Odds tell you two things: how much a winning bet pays, and how likely the bookmaker thinks the outcome is. British bookmakers traditionally quote fractional odds such as 5/2, betting exchanges use decimal odds such as 3.50, and many betting apps let you switch between the two. Both formats describe exactly the same price.
Fractional odds
Fractional odds show profit against stake. At 5/2 you win £5 for every £2 staked, so a £10 bet returns £35: £25 profit plus your £10 back. Odds-on prices such as 1/2 pay less profit than you stake, so £10 at 1/2 returns £15. Evens (1/1) doubles your money.
Decimal odds
Decimal odds show the total return for every £1 staked, stake included. 3.50 means £3.50 back for each £1, so £10 returns £35, the same as 5/2. To convert a fraction, divide the top number by the bottom and add 1: 5/2 is 2.5 plus 1, or 3.50.
| Fractional | Decimal | Implied probability |
|---|---|---|
| 1/2 | 1.50 | 66.7% |
| 4/5 | 1.80 | 55.6% |
| Evens | 2.00 | 50.0% |
| 11/10 | 2.10 | 47.6% |
| 6/4 | 2.50 | 40.0% |
| 2/1 | 3.00 | 33.3% |
| 5/2 | 3.50 | 28.6% |
| 4/1 | 5.00 | 20.0% |
American odds, used by US sportsbooks, show the profit on a $100 stake (+250) or the stake needed to win $100 (-200). +250 is the same price as 5/2, and -200 is the same as 1/2.
Implied probability
Divide 1 by the decimal odds and multiply by 100 to get the chance a price implies. At 2.10 that is 47.6%; at 1.50, 66.7%. With fractional odds, divide the bottom number by the sum of both numbers: 5/2 is 2 divided by 7, or 28.6%.
The bookmaker margin
Add up the implied probabilities of every outcome in a market and the total comes to more than 100%. The extra is the bookmaker's margin, also called the overround. Take this match-result market on a made-up fixture:
| Outcome | Odds | Implied probability |
|---|---|---|
| Riverside Rovers | 2.10 (11/10) | 47.6% |
| Draw | 3.40 (12/5) | 29.4% |
| Northgate City | 3.60 (13/5) | 27.8% |
| Total | 104.8% |
That extra 4.8% is built into the prices, which is why picking at random loses money over time. It is also why comparing bookmakers matters: a slightly bigger price on the same bet takes back part of the margin.
What value means
A bet has value when you think its chance of winning is higher than the price implies. If you rate Riverside Rovers a 52% chance and can back them at 2.10 (47.6%), the bet has value: across many bets like it you would expect £1.09 back for every £1 staked. If you rate them at 45%, it doesn't, however confident the preview sounds. Value is an estimate, not a promise, and value bets still lose often. How we pick our tips explains how we check whether our estimates hold up.
FAQs
How do you convert fractional odds to decimal?
Divide the first number by the second and add 1. So 5/2 becomes 2.5 plus 1, or 3.50; 4/5 becomes 0.8 plus 1, or 1.80; and evens (1/1) becomes 2.00. The 1 is added because decimal odds include your stake.
What does odds-on mean?
Odds-on means the price is shorter than evens, such as 1/2 or 4/5, so a winning bet returns less profit than the stake. The bookmaker rates the outcome as more likely than not: 1/2 implies a 66.7% chance.
What is the overround in betting?
The overround is the bookmaker's built-in margin: the amount by which the implied probabilities of every outcome in a market add up to more than 100%. A match-result market whose prices total 104.8% has an overround of 4.8%.