Fractional Odds, the UK Standard
Fractional odds are what British and Irish bookmakers show by default, and what every racecourse board has always used. The formula is short: profit equals your stake multiplied by the first number divided by the second, and your return is that profit plus the stake.
| Price | Profit on £10 | Return on £10 | Read as |
|---|---|---|---|
| 5/1 | £50.00 | £60.00 | Five to one against |
| 5/2 | £25.00 | £35.00 | Five to two |
| 11/10 | £11.00 | £21.00 | Just odds-against |
| Evens (1/1) | £10.00 | £20.00 | Profit equals stake |
| 1/2 | £5.00 | £15.00 | Odds-on |
Two words matter here. Odds-against means the first number is larger, so you win more than you stake. Odds-on means the reverse: at 1/2 a winning £10 bet gives you £5 profit, and short-priced favourites in racing and football are usually odds-on. Evens sits exactly between them.
One thing odds-on prices do to a bettor is worth naming. A run of short favourites feels safe because they win more often than they lose, but the margin is charged on every one of them, and the returns are small enough that a single upset wipes out several wins. Backing 1/2 shots means being right about two-thirds of the time just to break even.
Fractions look awkward next to decimals and they are, once you get past the common prices. Every UK bookmaker lets you switch the display in account settings, and most racing punters keep fractions while most football punters switch to decimals.
Decimal Odds
Decimal odds fold the stake into the number, which makes them easier to work with. Your return is stake multiplied by the decimal, and your profit is that return minus the stake.
| Price | Return on £10 | Profit on £10 |
|---|---|---|
| 6.00 | £60.00 | £50.00 |
| 3.50 | £35.00 | £25.00 |
| 2.10 | £21.00 | £11.00 |
| 1.50 | £15.00 | £5.00 |
Note that 6.00 and 5/1 are the same price, and 1.50 and 1/2 are the same price. Converting is mechanical: divide the fraction and add one to get the decimal, or subtract one and turn the result back into a fraction to go the other way.
| Fractional | Decimal | Implied probability | £10 returns |
|---|---|---|---|
| 1/5 | 1.20 | 83.3% | £12.00 |
| 1/2 | 1.50 | 66.7% | £15.00 |
| Evens (1/1) | 2.00 | 50.0% | £20.00 |
| 6/4 | 2.50 | 40.0% | £25.00 |
| 2/1 | 3.00 | 33.3% | £30.00 |
| 5/2 | 3.50 | 28.6% | £35.00 |
| 5/1 | 6.00 | 16.7% | £60.00 |
| 10/1 | 11.00 | 9.1% | £110.00 |
| 20/1 | 21.00 | 4.8% | £210.00 |
| 100/1 | 101.00 | 1.0% | £1,010.00 |
Decimals earn their keep on accumulators. Multiplying 2.00 by 1.80 by 2.50 takes a second; doing the same in fractions takes a pen. That is the whole reason exchanges and most European books use them.
American, or Moneyline, Odds
You will meet these on US-facing books and on some offshore sites. A positive number is the profit on a 100-unit stake, so +500 means 500 profit on 100 staked, the same price as 5/1 or 6.00. A negative number is the stake needed to win 100, so −200 means staking 200 to win 100, the same as 1/2 or 1.50.
Converting is two formulas. For a positive line, divide by 100 and add one: +500 becomes 6.00. For a negative line, divide 100 by the number and add one: −200 becomes 1.50. A price of −150 works out at 1.667, so £10 returns £16.67.
Implied Probability and the Bookmaker’s Margin
This is the part that turns reading a price into understanding one. Implied probability is what the odds say about the chance of an outcome, and there are two ways to get it. From decimals, divide one by the price: 4.00 gives 25%. From fractions, divide the second number by the sum of both: 5/1 gives 1 divided by 6, or 16.7%.
Now add up every outcome in a market. A fair book would total exactly 100%. It never does, and the excess is the bookmaker margin, known in the UK as the overround. Take a football match priced 1.95 for the home win, 3.50 for the draw and 4.00 for the away win.
| Outcome | Price | Implied probability |
|---|---|---|
| Home | 1.95 | 51.28% |
| Draw | 3.50 | 28.57% |
| Away | 4.00 | 25.00% |
| Total | — | 104.85% |
That 4.85% above 100 is the operator’s built-in edge on the market, and it is charged whichever side you back. Overround explained in one sentence: it is the price of doing business with a bookmaker rather than with another bettor.
Now the same match at a wider book: 1.85, 3.40 and 3.90 total 109.1%, an overround of 9.1%. On the away side that is 3.90 against 4.00, which looks trivial until you count it. Ten pounds at 4.00 returns £40 and at 3.90 returns £39, so across a hundred £10 bets the difference is £100 of your money. UK match-result markets commonly run between 5% and 9%, which is why comparing prices before you stake is the highest-value habit in betting.
Margin is one of the eight weighted criteria behind every score on this site, and how we assess it is set out on How We Rate. We compare match-result pricing on the same fixtures across the operators we cover rather than publishing a single margin figure per brand, and the licence, bonus and payment records behind our reviews are on Testing Data.
Exchanges work differently. There is no overround to speak of, because prices come from other bettors rather than from a trading team, and liquid markets sit under 1%. The exchange takes commission on net winnings instead, typically 2% to 5%, charged only when you win. A 2.10 exchange price against 2.00 at a bookmaker nets closer to 2.095 once standard commission applies, still better, but by less than the raw numbers suggest.
How Odds Move: Drift, Shortening and In-Play
Prices are not predictions, they are positions, and they move for four reasons. Weight of money on one side forces the trader to shorten it and push the other side out. Team news, a withdrawn horse or a late injury changes the underlying assessment. Sharp accounts taking a price signal that it was wrong, and the market follows. And in play, every goal, wicket and break of serve rewrites the whole board in seconds.
The vocabulary is worth having. A price that gets longer has drifted, and one contracting under money has shortened, or steamed if the move is sudden. The starting price, or SP, is the official price at the off of a race, formed from the on-course market. Best odds guaranteed, or BOG, is the racing concession that pays you the bigger of the price you took and the SP: back a horse at 4/1 with BOG and it starts at 6/1, and your £10 pays £70 instead of £50.
Understanding which way a price is heading changes when you bet rather than what you bet. Taking an early price protects you against a drift you did not see coming, and waiting protects you against paying for money that has already arrived. In racing, BOG removes most of that decision, because the worst you can do is the starting price.
A price boost is the same mechanic in reverse. The bookmaker moves one selection to a price with less margin than usual, or briefly none at all, and caps the stake. It is real value when the base price was competitive to start with, and cosmetic when a wide book boosts a price up to what a sharper rival was already offering.
Reading Odds in Multiples: Accas and Each-Way
The maths above applies to every bet you place, and the two most popular UK bets are where it bites hardest.
- Accumulators. Multiply the decimals. A £10 four-fold at 2.00, 1.80, 2.50 and 1.60 gives combined odds of 14.40, so the bet returns £144 for £134 profit. The implied probability of the whole thing is 1 divided by 14.40, or 6.9%. The catch is that margin compounds too: four legs each carrying around 5% overround leave the bookmaker roughly 21% on the combined bet, not 5%. Accas are the most profitable product on a UK sportsbook for exactly that reason.
- Each-way. Two equal bets, one on the win and one on the place, so a £5 each-way bet costs £10. Take 8/1 with place terms of a fifth the odds and three places.
| Outcome | Win part (£5 at 8/1) | Place part (£5 at 8/5) | Total return | Profit on £10 |
|---|---|---|---|---|
| Wins | £45.00 | £13.00 | £58.00 | £48.00 |
| Second or third | Stake lost | £13.00 | £13.00 | £3.00 |
| Fourth or worse | Stake lost | Stake lost | £0.00 | −£10.00 |
The place price comes from dividing the win odds by the fraction in the terms: 8/1 at a fifth becomes 8/5, which is 1.6 in decimals.
Odds Calculator
Developer note: embed a calculator taking a stake and a price in any of the three formats, with an output panel showing total return, profit, implied probability and the equivalent price in the other two formats. Add an accumulator mode accepting up to twenty legs and returning combined decimal odds, and an each-way mode taking the place fraction and number of places.
Until it is live, three formulas cover everything on this page. Return equals stake multiplied by the decimal price. Profit equals return minus stake. Implied probability equals one divided by the decimal price, expressed as a percentage.