How Betting Odds Work

Fractional vs decimal odds — same price, two formats

Fractional odds (the traditional UK format used heavily in horse racing) show profit relative to stake. 5/2 means you win £5 for every £2 staked, plus your stake back. 4/1 returns £4 profit per £1 staked. Evens (1/1) doubles your money.

Decimal odds show the total return per £1, stake included. 5/2 becomes 3.50 — a winning £10 bet returns £35 total (£25 profit + £10 stake). 4/1 becomes 5.00; evens becomes 2.00.

To convert fractional to decimal: divide the fraction and add 1. So 5/2 = 2.5 + 1 = 3.50. To go back, subtract 1 and express as a fraction: 3.50 − 1 = 2.5 = 5/2. Most UK apps let you toggle between the two in settings, so pick whichever you find quicker to read at a glance.

Turning odds into implied probability

Every price contains an implied probability — the percentage chance baked into the number. This is the single most useful skill in betting, because it lets you judge whether a price is generous or mean.

For decimal odds the formula is simple: implied probability = 1 ÷ decimal odds.

• 2.00 (evens) = 1 ÷ 2.00 = 50%
• 3.50 (5/2) = 1 ÷ 3.50 = 28.6%
• 5.00 (4/1) = 1 ÷ 5.00 = 20%
• 1.50 (1/2) = 1 ÷ 1.50 = 66.7%

For fractional odds: probability = denominator ÷ (numerator + denominator). So 5/2 = 2 ÷ (5 + 2) = 2 ÷ 7 = 28.6%. Same answer, as it should be.

The practical use: if you reckon a Premier League favourite has roughly a 60% chance of winning but the price implies only 50% (2.00), that's a bet worth taking. If the price implies 70% for the same view, it isn't.

The bookmaker margin — why the percentages add up to more than 100

Add the implied probabilities of every outcome in a market and you'll get a figure above 100%. That extra is the bookmaker's margin (also called the overround or the 'juice') — the price of doing business.

Take a simple match with two possible outcomes both priced at 1.90. Each implies 52.6%, so together they sum to 105.3%. That 5.3% is the margin. In a fair market with no margin, both sides would be 2.00 (50% each, totalling 100%).

The lower the margin, the more of the true probability is returned to you as value. Across the football markets we sample, margins vary noticeably between bookmakers — which is exactly why comparing prices matters on every bet, not just the big ones.

A worked example: comparing two real market samples

Here's the theory applied to sampled UK market data. Across the markets we track, Betfair returned the lowest average margin of the operators sampled at roughly 4.6% (0.046 across 278 samples), while Ladbrokes averaged about 5.9% (253 samples) and Coral about 5.7% (247 samples). At the higher end, LiveScore Bet averaged around 8.5% and Virgin Bet around 8.3%.

What does that gap mean in cash? Imagine a fair outcome that should be priced at 3.00 (33.3% implied). A book running a 4.6% margin might shave it to about 2.87; a book running an 8.5% margin might cut it to about 2.76.

Stake £50:

• At 2.87 your winning return is £143.50 (£93.50 profit)
• At 2.76 your winning return is £138.00 (£88.00 profit)

That's £5.50 difference on a single £50 bet from the same outcome — purely because one book takes a smaller cut. Repeat that across a season and the margin compounds. This is why the best-price share matters: in our sampled markets Betfair held the best available price on roughly 49% of selections, well ahead of the field, with Coral (about 27%) and Ladbrokes (about 20%) next. On raw price alone, bet365 rates 8.8 and Betfair 8.9 for odds in our scoring.

Odds move — and the movement tells you something

Prices aren't fixed. They drift out (get longer) or shorten (get shorter) as money comes in, team news lands, or a horse is fancied in the ring. A price shortening from 4.00 to 3.00 means the implied probability has jumped from 25% to 33.3% — the market now rates that outcome more likely.

For match-day betting this matters two ways. First, if you've spotted value early, taking the price before it shortens locks in better odds. Second, watching the drift can confirm or challenge your read. Our market movers page tracks where prices are shifting across today's fixtures, which is a quicker read than refreshing individual coupons.

Live betting takes this to the extreme: in-play odds recalculate second by second as the game state changes, so the implied probability of a 1-0 lead being held rises steadily as the clock runs down.

How to use odds knowledge on match day

Put it together and the workflow is straightforward:

• Read the price in whichever format you prefer — fractional or decimal, they're identical.
• Convert it to implied probability with 1 ÷ decimal odds.
• Compare that percentage to your own estimate of the chance. Bet only when your estimate is higher.
• Check two or three bookmakers for the same selection, because the margin gap is real money.
• Note whether the price is drifting or shortening before you commit.

Every UK bookmaker featured here is licensed by the UK Gambling Commission (UKGC), the minimum age is 18, and all quote prices in both formats. The difference between them is coverage of the schedule, the depth of markets and, crucially, how much margin they build into each price. Use our odds and comparison pages to check who's got the sharpest number on the fixture in front of you.