AI Odds basics · Updated 11 Aug 2026
Betting odds explained: fractional, decimal and American prices
Odds tell you two things at once: what a bet pays and how likely the bookmaker thinks the outcome is. Once you can switch between the formats, every bet slip reads the same way.
Betting odds do two jobs. They tell you how much a winning bet pays, and they reveal how likely the bookmaker thinks an outcome is. UK bookmakers show odds in fractional format by default, most also offer decimal odds in the account settings, and American odds turn up on US sports. All three describe the same price in a different way.
This guide shows how to read each format, how to convert between them, and how to use them to judge whether a price is short or generous. Every example uses round numbers so you can check the arithmetic yourself.
Fractional odds
Fractional odds show profit relative to stake. The number on the left is what you win, the number on the right is what you stake.
- 5/1 (five to one): stake £1, win £5 profit. A £10 bet returns £50 profit + £10 stake = £60.
- 5/2 (five to two): stake £2, win £5 profit. A £10 bet returns £25 profit + £10 = £35.
- 1/2 (two to one on): stake £2, win £1 profit. A £10 bet returns £5 profit + £10 = £15.
When the first number is bigger, the outcome is “odds-against” and pays more than the stake. When the first number is smaller, it is “odds-on”. Evens, 1/1, pays the same as the stake.
The general rule: profit = stake x (left number / right number). Return = profit + stake.
Decimal odds
Decimal odds show the total return for every £1 staked, stake included. They are easier to multiply, which is why exchanges and most international sites use them.
- 6.00: a £10 bet returns £60 in total, £50 of which is profit.
- 3.50: a £10 bet returns £35, £25 profit.
- 1.50: a £10 bet returns £15, £5 profit.
Return = stake x decimal odds. Profit = return minus stake.
Converting between the two takes one step either way:
- Fractional to decimal: divide and add 1. 5/2 = 2.5 + 1 = 3.50.
- Decimal to fractional: subtract 1 and express as a fraction. 3.50 - 1 = 2.5 = 5/2.
American odds
American odds, also called moneyline odds, use a plus or minus sign around a base of 100.
- Positive (+250): the profit on a £100 stake. +250 means £100 wins £250 profit, the same as 5/2 or 3.50.
- Negative (-150): the stake needed to win £100 profit. -150 means you stake £150 to make £100, the same as 4/6 or 1.67.
To convert to decimal: for a positive number, divide by 100 and add 1 (+250 becomes 3.50). For a negative number, divide 100 by the number without its sign and add 1 (-150 becomes 100 / 150 + 1 = 1.67).
Common prices in all three formats
| Fractional | Decimal | American | Implied probability | £10 bet returns |
|---|---|---|---|---|
| 1/2 | 1.50 | -200 | 66.7% | £15.00 |
| 4/6 | 1.67 | -150 | 60.0% | £16.67 |
| 10/11 | 1.91 | -110 | 52.4% | £19.09 |
| Evens (1/1) | 2.00 | +100 | 50.0% | £20.00 |
| 6/4 | 2.50 | +150 | 40.0% | £25.00 |
| 2/1 | 3.00 | +200 | 33.3% | £30.00 |
| 5/2 | 3.50 | +250 | 28.6% | £35.00 |
| 3/1 | 4.00 | +300 | 25.0% | £40.00 |
| 5/1 | 6.00 | +500 | 16.7% | £60.00 |
| 10/1 | 11.00 | +1000 | 9.1% | £110.00 |
Implied probability: what the price says about the chances
Every price carries an implied probability. Divide 1 by the decimal odds and multiply by 100.
- 3/1 = 4.00, so 1 / 4.00 = 25%.
- 4/6 = 1.67, so 1 / 1.667 = 60%.
In fractional form you can use: right number / (left number + right number). For 3/1 that is 1 / (3 + 1) = 25%. For 4/6 it is 6 / (4 + 6) = 60%.
This is the most useful reading of odds. If you think a team has a better chance than the implied probability, the price may be worth taking. If you think it is lower, the price is too short for you, however likely the win feels.
The bookmaker’s margin
Add up the implied probabilities of every outcome in a market and the total comes to more than 100%. The surplus is the bookmaker’s margin, also called the overround.
Worked example: a two-way market
A tennis match with both players at 10/11 (1.91).
- Each price implies 1 / 1.909 = 52.4%.
- Total: 52.4% + 52.4% = 104.8%.
- Margin: 4.8%.
If the market were fair, both players would be at evens (2.00), 50% each. The 10/11 prices are what the bookmaker charges for taking the bet.
Worked example: a three-way football market
Home 6/4 (2.50), draw 12/5 (3.40), away 2/1 (3.00).
- Implied: 40.0% + 29.4% + 33.3% = 102.7%.
- Margin: 2.7%.
Comparing the total across bookmakers on the same match is a quick way to see who is pricing tighter on that market. It changes from match to match, so one comparison tells you about that market, not the whole site.
How odds and offers fit together
Most welcome offers set a minimum price for the qualifying bet, usually evens, 2.00. That rules out odds-on selections. Bally Bet’s offer is one of the exceptions on our racecard, accepting a settled bet at 1/2 (1.50).
Free bets follow the same odds, with one important difference: the free bet stake is normally not returned. So a £10 free bet at 3.00 pays £20, not £30. In decimal terms, a free bet returns stake x (odds - 1). That is why a free bet works out better on longer prices than on short ones, and why the headline amount of an offer overstates its cash value. Our comparison of new betting sites offers lists the minimum odds and free bet rules for each offer.
Odds in multiples
In doubles, trebles and accas, the decimal odds of each leg are multiplied together. Two selections at 2.00 make a double at 4.00 (3/1), three make a treble at 8.00 (7/1). The implied probability shrinks the same way, and so does the margin: it is charged on every leg. Our bet calculator works out returns for singles, multiples and each-way bets in either format.
Switching the display on a betting site
Most UK betting sites let you change the odds display between fractional and decimal in the account or settings menu. It changes nothing about the bet itself, only how the price is shown. Many people find decimal easier for multiples and fractional easier for racing, where prices are still quoted that way on course.
For more on bet types, from each way to handicaps, see our betting guides. For where to put the knowledge to use, the comparison of new betting sites compares licence, offer terms and stated payout windows across new UK bookmakers.
Need to talk to someone?
National Gambling Helpline. Free, confidential, 24 hours a day.
Tools every UK licensed bookmaker must offer:
- Deposit limits
- Time-outs
- Reality checks
- Self-exclusion and GamStop
What does 5/2 mean in betting?
For every £2 you stake, you make £5 profit if the bet wins, and your stake comes back too. A £10 bet at 5/2 returns £35: £25 profit plus the £10 stake. In decimal odds 5/2 is 3.50.
How do I convert fractional odds to decimal?
Divide the first number by the second and add 1. So 6/4 is 1.5 + 1 = 2.50, and 1/2 is 0.5 + 1 = 1.50. Decimal odds include the stake, which is why you add 1.
What are evens?
Evens means your profit equals your stake: 1/1 in fractional odds, 2.00 in decimal and +100 in American. A £10 bet at evens returns £20. Many welcome offers set evens as the minimum odds for the qualifying bet.
What does odds-on mean?
Odds-on means the profit is smaller than the stake, because the bookmaker rates the outcome as more likely than not. 1/2, 4/6 and 10/11 are all odds-on. In decimal, anything below 2.00 is odds-on.
How do I work out the probability from the odds?
Divide 1 by the decimal odds. At 4.00 (3/1) that is 1 / 4.00 = 25%. This is the implied probability, and it includes the bookmaker's margin, so the bookmaker's real estimate of the chance is slightly lower.
Why do the probabilities add up to more than 100%?
Because each price carries a margin for the bookmaker. In a match where both teams are 10/11, each price implies 52.4%, a total of 104.8%. The 4.8% above 100 is the margin, sometimes called the overround.
Do free bets pay out at the same odds?
The odds are the same, but the stake is usually not returned. A £10 free bet at 2.00 pays £10 if it wins, while a £10 cash bet at 2.00 returns £20. That is why free bets are worth less than the same amount in cash.
