Bookmaker margin calculator: implied probability, overround and hold
Set home, draw and away prices and see their implied probabilities stacked against a fair 100% line. You will learn how the overround becomes an expected loss per $100 staked, and how it compounds when selections are combined.
How it works
Decimal odds imply a probability: implied probability = 1 ÷ decimal odds, so a price of 2.10 implies 47.6%. In a fair market the implied probabilities of all outcomes would total exactly 100%. The default prices of 2.10, 3.40 and 3.80 total 103.3%, and the 3.3 points above 100% are the overround, or margin. The expected-loss readout uses the hold instead: hold = 1 − 1 ÷ book total, which is $3.24 per $100 staked at the defaults. The two figures differ slightly because the margin is measured in probability and the hold in money staked.
After a quick prediction, three sliders set the home, draw and away prices anywhere from 1.20 to 12.00, with the implied percentage shown beside each. The bar stacks the three probabilities against a marker labelled '100% · fair', and the band past it is the margin. 'Strip the margin' divides each probability by the book total, so the bar fits exactly 100% and shows the market with the margin removed in proportion. Below, the parlay panel applies the same hold to one to five legs, using combined hold = 1 − (1 − hold) to the power of the number of legs.
Odds are easy to read as the bookmaker's honest estimate of each outcome. Implied chances that add up to more than 100% cannot all be true probabilities, so on average the prices are set below fair odds. Combining selections multiplies the effect, from a 3.2% hold on one leg to 15.2% on five at the default prices. Real results show a similar gap: New Jersey's Division of Gaming Enforcement reported that in 2025 operators kept 19.2% of the money staked on parlays for completed events, against 3.8% to 5.2% on football, basketball and baseball bets.
What it leaves out: The figures assume the margin is spread across outcomes in proportion to their prices and that parlay legs are independent with the same hold, while research on US football betting found bookmakers set prices and then take positions rather than simply balancing their books. This is an educational calculator for adults (18+), and it cannot alter any bookmaker's margin.
Sources
Checked on 2026-11-05. Independent, not affiliated. 18+.
- New Jersey Division of Gaming Enforcement: DGE Announces December 2025 Gaming Revenue Results, sports wagering statistics, completed events by category (16 January 2026, PDF)nj.gov
- NBER: Levitt, How Do Markets Function? An Empirical Analysis of Gambling on the National Football League, Working Paper 9422 (January 2003); published in the Economic Journal 114(495), 2004nber.org
Embed it
Free to embed on your own page, with no tracking or cookies in the frame. Keep the attribution link.