Fractional odds are the traditional British and Irish format. They state the profit relative to the stake: at 3/2 a winning stake of 2 earns 3 profit (plus the stake back), and at 5/1 a stake of 1 earns 5. The first number is the “against” side and the second the “for” side, so 3/2 means 3 chances of losing for every 2 of winning – an implied probability of 2 ÷ 5 = 40%.
Odds shorter than evens, such as 1/2 (“two to one on”), belong to favourites: you must stake 2 to win 1, and the implied chance is 2/3 = 66.67%. Evens is 1/1. The calculator converts any probability, pair of chances, decimal or moneyline price into the simplest fraction (approximated with denominators up to 100 when the price is not a neat fraction).
It also shows the equivalent decimal and moneyline odds, a pie chart of win and lose chances, the return and profit for your stake and the probability of a run of identical results. Remember that bookmakers’ fractions include a margin, so the implied probabilities overstate the real chances a little.
How to use the Fractional Odds Calculator
- I know the odds as
- Chances for (winning outcomes)
- Chances against (losing outcomes)
- Probability of winning
- Fractional odds – first number (profit)
- Fractional odds – second number (stake)
- Decimal odds
- Moneyline odds (minus for favourites) — Type −150 for a favourite or 250 for +250.
- Stake (any currency)
- Times in a row
Everything is calculated in your browser, so results appear instantly and nothing you type is sent to a server.
How it works
The win probability p is computed from whatever you enter; (1 − p)/p is then expressed as the closest simple fraction, exact when your inputs are whole-number chances or a fraction. Profit = stake × (1 − p)/p.
Worked example
You estimate a 40% chance of winning. The chance of losing is 60%, so fractional odds are 60/40 = 3/2. The equivalent decimal odds are 2.50 and the moneyline is +150. A 10 stake would win 15 profit and return 25. Two wins in a row have probability 0.4² = 16%.
Common fractional prices
| Fractional | Implied probability | Decimal |
|---|---|---|
| 1/4 | 80% | 1.25 |
| 1/2 | 66.67% | 1.50 |
| 4/5 | 55.56% | 1.80 |
| 1/1 (evens) | 50% | 2.00 |
| 6/4 | 40% | 2.50 |
| 5/1 | 16.67% | 6.00 |
| 10/1 | 9.09% | 11.00 |
Formula
Fractional odds a/b = (1 − p) ÷ p written as a fraction = chances against ÷ chances for
Probability = b ÷ (a + b) · Profit = stake × a ÷ b
Worked example
Inputs — I know the odds as: Probability of winning; Probability of winning: 40 %; Stake (any currency): 10; Times in a row: 2.
Results — Fractional odds (profit/stake) = 3/2; Probability of winning (implied) = 40 %; Probability of losing = 60 %; Decimal odds = 2.5; American moneyline odds = +150; Odds in favour (win : lose) = 2 : 3.
Key terms
- Decimal odds
- The total payout per 1 unit staked, stake included; 1 ÷ decimal odds is the implied probability.
- Fractional odds
- Profit ÷ stake, written like 5/2 – a winning 2 pays 5 profit.
- Moneyline odds
- American format: +250 is the profit on a 100 stake, −150 is the stake needed to win 100.
- Implied probability
- The chance of winning that a price represents, before any bookmaker margin is removed.
Frequently asked questions
How do fractional odds work?
a/b means a profit of a for every b staked. At 3/2 a 10 stake wins 15 profit and returns 25.
How do I convert fractional odds to probability?
For a/b the probability is b ÷ (a + b): 3/2 → 40%, 5/1 → 16.67%, 1/2 → 66.67%.
How do I convert a probability to fractional odds?
Divide the chance of losing by the chance of winning: 40% → 60/40 = 3/2.
What does "odds-on" mean?
Odds shorter than evens, such as 4/6 or 1/2, where the favourite is more likely to win than lose.
What are evens?
1/1 – stake 1, win 1 profit; implied probability 50%; decimal 2.00; moneyline +100.
How do fractional odds relate to decimal odds?
Decimal = fraction + 1, so 3/2 = 1.5 + 1 = 2.50.