Betting odds explained

Fractions and decimals are the same number

A fractional price tells you the profit relative to the stake: at 5/2 you win £5 for every £2 you put on, and you get your £2 back as well. A decimal price tells you the total return per £1, stake included. Converting is one step in either direction: divide the fraction and add 1 to get the decimal, subtract 1 from the decimal to get the fraction.

So 5/2 = 2.5 + 1 = 3.50. A £10 bet returns £35, of which £25 is profit. Evens is 1/1 or 2.00. Odds-on prices, where the fraction is the other way round — 4/7, say — come out below 2.00 in decimals, which is why the decimal format makes favourites easier to compare at a glance.

From a price to a probability

Divide 1 by the decimal price and you have the implied probability the bookmaker has attached to that outcome. It is the single most useful thing you can do with a price, because it converts every market into the same unit.

• 1.50 → 1 ÷ 1.50 = 66.7%
• 2.10 → 47.6%
• 3.50 → 28.6%
• 9.00 → 11.1%

One caveat, and it matters: this is not the true probability of the event. It is the bookmaker's estimate with the margin already baked in, so it overstates every outcome slightly.

The overround: where the margin hides

Add up the implied probabilities of every outcome in a market and the total comes to more than 100%. That excess is the overround — the bookmaker's theoretical margin, spread across the prices rather than charged as a fee.

Take a match priced 1.95 / 3.60 / 3.90. The implied probabilities are 51.3%, 27.8% and 25.6%, adding up to 104.7%. The overround is 4.7% and the theoretical payout is 1 ÷ 1.047 = 95.5%. On a market priced 1.85 / 3.40 / 4.20 the same sum comes to 107.1% — an overround of 7.1% and a payout of 93.4%. Same sport, same fixture type, meaningfully different value.

Why one headline price tells you nothing

Bookmakers know which markets get compared. A generous price on the Premier League match result sits happily alongside a fat margin on the same match's corners or cards. If you want to judge a sportsbook on price, work out the overround on the markets you actually bet, over several fixtures — not on the one price in the advert.

The same arithmetic explains why long accumulators are expensive: the margin compounds with every leg you add. Three legs at 4.7% each leave a payout of about 87%, not 95%.

Where to go next

Prices are only half the picture — the rules that settle your bet are the other half. See each-way betting explained, and our ranked UK betting sites, all UKGC-licensed.