number can and cannot tell you

A smaller overround means the two displayed prices sit closer to a 100% implied total. It does not identify which club will win. It also does not mean every selection at that bookmaker is better than the alternatives: compare the exact club price across books for the bet you are considering.

Check how the bookmaker treats any exceptional result or abandoned match. The arithmetic below assumes a standard two-outcome market with both club prices displayed. If a side is missing, there is no complete quoted market for this calculation.

Use the round board carefully

Confirm the official AFL fixture, keep the selected club and head-to-head market fixed, and check price capture time. The round board can help find a higher price on one club; the margin calculation gives context for one bookmaker's full market.

• Do not mix prices from different books or timestamps.
• Calculate with both displayed sides of one complete market.
• Treat overround as pricing context, not a probability forecast.