How Betting Odds Work

Fractional odds: the traditional UK format

Fractional odds are still the default on horse racing and in many high-street shops. They're written as two numbers with a slash, such as 5/2, 6/4 or 4/1.

The left number is the profit; the right number is the stake it relates to. So 5/2 means: for every £2 you stake, you win £5 profit if the selection wins — plus your £2 back. A £10 bet at 5/2 returns £35 total (£25 profit + £10 stake).

• Odds-against (e.g. 4/1, 5/2): the left number is bigger, so profit exceeds stake. These are the less-likely outcomes.
• Evens (1/1): you double your money. A £10 bet returns £20.
• Odds-on (e.g. 1/2, 4/6): the right number is bigger, so profit is smaller than the stake. These are the favourites. At 1/2, a £10 bet returns £15.

Decimal odds: the format most apps default to

Decimal odds are the standard across in-play markets and most modern apps, and many UK bookmakers let you switch your display to them in settings. They're a single number, such as 3.50 or 1.50.

The decimal figure is your total return per £1 staked, stake included. So multiply your stake by the decimal number to get the full return:

• 3.50 × £10 = £35 total return (£25 profit)
• 1.50 × £10 = £15 total return (£5 profit)
• 2.00 is exactly evens — double your money

Converting between the two is straightforward. Fractional to decimal: divide the left by the right, then add 1. So 5/2 = (5 ÷ 2) + 1 = 3.50. Decimal to fractional: subtract 1, then express as a fraction. 3.50 − 1 = 2.50 = 5/2. Once you're comfortable with decimals, comparing prices across bookmakers is faster because you're reading one number instead of doing fraction arithmetic in your head.

Turning odds into implied probability

Every price carries an implied probability — the chance of winning the odds suggest. This is the single most useful skill for judging whether a bet is worth taking.

From decimal odds: implied probability = 1 ÷ decimal odds.

• 2.00 → 1 ÷ 2.00 = 50%
• 3.50 → 1 ÷ 3.50 = 28.6%
• 1.50 → 1 ÷ 1.50 = 66.7%

From fractional odds: probability = right ÷ (left + right).

• 5/2 → 2 ÷ (5 + 2) = 28.6%
• 1/2 → 2 ÷ (1 + 2) = 66.7%

If you think an outcome is more likely than its implied probability, the price is in your favour. If you rate Arsenal's chance of beating a mid-table side at 70% but the odds imply 66.7%, there's a small edge in backing them — before the margin is taken into account.

The bookmaker margin (the overround)

Add up the implied probabilities for every outcome in a market and they'll total more than 100%. That extra slice is the bookmaker's built-in margin, also called the overround or the vig. It's how a bookmaker profits regardless of the result, and it's the reason odds are always shorter than the 'true' probability.

Example. In a two-way match with no draw, if both sides were priced at true even money they'd be 2.00 each — 50% + 50% = 100%. A bookmaker might instead offer 1.90 each. That's 52.6% + 52.6% = 105.3%, so the margin is 5.3%. The lower the total percentage, the better value the market is for you.

We hold sampled pricing data for many UK bookmakers, and the average margin varies noticeably by operator. Across sampled markets, Betfair showed the lowest average margin in our dataset at roughly 4.6%, with Coral around 5.7% and Ladbrokes around 5.9%. At the tighter-priced end for a large sample, bet365 averaged about 7.4%. These are averages across a sample, not a guarantee on any single market — but as a rule, a lower margin means more of the true probability is returned to you. You can compare live prices on our betting odds United Kingdom pages before you stake.

A full worked example

Say you want to back a Premier League home win at 3.50 (5/2) with a £20 stake.

• Implied probability: 1 ÷ 3.50 = 28.6%
• Total return if it wins: 3.50 × £20 = £70
• Profit: £70 − £20 = £50
• If it loses: you lose the £20 stake

Now compare two bookmakers. Operator A prices the same result at 3.50; Operator B at 3.75. On a winning £20 bet, Operator A returns £70 and Operator B returns £75 — an extra £5 for the identical outcome. Over a season of dozens of bets, consistently taking the bigger price is the difference between a losing and a break-even bettor. This is why line-shopping across accounts matters far more than most people assume, and why our comparisons and market movers pages exist.

Accumulators: how odds multiply

An accumulator (acca) combines several selections into one bet; every leg must win. With decimal odds you simply multiply the prices together.

Three legs at 1.50, 2.00 and 1.80: 1.50 × 2.00 × 1.80 = 5.40. A £10 acca returns £54 (£44 profit). The upside is a big return from a small stake; the downside is that one losing leg voids the whole bet. Note too that the margin compounds across legs — every added selection stacks another slice of overround onto your bet, which is why long accas are hard to beat over time.

Many UK operators offer a Bet Builder for combining multiple markets within a single match (goalscorers, cards, corners), and most support Cash Out so you can settle an acca early for a guaranteed figure rather than riding it to the final leg.

Odds movement and why prices change

Odds are not fixed. They shift as money comes in, team news breaks and, in-play, as the score changes. If a favourite drifts (odds lengthen), the market rates their chance lower than before; if they shorten, the market rates it higher.

For in-play betting this matters most, because prices update second by second. Operators with strong live capability in our ratings — bet365 (9.4) and Betfair (8.7) among the highest — recalculate markets quickly, which affects how fast Cash Out values and live prices adjust. You can track notable shifts on our market movers feed and see live prices on today's matches.