How Betting Odds Work

Fractional odds: the traditional UK format

Fractional odds are written as a fraction such as 5/2, 4/1 or 1/3. The left number is the profit you win, the right number is the stake required.

• 4/1 means a £10 stake returns £40 profit plus your £10 back = £50 total.
• 5/2 means a £10 stake returns £25 profit plus £10 = £35 total.
• 1/3 (odds-on) means you must stake £30 to win £10 profit.

The rule of thumb: divide the left number by the right, multiply by your stake, then add the stake back. Fractional odds dominate horse racing markets and remain the default display on most UK bookmaker sites, though nearly all now let you switch to decimal in settings.

Decimal odds: the cleaner calculation

Decimal odds express the total return per £1 staked, including your stake. This makes them easier to compare across bookmakers and easier to use in a worked value calculation.

• 4/1 = 5.00 decimal (£10 × 5.00 = £50 total return).
• 5/2 = 3.50 decimal.
• 1/3 = 1.33 decimal.

To convert fractional to decimal: divide the fraction and add 1. So 5/2 = 2.5 + 1 = 3.50. To go back the other way, subtract 1 and express as a fraction. Because decimal odds are a single number, they are the format used for line comparison on our /en-gb/odds/ pages and in margin calculations.

Implied probability: what the odds are really saying

Every price contains an implied probability — the bookmaker's estimate of how likely the outcome is. The formula with decimal odds is simple:

Implied probability = 1 ÷ decimal odds

• Odds of 2.00 → 1 ÷ 2.00 = 50%.
• Odds of 3.50 → 1 ÷ 3.50 = 28.6%.
• Odds of 5.00 → 1 ÷ 5.00 = 20%.

If your own assessment of an outcome's chance is higher than the implied probability the odds give, the bet carries value. If it is lower, the price is against you. This single comparison — your estimate versus implied probability — is the core of every value-betting decision. Everything else is arithmetic.

The bookmaker margin: why probabilities add up to more than 100%

In a fair market the implied probabilities of all outcomes would sum to exactly 100%. Bookmakers build in a margin (also called overround or the 'vig') so the total exceeds 100%. That excess is their theoretical edge.

Worked example — a two-way market (tennis, two players):

• Player A priced at 1.80 → implied 55.6%
• Player B priced at 1.90 → implied 52.6%
• Total = 108.2%

The 8.2% above 100% is the margin. A fair, margin-free market would price these at roughly 1.95 each. The wider the margin, the worse the prices for you.

In our sampling, average margins ranged widely: Betfair 4.64%, BetMGM 5.24%, Coral 5.66%, Ladbrokes 5.88% and Unibet 5.89% sat at the sharper end, while Virgin Bet (8.30%) and LiveScore Bet (8.46%) ran the widest of the books with meaningful sample sizes. Over hundreds of bets, a 3–4 percentage-point margin difference is the difference between prices that grind you down slowly and prices that grind you down fast.

Best-price share: which books post the top number most often

Margin tells you the average quality of a book's prices; best-price share tells you how often that book actually posts the highest odds in a market versus its rivals. The two usually correlate, but not perfectly.

From the same sampling:

• Betfair topped 48.6% of markets sampled (278 markets) — by far the highest, driven by its exchange-influenced pricing.
• Coral: 27.1% (247 markets).
• Ladbrokes: 20.2% (253 markets).
• Sky Bet: 15.7%, Unibet: 15.2%, 888sport: 15.1%, BetMGM: 14.8%.
• William Hill: 14.7%, Betway: 12.8%.
• BetVictor sat lowest of the larger samples at 6.2%.

A best-price share near 49% means that if you only ever bet at that one book, you would already be on the top price roughly half the time. To capture the rest you would need accounts at several bookmakers and check the line before each bet. That is the practical case for line-shopping rather than loyalty.

A full worked example: stake, return and value

Take a Premier League match. You want to back the home win.

1. bet365 prices the home win at 2.10 (11/10 fractional). Implied probability = 1 ÷ 2.10 = 47.6%. 2. Your own model gives the home side a 52% chance. 3. Because 52% > 47.6%, the bet has positive expected value at that price.

Expected value on a £10 stake: - Win: profit = £10 × (2.10 − 1) = £11, occurring 52% of the time. - Lose: −£10, occurring 48% of the time. - EV = (0.52 × £11) − (0.48 × £10) = £5.72 − £4.80 = +£0.92 per £10 staked.

Now shop the line. If another book posts 2.25 for the same selection, implied probability drops to 44.4% and your EV rises to (0.52 × £12.50) − (0.48 × £10) = +£1.70. Same opinion, same stake — a higher price nearly doubled the expected return. This is why margin and best-price share matter more than any single flashy headline number.

Compare live prices on our /en-gb/odds/ page and track how they shift on /en-gb/market-movers/.

Odds movement: opening versus current lines

Prices are not static. An opening line reflects the bookmaker's initial estimate; the current line reflects money placed, team news and market sentiment. If a price shortens (say 3.50 drifts to 2.80), implied probability has risen — the market now rates the outcome more likely. If it lengthens, the reverse.

For value bettors the useful signal is the direction and speed of movement. Backing a price before it shortens locks in a number the market later agrees was too generous. Our futures and market-mover tools exist to make this movement visible rather than guessed at.

How odds ratings feed our operator scores

On BETAXIO an operator's odds rating is a data-backed figure, not a subjective impression. Betfair (odds rating 8.9) and bet365 (8.8) lead the field, consistent with Betfair's market-leading 4.64% margin and 48.6% best-price share. BetVictor (odds rating 8.0) scores well on average price but posts the top number less often (6.2% best-price share), which is exactly the margin-versus-frequency distinction covered above.

Ratings for market coverage, live capability and Bet Builder are separate axes. A book can price sharply yet offer fewer markets, or offer huge depth at slightly wider margins — bet365 scores 9.6 for market coverage, the highest sampled. Read the odds rating and margin together: the rating summarises average price quality, the margin and best-price share explain why.