Betting Odds Explained: Decimal, Fractional, American and Implied Probability
6 min read · updated 2 September 2026
Odds are just a price. They tell you what a bookmaker will pay if your bet wins, and — read the other way — what chance that bookmaker's price implies. Every format you will meet is the same information wearing different clothes: decimal, fractional and American odds all describe one number, the probability the outcome is priced at.
Learning to move between the formats and, more importantly, to read the probability behind them is the first real skill in betting. It is what lets you compare a price against your own opinion and decide whether there is any value in the bet at all.
Decimal odds
Decimal odds show the total return per unit staked, stake included. A price of 2.50 means a 1 stake returns 2.50 — your 1 back plus 1.50 profit. Multiply your stake by the decimal odds and that is what lands in your account if the bet wins.
Decimal is the format used across most of Europe and Australia and on every betting exchange, because it makes accumulators and staking maths trivial: multiply the legs together for the combined price, divide 1 by the odds for the implied probability.
Fractional odds
Fractional odds, traditional in the UK and Ireland, show profit relative to stake. 3/2 ("three to two") means a 2 stake wins 3 profit, returning 5 in total. 6/4 is the same price shown with a different denominator; 10/11 is an odds-on price where you stake 11 to win 10.
To convert to decimal, divide the fraction and add one: 3/2 becomes 1.5 + 1 = 2.50. Sportsbooks round fractional odds to the nearest conventional fraction, so they are slightly less precise than the decimal or American equivalents.
American (moneyline) odds
American odds use a positive or negative number based around a 100 stake. A positive number is the profit on a 100 stake: +150 wins 150 profit. A negative number is the stake needed to win 100: -200 means risking 200 to win 100.
Favourites carry negative odds, underdogs positive. To convert: for positive odds, decimal = (american / 100) + 1; for negative odds, decimal = (100 / -american) + 1. So +150 is 2.50 and -200 is 1.50.
Implied probability and the bookmaker's margin
Divide 1 by the decimal odds and you get the implied probability — the chance the price corresponds to. 2.50 implies 40%, 1.50 implies about 66.7%, 5.00 implies 20%.
Add up the implied probabilities of every outcome in a market and you will get a number above 100%. That excess is the bookmaker's margin, or overround — the built-in edge. A two-way market priced 1.91 / 1.91 sums to about 104.7%, so the bookmaker holds roughly a 4.5% margin. Removing that margin gives you the "fair" odds, which is what you should actually compare your opinion against.
- Decimal 2.00 = fractional 1/1 (evens) = American +100 = 50% implied
- Decimal 1.50 = fractional 1/2 = American -200 = 66.7% implied
- Decimal 3.00 = fractional 2/1 = American +200 = 33.3% implied
FAQ
- Which odds format is best to use?
- Decimal, for anyone doing the maths. It makes implied probability, accumulator prices and stake sizing a single calculation. Fractional and American are mostly regional conventions on top of the same number.
- What does a 'fair price' mean?
- The odds with the bookmaker's margin stripped out, so the implied probabilities of all outcomes sum to exactly 100%. It is the true break-even price for the bet, and the benchmark you compare your own estimate against.
- How do I convert odds to a percentage quickly?
- Divide 1 by the decimal odds and multiply by 100. For fractional a/b, the implied probability is b / (a + b). For American, positive odds give 100 / (odds + 100) and negative give -odds / (-odds + 100).