Fractional odds show profit relative to stake. Convert the fraction to decimal odds, then take its reciprocal for raw implied probability.
Profit over stake
Fractional odds state the profit relative to the stake. At 5/2, every $2 staked earns $5 profit if it wins, so a $10 bet earns $25 profit and returns $35 including the stake. The first number is the profit, and the second is the stake that earns it.
When the first number is smaller, the selection is odds-on, meaning a favorite: at 1/2, a $10 stake earns $5 profit. When the first number is larger, it is odds-against: 11/10 pays $11 for every $10 staked. Evens, written 1/1, pays a profit equal to the stake.
Convert fractions to decimal and American odds
For fractional odds a/b, decimal odds are a ÷ b + 1. American odds are +100 × a ÷ b when the fraction is 1/1 or longer, and −100 × b ÷ a when it is shorter than 1/1.
| Fractional | Decimal | American |
|---|---|---|
| 1/2 | 1.50 | −200 |
| 4/6 | 1.67 | −150 |
| 8/11 | 1.73 | −137.5 |
| 11/10 | 2.10 | +110 |
| 5/2 | 3.50 | +250 |
| 25/1 | 26.00 | +2500 |
Converting also settles which of two fractions pays more. Is 9/4 better than 11/5? In decimal, 9/4 is 3.25 and 11/5 is 3.20, so a $25 stake returns $81.25 at 9/4 and $80.00 at 11/5.
Implied probability from a fraction
The implied probability of a/b is b ÷ (a + b), the stake as a share of the total return. For 5/2 that is 2 ÷ 7 = 28.57%; for 8/11 it is 11 ÷ 19 = 57.89%. At the extremes of the table, 1/2 implies 66.67% and 25/1 implies 1 ÷ 26 = 3.85%. Like any quoted price, it includes the bookmaker’s margin, so the implied probabilities across a complete market add up to more than 100%.
A quick sense check: anything shorter than evens implies more than 50%, and anything longer implies less. A price of 4/6 is a 60% break-even, not a 40% one, because the stake (6) is the larger share of the total return (10).
Where fractions still appear
Fractional odds remain standard in British and Irish horse racing and are common on US futures and novelty markets, where “25/1 to win the title” reads more naturally than +2500. Futures markets with dozens of outcomes usually carry much larger total margins than a two-way game line, so the implied probabilities of every team can add up well past 100%. Removing that margin is covered in the no-vig odds guide.
Fractions also follow a conventional ladder of prices, such as 6/4, 13/8, and 15/8, rather than every possible value. A price worth 2.62 in decimal is typically shown as 13/8, which is 2.625, so a fraction can be slightly rounded from the underlying price. When precision matters, convert both quotes to decimal before comparing books.
Does this topic predict the next result?
No. A definition, historical statistic, market price, or model output is not a guarantee. Treat estimates as conditional on their data and assumptions, and keep the uncertainty visible.