Betting Margin Calculator: See What the Bookmaker Keeps
You can read a price, but not the cut inside it. Enter the decimal odds of one betting market and see the bookmaker's margin, the fair odds without it, and the same prices after exchange commission.
Enter the odds
A betting margin (or overround) is the bookmaker's cut, built into the odds. Divide 1 by each decimal price, add the results, and anything above 100% is the margin. Take a match priced 1.95, 3.60 and 4.20: 51.28% + 27.78% + 23.81% = 102.87%, a margin of 2.87%. Divide each share by 102.87% to remove it and the fair odds are 2.01, 3.70 and 4.32. On Betfair Exchange, commission is charged on your net winnings in a market, so the calculator adds a third slip: each price as a single bet after an exchange commission rate that you type. Decimal odds, one market at a time.
Reading the overround, the fair odds and the total after exchange commission
The result shows three slips for the same prices: as typed, with the margin removed, and after exchange commission with each price treated as a single bet. Each has a big figure, its total, and a line per outcome: the odds, then the implied probability.
- Implied probability. 1 ÷ the odds.
- Overround and margin. The implied probabilities added up. A fair market adds to 100%, and the margin is the total minus 100 points.
- Fair odds. Each implied probability divided by the total, turned back into odds, which equals your odds × the overround. This proportional method takes the same share off every price. Other methods exist and give different fair odds for long shots.
- Total after exchange commission. Commission taken off each price's winnings, then the same sum. The formula is printed under the result.
The rate box starts at 2, the Basic package rate in Betfair's own worked example, where £100 of net profit costs £2.00 of exchange commission. It is an example, so type your own. In regions with Betfair's rewards packages the rate follows the package you pick, and if you choose none Betfair can move you to another, so check the current terms. Elsewhere it is a market base rate reduced by a discount.
Two worked examples to repeat
Both use invented prices (no bookmaker quoted them). Type them in and the tool prints exactly these figures.
Example 1: a football result at a bookmaker
| Outcome | Odds | Implied probability | Fair probability | Fair odds |
|---|---|---|---|---|
| 1, home | 1.95 | 51.28% | 49.85% | 2.01 |
| 2, draw | 3.60 | 27.78% | 27.00% | 3.70 |
| 3, away | 4.20 | 23.81% | 23.15% | 4.32 |
| Total | 102.87% | 100.00% |
The margin is 102.87% minus 100%, so 2.87%. The home price is 1.95 where the fair price is 2.01: that gap is the cut. Leave the rate box at 2 and the third slip reads 104.15%. Read that as a what-if: a bookmaker pays no exchange commission, so example 2 uses exchange prices.
Example 2: one tennis match, bookmaker against exchange
| Line | Odds | Overround before exchange commission | Total after 2% exchange commission | Total after 5% exchange commission |
|---|---|---|---|---|
| Bookmaker | 1.70 and 2.30 | 102.30% | not charged | not charged |
| Exchange, best back prices | 1.72 and 2.36 | 100.51% | 101.50% | 103.01% |
Before commission the exchange line looks better: 100.51% against 102.30%. After 2% exchange commission on each price as a single bet the total is 101.50%, still below; at 5% it is 103.01%, above. The total ties with the bookmaker at about 3.6%. Selection by selection the tips differ: about 2.8% for 1.72 against 1.70, about 4.4% for 2.36 against 2.30. Compare the odds after exchange commission line by line as well as the total. Back both outcomes and no exchange commission is due: the cost is the 0.51% overround.
Picture a bettor who backs the 1.72 on the exchange because it beats the bookmaker's 1.70, pays a rate of 5%, and only afterwards works out that after exchange commission the price was worth 1.684, below the 1.70, and that the 2.36 would have been worth 2.292, below the 2.30. The comparison takes ten seconds, so do it before the bet.
Nobody bills you for the margin. You accept it with the price.
Where the sums stop
Start with the third slip, because it is the easiest to misread. Exchange commission is charged on your net winnings in a market, not bet by bet, and none is due on a market you lose. The slip asks what a price is worth if it wins. Back every outcome of one market at prices totalling 100% or more and you lose whichever wins, so no exchange commission is due; this total overstates the cost of covering the market. Use it to compare one selection at a time.
The rate is yours to type, and it covers exchange commission only: the calculator adds no deposit or withdrawal fee, no Expert Fee (a separate weekly charge that only very profitable accounts meet) and none of a broker's own fees.
What to do with the number
If the exchange price still looks better after commission, the next question is how to reach an exchange at all. The broker comparison sets out the routes and the limit on each, and getting into Betfair Exchange, step by step covers the account itself. Every route involves ID checks, and betting is for adults only.
The tools page has a worked case of exchange commission netted across two bets in one market, and the access route finder sorts the routes in five questions.
Questions people ask
How do I calculate overround, and is it the same as margin?
Turn each decimal price into a share, 1 ÷ the odds, then add the shares. The total is the overround and the margin is what sits above 100%: 102.87% and 2.87% for 1.95, 3.60 and 4.20. The two words are used loosely, so check which figure a page means.
How do I get fair odds, or no-vig odds, from bookmaker odds?
Divide each implied probability by the total and turn the result back into odds, which is the same as multiplying each price by the overround. For 1.95, 3.60 and 4.20 that gives 2.01, 3.70 and 4.32. No-vig odds are the same idea under another name.
Do betting exchanges have a margin, and how do I add commission?
Not one that a bookmaker builds in: the prices come from other customers, and Betfair calls itself a facilitator, with a few stated exceptions. On an exchange, a total above 100% mostly shows the gap between other customers' back and lay offers, and the fee is the exchange commission you enter. Type the exchange's back prices into the tool and add your own exchange commission rate; it depends on your account and region, so check the current terms. How exchanges work has more.
Which odds formats does it take?
The calculator reads decimal odds, so convert fractional odds first: 1 + the top number ÷ the bottom number, so 6/4 is 2.50. American +150 is 2.50 and -200 is 1.50.