Decimal, fractional and American odds all say the same thing in different ways: how much you win and how likely the bookmaker thinks it is. Here's how to read and convert them, and how to spot the margin.
3
formats
Implied probability
the key idea
Margin
what you pay
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Formats, probability and margin
Every odds format can be converted to an implied probability. Add up the implied probabilities of all outcomes in a market and you'll get more than 100%; the excess is the bookmaker's margin.
Decimal 2.50. Stake × 2.50 = total return. Implied probability 40%
Fractional 6/4. Win 6 for every 4 staked, plus stake back. Same as 2.50
American +150. Win 150 on a 100 stake. Same as 2.50
American −120. Stake 120 to win 100. Decimal 1.83, implied 54.5%
Decimal to probability: 1 ÷ odds. Fractional to decimal: (numerator ÷ denominator) + 1. American positive: (odds ÷ 100) + 1. American negative: (100 ÷ odds) + 1.
A tennis match at 1.90 each way: 52.6% + 52.6% = 105.3%, so the margin is about 5%. At 1.95 each way the margin is 2.6%. Lower margins mean better value; compare books on the same market.
Odds reflect money and opinion, not certainty. Short odds lose often enough to matter; long odds win occasionally. Expected value, not the price alone, decides whether a bet is worth it.
Bookmakers' margins vary by sport and market. Major football and NHL markets are tightest; niche markets and props are widest.
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Reviews are written and fact-checked by our editors. Last checked 25 October 2026.
4 questions
What's the easiest odds format?
Decimal: stake times odds equals total return.
What is implied probability?
The chance of an outcome implied by the odds: 1 divided by decimal odds.
What's a bookmaker margin?
The amount by which implied probabilities exceed 100%, typically 2–8%.
Which bookmakers have the lowest margins?
Large regulated books such as bet365 and the Betfair exchange tend to be tightest.
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