By Callum Rennie, Odds & Markets Editor. Last updated
The two formats you will see in the UK
British betting sites traditionally show fractional odds, such as 5/2 or 11/4, and almost all of them let you switch to decimal odds in your account settings. Both describe exactly the same price. Fractions show your profit relative to your stake; decimals show your total return, stake included, for every one pound staked.
A 5/2 price means you win £5 profit for every £2 staked. A £10 bet returns £25 profit plus your £10 stake, a total of £35. In decimal format the same price is 3.50, because £10 multiplied by 3.50 gives £35. To convert fractional to decimal, divide the first number by the second and add one: 5 divided by 2 is 2.5, plus one makes 3.50.
| Fractional | Decimal | Implied probability | Return on £10 |
|---|---|---|---|
| 1/2 | 1.50 | 66.7% | £15.00 |
| 10/11 | 1.91 | 52.4% | £19.09 |
| Evens | 2.00 | 50.0% | £20.00 |
| 6/4 | 2.50 | 40.0% | £25.00 |
| 5/2 | 3.50 | 28.6% | £35.00 |
| 4/1 | 5.00 | 20.0% | £50.00 |
| 10/1 | 11.00 | 9.1% | £110.00 |
From price to implied probability
Implied probability is the chance of an outcome that a price suggests. For decimal odds, divide one by the price: 1 divided by 3.50 is 0.286, or 28.6%. For fractional odds, divide the second number by the sum of both numbers: for 5/2, that is 2 divided by 7, which also gives 28.6%.
This conversion is the most useful skill in betting. It lets you ask a simple question about any bet: do I think this outcome is more likely than the price implies? If you believe a team wins 45% of the time and the price implies 40%, the price looks generous by your estimate. If you think it is nearer 35%, the price looks poor.
Working out the bookmaker’s margin
Add up the implied probabilities for every outcome in a market. In a perfectly fair book the total would be exactly 100%. Real markets always total more, and the excess is the overround.
Take a tennis match with both players at 10/11. Each price implies 52.4%, so the book totals 104.8%. To express the margin as the share of stakes the operator expects to keep, divide the excess by the total: 4.8 divided by 104.8 is about 4.6%. Now imagine another site offering 20/21 on both players, which is 1.95 in decimal. Each implies 51.3%, the book totals 102.6%, and the margin falls to about 2.5%. Over hundreds of bets, that gap matters a great deal.
A quick rule of thumb
On two-way markets, a book under 104% is competitive and anything over 107% is expensive. On a three-way football match result, under 105% is good. Racing and specials markets usually carry much higher totals.
How each-way terms change the maths
An each-way bet splits your stake into a win bet and a place bet of equal size. If the win terms are 12/1 and the place terms are one fifth of the odds for four places, the place part pays at 12/5. A £5 each-way bet costs £10. If the horse finishes third, you lose the £5 win stake but the place bet returns £5 plus £12 profit, a total of £17.
Because the place fraction and the number of places vary between betting sites, two operators offering the same win price can offer very different each-way value. Compare both parts before you place the bet, especially in large handicap fields where extra places are sometimes offered.
Why prices move
Prices shift for two main reasons: new information, such as team news or going changes, and the weight of money placed by other customers. A price that shortens from 4/1 to 5/2 before an event suggests the market now rates that outcome more highly. Being aware of this helps you understand why a price you saw in the morning may not be available in the afternoon.
For a wider view of how odds fit into choosing a site, read our guide on how to compare betting sites. Remember that understanding odds does not remove the house edge; it simply helps you see what you are paying.
Questions readers ask
An odds-on price is shorter than evens, such as 1/2 or 4/6. It implies a probability above 50%, and your profit will be smaller than your stake if the bet wins.
Neither is better; they show the same price. Decimals make comparing prices and calculating returns easier, which is why many regular bettors switch to them.
When a horse is withdrawn after you have taken a price, the remaining runners’ chances improve. A Rule 4 deduction reduces winnings on bets struck at earlier prices to reflect this, based on the withdrawn horse’s odds.