Fractional Odds Explained 2026 — Converting to Decimal and Implied Probability (With Examples)

Fractional odds such as 5/2 or 8/13 tell you how much profit you make for each unit you stake. They are still the default on British racecourses and in a lot of UK football coverage, even though most betting apps now let you switch to decimal. Once you know three small formulas you can read any price, compare it with decimal odds and see what chance the bookmaker is really giving.

All the numbers in this guide were checked with a short script, and the worked examples use a £10 stake.

How do you read fractional odds?

Odds of a/b mean you win a in profit for every b you stake, and you get your stake back on top. At 5/2, every £2 you bet makes £5 profit. A £10 bet at 5/2 returns £35: £25 profit plus your £10.

Two terms come up all the time:

• Odds-against: the first number is bigger (5/2, 11/4). You win more than you staked.
• Odds-on: the first number is smaller (4/6, 8/13). You win less than you staked, because the outcome is seen as more likely than not.

Evens (1/1) sits between the two: stake £10, win £10.

The profit formula is simply:

Profit = stake × (a ÷ b)

So £10 at 8/13 is £10 × 8 ÷ 13 = £6.15 profit, and £16.15 back in total.

How do you convert fractional odds to decimal?

Divide the first number by the second, then add 1. The extra 1 is your stake, because decimal odds show the total return per £1, not just the profit.

Decimal = (a ÷ b) + 1

To check a decimal price, multiply it by your stake. £10 at 3.50 returns £35, the same as £10 at 5/2. Good: the two formats agree.

How do you convert decimal odds back to fractional?

Take 1 off the decimal price and write what is left as a simple fraction. 2.75 − 1 = 1.75, which is 7/4. 1.50 − 1 = 0.5, which is 1/2. 3.20 − 1 = 2.2, which is 11/5.

Some decimal prices don't map to a neat fraction, which is one reason bookmakers keep using traditional ones like 8/13 or 100/30. When you convert, round to the nearest familiar fraction and check the return on a £10 stake.

How do you work out implied probability from fractional odds?

Divide the second number by the sum of both numbers. That gives the chance of the outcome built into the price.

Implied probability = b ÷ (a + b)

With decimal odds it is even quicker: implied probability = 1 ÷ decimal. 1 ÷ 3.50 = 28.57%, which matches the 5/2 line above.

Implied probability is not the real chance of something happening. It is the bookmaker's price turned into a percentage, and that price includes their margin.

How do you spot the bookmaker's margin?

Add up the implied probabilities of every outcome in a market. Anything above 100% is the bookmaker's margin, also called the overround. Take this illustrative home, draw, away football market:

The market adds up to 106.53%, so roughly 6.5 percentage points are built in for the bookmaker. A fair book, with no margin, would add up to exactly 100%. The lower the total, the better the prices are for you as a bettor. Comparing the same market across licensed bookmakers this way is a quick check on value.

Fractional, decimal and American odds side by side

All three formats show the same price in different ways. The UK tends to use fractional, most of Europe uses decimal and the US uses American (moneyline) odds.

For odds-against prices, American odds are the fractional profit on a 100 stake (5/2 becomes +250). For odds-on prices, they show how much you must stake to win 100 (1/2 becomes −200).

Common mistakes

• Forgetting the stake. 5/2 is 3.50 in decimal, not 2.50. The fraction shows profit only.
• Flipping the formula. Implied probability for 5/2 is 2 ÷ 7, not 5 ÷ 7. The second number goes on top.
• Treating implied probability as the real chance. It includes the margin, so it is always a bit higher than a fair price would be.
• Reading 100/30 as 10/3 and assuming something odd. They are the same price (4.33 decimal). Bookmakers use 100/30 by tradition.

Responsible gambling

Knowing how odds work does not change the fact that the bookmaker's margin is built into every price. Only bet with money you can afford to lose, and only with operators licensed by the UK Gambling Commission. If gambling is causing you or someone close to you harm, the National Gambling Helpline, run by GamCare, is free and open 24 hours a day, 7 days a week on 0808 8020 133 (England and Scotland) or 0808 2819 265 (Wales). GAMSTOP (gamstop.co.uk) is the free national self-exclusion scheme that blocks access to UK-licensed gambling websites and apps. 18+.

Sources

• Worked examples and conversions: calculated by Scorely with a Python script, 6 October 2026. The home, draw, away market is illustrative and not taken from a real bookmaker.
• Gambling Commission, where to get help and support for a gambling problem: https://www.gamblingcommission.gov.uk/public-and-players/guide/page/where-to-get-help-and-support-for-a-gambling-problem (accessed 6 October 2026).
• NHS, gambling addiction support: https://www.nhs.uk/live-well/addiction-support/gambling-addiction/ (accessed 6 October 2026).
• GAMSTOP: https://www.gamstop.co.uk/ (accessed 6 October 2026).
• UK Gambling Commission public register: https://www.gamblingcommission.gov.uk/public-register/businesses (accessed 6 October 2026).