Implied probability calculator: odds to %, margin and fair odds

Implied probability and margin calculator

Key facts

Implied probability = 1 / decimal odds. $2.80 implies 35.71%; $1.45 implies 68.97%.
Add the implied chances of every outcome: the amount over 100% is the overround. $1.45 and $2.80 add up to 104.68%.
The share of all stakes the bookmaker keeps on a balanced book is 1 - 1 / total: 4.47% for that market.
Fair price = odds x market total. Without the margin, $2.80 becomes $2.93 and $1.45 becomes $1.52.
Betfair says its average overround on 2021 Australian Group 1 races before the jump was about 103%, against 115% to 120% at some large bookmakers.

How to use the implied probability calculator

Enter the decimal price of every outcome in one market: both teams in a head to head, all three results in a soccer match, or every runner in a race. Add a row for each extra price. The calculator shows what the market adds up to, the bookmaker margin, the implied chance of each price, that chance with the margin removed, and the fair price that goes with it.

It only works when you enter the whole market. Leave out a runner and the total drops, which makes the margin look smaller than it is.

The implied probability formula

Implied probability is 1 divided by the decimal odds. Australian prices include your stake, so $2.80 means $2.80 back for every $1, and 1 / 2.80 = 0.3571, or 35.71%.

That number does two jobs. It is the chance the price suggests, and it is the strike rate you need to break even. If you only ever back runners at $2.80, you need to win more than 35.71% of the time to come out ahead. Use the odds converter if your price is in fractional or American format.

Bookmaker margin calculator: worked example

Take an AFL head to head at $1.45 and $2.80. Each price on its own looks reasonable. Added together, the market shows what you are paying.

Price	Implied chance	Chance without margin	Fair price
$1.45	68.97%	65.88%	$1.52
$2.80	35.71%	34.12%	$2.93
Total	104.68%	100%

The market adds up to 104.68%, so the overround is 4.68%. To strip the margin, divide each implied chance by 1.0468, or multiply each price by 1.0468. The calculator spreads the margin across outcomes in proportion to their chance, which is the simplest of several methods.

Overround or hold: two ways to state the margin

The same market can be described by two different numbers, and both are right. The overround is the total minus 100%. The hold is what the bookmaker keeps from all the money if bets come in exactly in line with its prices, and it is 1 - 1 / total.

Market adds up to	Overround	Hold (share of all stakes kept)
103%	3%	2.91%
104.68%	4.68%	4.47%
105.26%	5.26%	5.00%
111.12%	11.12%	10.01%
115%	15%	13.04%
120%	20%	16.67%

The hold is the "implied odds with vig" figure that matters to your wallet: it is what you would lose, on average, if you dutched every outcome. The dutching calculator shows the same thing from the punter's side.

Racing markets can carry much more margin

A race has many outcomes and every price carries part of the margin, so the total can climb well past a two-way footy market. Here is an example six runner race.

Runner	Price	Implied	Without margin	Fair price
1	$2.40	41.67%	37.50%	$2.67
2	$3.80	26.32%	23.68%	$4.22
3	$5.50	18.18%	16.36%	$6.11
4	$8.00	12.50%	11.25%	$8.89
5	$13.00	7.69%	6.92%	$14.45
6	$21.00	4.76%	4.29%	$23.33

This market totals 111.12%, more than double the margin in the footy example. Betfair publishes its own comparison: it says its average overround on 2021 Australian Group 1 thoroughbred races before the jump was about 103%, against somewhere between 115% and 120% at some large bookmakers. Keep in mind that Betfair charges commission on net winnings, so a low overround on the exchange is not free money either.

Pundit tip: compare the market total between two bookmakers before you compare one price. A bookmaker whose race adds up to 112% is charging more across the whole card, even if one runner happens to be longer.

Turning implied probability into a decision

A price is only good value if your own estimate of the chance beats the implied chance. If you rate a team at 40% and the price is $2.80 (35.71%), the price is generous by your numbers; at $2.20 (45.45%) it is not. The expected value calculator shows what that gap is worth per bet, the Kelly criterion calculator turns it into a stake size, and our bankroll management guide compares staking plans.

Mixing the best prices from several bookmakers can push a market total under 100%. When that happens, the arbitrage calculator splits your stake so every outcome returns the same amount. For more on reading prices, see how to read odds, and for every calculator in one place, our bet calculators page.

Frequently asked questions

How do you calculate implied probability from odds?

Divide 1 by the decimal odds and multiply by 100. $4.00 is 25%, $2.50 is 40% and $1.80 is 55.56%. That figure is also your break-even strike rate: back $4.00 shots and you need more than 1 in 4 to win to show a profit.

What is a bookmaker margin?

It is the amount by which the implied chances in a market add up to more than 100%. Two teams at $1.90 each imply 52.63% apiece, 105.26% in total, so the margin is 5.26%. It is how the bookmaker gets paid whichever side wins.

How do I remove the vig from odds?

Divide each implied chance by the market total, or multiply each price by the market total. In a $1.90 and $1.90 market the total is 1.0526, so each fair price is $1.90 x 1.0526 = $2.00.

What does an overround of 110% mean?

It means the prices imply chances totalling 110%. If you backed every runner to collect $100, you would outlay $110. On a balanced book the bookmaker keeps 1 - 1 / 1.10 = 9.09% of all stakes.

Can a market add up to less than 100%?

Not at one bookmaker in normal conditions. If you take the best price for each outcome from different bookmakers, the total can dip under 100%, which is an arbitrage. Our arbitrage calculator works out the stakes.