Overround and margin: what a sportsbook really keeps on an over/under totals line
Economics
Priced at 1.87, 1.95, the market has an overround of 4.76%, and a book that takes money in proportion keeps 4.54% of all stakes.
- 4.76%overround
- 4.54%margin of a balanced book
- 1.96fair odds of outcome 1
- 2.04fair odds of outcome 2
Overround versus actual margin
The lead shows an overround for quoted odds on both sides. This figure represents the sum of implied probabilities exceeding unity. The margin kept by a balanced book is lower. The table below details these values alongside the fair odds derived from them. The difference arises because the overround is a static calculation from quoted prices, while the margin reflects the actual retention when stakes are perfectly balanced across outcomes. Traders use this distinction to assess pricing efficiency against theoretical expectations.
| Outcome | Quoted odds | Implied probability | Fair odds |
|---|---|---|---|
| Outcome 1 | 1.87 | 53.5% | 1.96 |
| Outcome 2 | 1.95 | 51.3% | 2.04 |
Deriving fair odds
Fair odds are calculated by removing the margin proportionally from the implied probabilities. As the table above shows, the fair odds for outcome one are higher than the quoted price. Outcome two has fair odds higher than its quoted price. This adjustment normalizes the probabilities to sum to unity. The result provides a benchmark for the true likelihood of each outcome without the bookmaker's built-in advantage. These figures illustrate the formula rather than serving as market averages.
Practical application for traders
Traders compare quoted prices against fair odds to identify value. The fair odds for outcome two are larger than those for outcome one, reflecting the different implied probabilities. A balanced book is an idealisation; real books are rarely perfectly balanced. Consequently, actual hold often differs from the priced margin. See the regulation tracker for further context on compliance standards affecting these calculations.
Questions
Why is the margin lower than the overround?
The overround sums implied probabilities from quoted odds. The margin reflects actual retention when stakes are balanced. Perfect balance is rare, so actual hold varies from the theoretical overround calculation.
How are fair odds calculated?
Fair odds are derived by removing the margin proportionally from implied probabilities. This normalizes the probabilities to sum to unity, providing a benchmark without the bookmaker's advantage.
Are these odds typical market averages?
No. The odds provided illustrate the calculation formula. They are not representative of industry averages or specific market conditions.
Every figure on this page is computed by code from standard industry formulas and the facts of our regulation tracker, each checked a second way. See the methodology.