Overround
The amount by which a market's implied probabilities add up to more than 100%, showing the margin built into the odds.
In a fair market, the implied probabilities of every possible outcome would add up to exactly 100%. Sportsbooks price slightly short on each outcome, so the total comes out higher. That excess is the overround, sometimes called the book percentage or margin.
Worked example: three-way NHL line
A regulation-time (60-minute) market has three outcomes because a tie after regulation is possible. Illustrative prices: home 2.20, tie 4.00, away 3.00.
| Outcome | Decimal | Implied |
|---|---|---|
| Home win | 2.20 | 45.45% |
| Tie | 4.00 | 25.00% |
| Away win | 3.00 | 33.33% |
| Total | 103.79% |
The overround is 3.79%. The payout rate on the market is 100 / 103.79 = 96.4%, so for every $100 spread across all three outcomes in proportion to their implied chances, about $96.35 comes back.
Removing it
To estimate fair odds, divide each implied probability by the total. Home: 45.45 / 103.79 = 43.8%, fair odds about 2.28. Tie: 24.1%, about 4.15. Away: 32.1%, about 3.11.
Why it matters
Overround is the quickest way to compare books. Futures and props usually carry far more than main-game lines, which is useful to know before you bet on long shots. See also vig.