How to use
- 1Type the odds you know into the matching box: decimal (2.50), fractional (3/2), American (+150) or probability (40).
- 2The other formats update as you type.
- 3Enter a stake to see the total return and the profit.
- 4In the margin section, enter the odds for all 2 or 3 outcomes of a market to get the margin, the payout and the fair odds.
How the odds formats relate
Decimal odds show the total return per 1 unit staked and are standard in Europe, Australia and Canada. Fractional odds, traditional in the UK and Ireland, show only the profit: 3/2 means 3 units of profit for every 2 staked, which is 2.50 in decimal. American moneyline odds use 100 as the base: +150 wins 150 on a 100 stake, while −200 means you must stake 200 to win 100 (decimal 1.50). Implied probability is 1 ÷ decimal odds, so 2.50 implies 40%.
Bookmaker margin and fair odds
Add up the implied probabilities of every outcome in a market. Odds of 2.10, 3.40 and 3.60 give 47.62% + 29.41% + 27.78% = 104.81%; the 4.81% above 100% is the bookmaker’s margin, or overround. The payout is 100 ÷ 104.81 ≈ 95.41%. The tool removes the margin in proportion to each probability to show fair odds (2.20, 3.56 and 3.77 in this example). This proportional method is the common, simple approach — bookmakers may spread their margin differently in practice.
18+ and responsible gambling
This is a calculator for understanding odds; it is not betting advice and does not promise any profit. Gambling is only for adults — 18+ in the UK and 21+ in most US states — and only with licensed operators where it is legal. Never bet money you cannot afford to lose. If gambling stops being fun, free help is available from the National Gambling Helpline (GamCare) in the UK or 1-800-GAMBLER in the US.
Frequently asked questions
How do I convert fractional odds to decimal?
Divide the first number by the second and add 1. For 5/2: 5 ÷ 2 + 1 = 3.50. For 4/6 (odds-on): 4 ÷ 6 + 1 ≈ 1.67.
What does +200 mean in American odds?
A 100 stake wins 200 profit, so the total return is 300. In decimal that is 3.00, an implied probability of 33.3%. A minus figure such as −200 is the amount you stake to win 100.
How do you calculate implied probability from odds?
Divide 100 by the decimal odds: 2.00 implies 50%, 1.25 implies 80%. The figure includes the bookmaker’s margin, so the true probability the market assigns is slightly lower.
How is the bookmaker margin calculated?
Sum 1 ÷ odds for every outcome and subtract 1. A two-way market at 1.90 and 1.90 gives 1/1.90 + 1/1.90 = 1.0526, a margin of 5.26%.
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