The three odds formats
Fractional odds such as 5/2 show your profit against your stake: win £5 for every £2 staked. They're the traditional format in UK and Irish racing.
Decimal odds such as 3.50 show your total return for every £1 staked, including the stake. They're the simplest to work with, which is why betting exchanges and most of Europe use them.
American odds are based on £100 (or $100). A plus sign such as +150 shows the profit on a 100 stake, and a minus sign such as −200 shows how much you need to stake to win 100.
How to convert odds yourself
To turn fractional into decimal, divide the first number by the second and add 1: 5/2 is 2.5 plus 1, so 3.50. To go the other way, take 1 off the decimal odds and turn what's left into a fraction: 3.50 minus 1 is 2.5, which is 5/2.
For American odds from decimal, prices of 2.0 or more are (decimal − 1) × 100 with a plus sign, so 3.50 is +250. Prices under 2.0 are 100 ÷ (decimal − 1) with a minus sign, so 1.50 is −200.
Implied probability is 1 divided by the decimal odds. At 3.50, that's 1 ÷ 3.5, or 28.6%.
Odds conversion chart
The most common prices in all three formats, with the chance each one implies:
| Fractional | Decimal | American | Implied probability |
|---|---|---|---|
| 1/5 | 1.20 | -500 | 83.3% |
| 1/4 | 1.25 | -400 | 80.0% |
| 1/3 | 1.33 | -300 | 75.0% |
| 2/5 | 1.40 | -250 | 71.4% |
| 1/2 | 1.50 | -200 | 66.7% |
| 4/7 | 1.57 | -175 | 63.6% |
| 4/6 | 1.67 | -150 | 60.0% |
| 4/5 | 1.80 | -125 | 55.6% |
| 10/11 | 1.91 | -110 | 52.4% |
| Evens (1/1) | 2.00 | +100 | 50.0% |
| 11/10 | 2.10 | +110 | 47.6% |
| 6/5 | 2.20 | +120 | 45.5% |
| 5/4 | 2.25 | +125 | 44.4% |
| 11/8 | 2.38 | +137.5 | 42.1% |
| 6/4 | 2.50 | +150 | 40.0% |
| 13/8 | 2.63 | +162.5 | 38.1% |
| 7/4 | 2.75 | +175 | 36.4% |
| 15/8 | 2.88 | +187.5 | 34.8% |
| 2/1 | 3.00 | +200 | 33.3% |
| 9/4 | 3.25 | +225 | 30.8% |
| 5/2 | 3.50 | +250 | 28.6% |
| 11/4 | 3.75 | +275 | 26.7% |
| 3/1 | 4.00 | +300 | 25.0% |
| 100/30 | 4.33 | +333.33 | 23.1% |
| 7/2 | 4.50 | +350 | 22.2% |
| 4/1 | 5.00 | +400 | 20.0% |
| 9/2 | 5.50 | +450 | 18.2% |
| 5/1 | 6.00 | +500 | 16.7% |
| 11/2 | 6.50 | +550 | 15.4% |
| 6/1 | 7.00 | +600 | 14.3% |
| 7/1 | 8.00 | +700 | 12.5% |
| 8/1 | 9.00 | +800 | 11.1% |
| 9/1 | 10.00 | +900 | 10.0% |
| 10/1 | 11.00 | +1000 | 9.1% |
| 12/1 | 13.00 | +1200 | 7.7% |
| 14/1 | 15.00 | +1400 | 6.7% |
| 16/1 | 17.00 | +1600 | 5.9% |
| 20/1 | 21.00 | +2000 | 4.8% |
| 25/1 | 26.00 | +2500 | 3.8% |
| 33/1 | 34.00 | +3300 | 2.9% |
| 50/1 | 51.00 | +5000 | 2.0% |
| 100/1 | 101.00 | +10000 | 1.0% |
Implied probability and the bookmaker's margin
Implied probability tells you how likely the odds say something is. It's the bookmaker's view plus their margin, not the true chance. In a two-way market priced at 10/11 on each side, each price implies 52.4%, which adds up to 104.8%. The extra 4.8% is the margin that makes the market profitable for the bookmaker, and it's why the true chance of any outcome is usually a little lower than its implied probability.
Common questions
Which odds format is best?
None is better, they just show the same price differently. Fractional odds are traditional in UK racing, decimal odds are easiest for working out returns, and American odds are used by US bookmakers. Most bookmakers let you switch formats in your account settings.
Why don't the implied probabilities in a market add up to 100%?
Because the bookmaker builds a margin into the odds. Add up the implied probabilities of every outcome in a market and you'll usually get more than 100%. The amount over 100% is roughly the bookmaker's edge.
What does odds-on mean?
Odds shorter than evens, such as 1/2 or 4/5. You win less than you stake, and the implied probability is over 50%. In decimal, odds-on prices are below 2.0, and in American format they start with a minus sign.