This bookmaker margin calculator reads the margin out of a market from its prices alone. Type the decimal odds for every selection, one box each, and it shows the chance each price implies, the percentage they add to, and the margin inside.
Everything starts from one formula: implied probability = 1 / decimal odds. Add that across every selection and a market carrying no margin comes to exactly 100%. Anything above is the bookmaker's margin, and the maths behind it is set out in how betting odds work.
How to use the margin calculator
The tool opens with two boxes. Type a decimal price in each, press Add a selection for a third and Remove to drop one, or paste a column of prices to fill several boxes at once. It reads up to 30 selections.
Each box takes a decimal price from $1.01 to $10001, typed without a thousands comma, and prints the chance it implies. A blank box is skipped rather than refused, which is how you model a scratched runner or a part-built market, and the line under the boxes says how many prices were measured. A price outside the band turns red and is left out; under two readable prices leaves the tiles empty. The odds converter turns a fractional or American price, or a chance, into a decimal price.
Three tiles carry the answer: Market percentage is the total, Margin (overround) is that total less 100, and Kept on a balanced book is what a bookmaker keeps of turnover when the money arrives in the right proportions. Over 100% it is the smaller of the two.
The formulas behind the result
- implied probability = 1 / decimal odds
- market percentage = the sum of every selection's implied probability, x 100
- margin, or overround = market percentage - 100
- kept on a balanced book = 1 - 100 / market percentage
The last line is what a book balanced at those prices keeps of the money staked, and bookmaker margin explains why it sits under the overround.
A two way market worked through
With illustrative prices of $1.86 and $1.86:
| Selection | Price | Implied probability |
|---|---|---|
| Team A | $1.86 | 53.8% |
| Team B | $1.86 | 53.8% |
| Market | 107.5% |
The rows are rounded: they add to 107.6% while the tile reads 107.5%.
Working: 1 / 1.86 = 0.5376, so each price implies 53.8% and the two add to 1.0753, or 107.5%. The margin is 107.5 - 100 = 7.5%. The share kept on a balanced book is 1 - 100 / 107.5269 = 7.0%, the same as 1 - 1.86 / 2: $100 on each side is $200 held, and the winning side is paid $100 x 1.86 = $186, leaving $14, or 7.0% of the $200.
A race market worked through
Eight runners, with illustrative prices:
| Runner | Price | Implied probability |
|---|---|---|
| 1 | $2.80 | 35.7% |
| 2 | $4.50 | 22.2% |
| 3 | $6.00 | 16.7% |
| 4 | $9.00 | 11.1% |
| 5 | $11.00 | 9.1% |
| 6 | $15.00 | 6.7% |
| 7 | $21.00 | 4.8% |
| 8 | $34.00 | 2.9% |
| Market | 109.2% |
The eight chances add to 109.2%, a margin of 9.2%, and a book balanced there keeps 1 - 100 / 109.1749 = 8.4% of turnover. One price alone is implied probability; what a race total says is market percentage in horse racing.
Leaving a box blank for a scratched runner
Blank out the $9.00 runner above and the seven prices left add to 98.1%, because that price implied 11.1% and the book loses those 11.1 points: 109.2 - 11.1 = 98.1. The count under the boxes drops to 7, so a missing runner is visible.
After a real scratching the bookmaker frames the market again. The method used in New South Wales, the Northern Territory, Queensland and Victoria since 1 August 2011 restores the market percentage the field had before the withdrawal (Racing NSW, checked October 2026). Scratching deductions covers the deduction taken from bets already struck, and your bookmaker's terms say how it applies one.
Reading a market that adds to less than 100%
A total under 100% is a negative margin, and the calculator keeps the minus sign. The common cause is the incomplete market above, so check the count first.
The other is prices taken from more than one bookmaker, which is no longer one market at all: prices from two books can add to under 100%, the starting point of arbitrage betting in Australia. An arbitrage only works if every bet is accepted at the price shown, and bookmakers can limit stakes, void bets or close accounts under their terms.
How the calculator rounds
Every figure rounds once, at the end, to one decimal place, or two below 1%, so a $151 outsider shows 0.66% rather than 0.7%. The tiles add the unrounded chances, so the figures under the boxes can add to a different total.
With illustrative soccer prices of $2.30, $3.50 for the draw and $2.90, the boxes show 43.5%, 28.6% and 34.5%, adding to 106.6%. The tile reads 106.5%, because 1 / 2.30 + 1 / 3.50 + 1 / 2.90 is 43.478 + 28.571 + 34.483 = 106.532: a margin of 6.5%, and 1 - 100 / 106.532 = 6.1% kept.
Rounding early can shift the last digit at any margin, so the tool rounds only what it prints.
Mistakes that make a margin look wrong
| Mistake | What it does to the number | The fix |
|---|---|---|
| Leaving a runner out | Blanking the $9.00 runner above reads 98.1%, not 109.2% | Check the count against the field |
| Mixing two bookmakers' prices | Describes a market nobody offers, and can fall under 100% | Measure each book on its own |
| Reading the overround as the share kept | The race above is a 9.2% overround, but a balanced book keeps 8.4% | Read the third tile |
| Typing place prices | A fair three place market still shows a 200% margin | Read the total against 100% x the places |
| Comparing a two way total with a race total | Two different markets, so the totals are not comparable | Compare one market across bookmakers |
Lining up a market's prices side by side
The B337 Terminal shows prices from 40+ bookmakers across racing and sports side by side, with Betfair back and lay beside them, so one market's spread is in front of you already. Not every bookmaker prices every market, so check a column is complete before typing it into the tool above. A free account opens a limited view of the Terminal with live odds. Prices are read on a repeating cycle, not tick by tick, so a price on screen can trail the bookmaker's own: confirm it in your bookmaker account before you bet. Bookmaker names are trade marks of their owners. B337 is not affiliated with them.
Taking that margin back out to estimate a fair price is the next step: the fair odds calculator does it by four methods.
Risk: Betting involves risk. A low margin is not a good bet, a fair price taken from a market is an estimate, and a generous-looking price can still lose. See responsible gambling for limits and support.