A bookmaker margin calculator tells you how much of every pound staked the bookie expects to keep, and what the odds would be if they kept nothing. Type the price of every outcome in a market, two for a tennis match or three for a football match, and the free calculator above gives the overround, the margin and the fair odds for each outcome.
It starts on a classic two-way market: 1.91 on both sides. That’s a 4.71% margin, and the fair price for each side is 2.00.
Bookmaker margin calculator: the maths in two steps
Every decimal price implies a chance: 1 ÷ odds. 1.91 implies 52.36%. Add up the implied chances for every outcome in the market and you get the overround, sometimes called the book percentage. If the market were fair it would be exactly 100%. Anything above that is the margin.
A football example: home 2.10, draw 3.40, away 3.60.
- Implied chances: 47.62% + 29.41% + 27.78% = 104.81%
- Margin: 4.81%
- Payout: 100 ÷ 104.81 = 95.41%. Stake £100 across the market in the right proportions and £95.41 comes back.
Getting the fair odds
To take the margin out, scale each implied chance down so the total is 100%. 47.62% ÷ 1.0481 is 45.43%, so the fair price of the home win is 2.20 rather than 2.10. The draw becomes 3.56 and the away win 3.77. A quick shortcut: fair odds = odds × overround.
That’s the proportional method, and it’s the one this calculator uses. It isn’t perfect. Bookmakers tend to load more of their margin onto longshots than favourites, so the true price of an outsider is often a little longer than the proportional figure. For two or three outcomes near the same price the difference is small. In a 20-runner horse race it can matter.
Overround, margin and no-margin odds
The same numbers turn up under a handful of names, which is why people search for an overround calculator, a margin calculator or a fair odds calculator and land on the same sum. Overround, or book percentage, is the total: 104.81% in the football example. The margin is the part above 100%, here 4.81%. In North America the margin is usually called the vig or juice, and fair odds are called no-vig odds. Whatever the name, this page works them all out at once.
One thing to watch: the overround only means something when every outcome in the market is included. Leave out the draw in a football match and 2.10 and 3.60 add up to just 75%, which looks like a huge bargain and isn’t.
Why the margin is worth checking
The margin is the cost of the bet. The same match can carry 3% at one bookie and 8% at another, and over a season that gap is far bigger than most people’s edge. Comparing margins is also how you spot a good price: if one bookie’s odds are longer than the fair odds you’ve worked out from a sharp market, that’s a bet with value.
If the overround comes out under 100% across bookies, you’ve found an arbitrage. The arbitrage calculator works out the stakes. The odds converter switches between fractional, decimal and American, and the Asian handicap calculator settles two-way handicap lines.
A lower margin makes a bet cheaper. It doesn’t make it a winner. BeGambleAware.org is free and confidential if it stops being fun.