How to Calculate Bookmaker Overround

Step-by-step calculation

For a three-selection market at 1.80, 3.60 and 4.20:

1. Home: 1 / 1.80 x 100 = 55.56%.
2. Draw: 1 / 3.60 x 100 = 27.78%.
3. Away: 1 / 4.20 x 100 = 23.81%.
4. Total: 55.56% + 27.78% + 23.81% = 107.15%.
5. Overround: 107.15 - 100 = 7.15 percentage points.

Before interpreting the result, confirm that all selections were available, current and actually bettable at those prices.

A more typical comparison

Book B has the lower displayed overround for this snapshot. That does not mean every individual selection is better at Book B, so compare the specific price as well as the market total.

Conditions for a valid comparison

• Use the same market definition and settlement period.
• Include every mutually exclusive selection.
• Capture prices at the same time.
• Distinguish available prices from indicative or stale prices.
• Treat exchange commission separately. Betfair's commission guide describes one exchange model in which commission applies to net market winnings rather than being embedded as one fixed overround.

What overround cannot tell you

Overround does not identify how margin is distributed across selections. It does not prove the true probabilities, operator profit on the event, or the expected value of a particular accepted bet. Expected value still requires a probability estimate and the actual payoff; OpenStax gives the underlying expected-value definition.

Compare the market and the selection separately

A lower market overround does not guarantee that the desired selection has the higher price. Consider two complete three-way books. Book A may distribute more of its margin to the favourite while Book B applies more to the draw. The total summarizes the book, but the accepted payoff is determined by the individual selection. A practical comparison therefore records both the full-market total and the target price.

When prices are changing, save one timestamp for the snapshot. Combining the home price from 10:00, the draw from 11:00 and the away price from 12:00 creates a synthetic book that may never have existed. Likewise, omitting an unappealing or suspended selection understates the sum.

Audit a reported overround

1. Confirm the selections are exhaustive and mutually exclusive.
2. Convert every price with the same precision.
3. Add the raw reciprocals before rounding the displayed total.
4. Subtract exactly 100 percentage points.
5. Record source, timestamp, available size and settlement period.
6. Recalculate after any price changes rather than editing one cell in isolation.

If an exchange is included, embedded overround and commission are different cost structures. The comparison should model the applicable commission on the net market outcome, as described for one product in Betfair's commission guidance, instead of pretending it is one fixed extra percentage on every quoted price.

Related resources

Continue with implied probability or the odds comparison workflow.

Continue learning

• Next guide: How to Calculate Implied Probability from Betting Odds
• Related guide: How to Investigate an Outlier Odds Price
• Definition: Overround Explained

Assumptions and limitations

Examples are illustrative snapshots and may not resemble a current market. Rounding can shift the reported total by a few hundredths of a percentage point. Promotional prices, maximum stakes and commission need separate treatment.