How Betting Odds Work

Fractional vs decimal odds

Fractional odds (5/1, 5/2, 11/8) show profit relative to stake. 5/1 means €5 profit per €1 staked; your original stake comes back on top, so €1 returns €6 in total.

Decimal odds (6.00, 3.50, 2.375) show total return per €1 staked, stake included. So 6.00 returns €6 total on a €1 bet — the same price as 5/1.

To convert fractional to decimal: divide the fraction and add 1. - 5/1 → 5 ÷ 1 = 5, + 1 = 6.00 - 5/2 → 5 ÷ 2 = 2.5, + 1 = 3.50 - 11/8 → 11 ÷ 8 = 1.375, + 1 = 2.375

To convert decimal back to fractional: subtract 1 and express as a fraction. 3.50 − 1 = 2.5 = 5/2.

Decimal is easier for accumulators (multiply the prices together) and for comparing prices across bookmakers, which is why most online odds pages default to it. Fractional survives in Irish racing because the tradition is deep and the on-course boards still use it.

Odds as implied probability

Every price converts to an implied probability — the chance the outcome must have for the bet to be a break-even proposition.

Decimal formula: implied probability = 1 ÷ decimal odds. - 2.00 → 1 ÷ 2.00 = 50% - 3.50 → 1 ÷ 3.50 = 28.6% - 6.00 → 1 ÷ 6.00 = 16.7%

Fractional formula: probability = denominator ÷ (numerator + denominator). - 5/2 → 2 ÷ (5 + 2) = 28.6% - 11/8 → 8 ÷ (11 + 8) = 42.1%

This is the single most useful skill in betting. Once a price is a percentage, you can compare it against your own estimate. If you think a team wins 35% of the time but the price implies 28.6% (3.50), the bet carries positive expected value. If your estimate is lower than the implied probability, the price is short and you pass.

The bookmaker margin — why the percentages exceed 100%

Add up the implied probabilities of every outcome in a market and the total should equal 100% in a fair market. It never does. The excess over 100% is the bookmaker's margin (also called the overround or the vig).

Take a two-way market priced at 1.90 / 1.90. Each side implies 1 ÷ 1.90 = 52.6%. Together: 105.3%. That 5.3% over 100% is the margin — the built-in edge the operator holds regardless of result.

Margin directly reduces your long-run return. A lower margin means prices are closer to true probability, which means more of your stake is returned over time. Across the markets sampled for this guide, average margins ranged from roughly 4.3% to 8.4%. On a market you bet repeatedly, the gap between a 4.3% book and an 8.4% book is the difference between a viable strategy and a slow drain.

Measured margins across Irish-facing bookmakers

These are the average margins measured across the sampled markets for each operator, lowest (best for the bettor) first. A lower margin means the book is priced tighter and more of a winning bet's expected value stays with you.

• Betfair — 4.29% avg margin (213 markets), best-price share 53.1%
• Unibet — 5.73% avg margin (191 markets), best-price share 17.3%
• Ladbrokes — 5.83% avg margin (193 markets), best-price share 22.3%
• Betway — 6.32% avg margin (195 markets), best-price share 13.8%
• Paddy Power — 6.93% avg margin (195 markets), best-price share 12.3%
• William Hill — 6.96% avg margin (219 markets), best-price share 17.4%
• 888sport — 7.13% avg margin (219 markets)
• bet365 — 7.44% avg margin (30 markets sampled)
• BoyleSports — 7.51% avg margin (195 markets)
• BetVictor — 7.51% avg margin (188 markets)
• LiveScore Bet — 8.37% avg margin (193 markets)

Best-price share is the percentage of sampled markets where that operator posted the top price. Betfair leads on both metrics — its 53.1% best-price share reflects the exchange model, where prices are set by backers and layers rather than a fixed margin, so the effective overround is thinner. Among traditional fixed-odds books, Ladbrokes (22.3%) and William Hill (17.4%) topped the best-price count most often. Bet365 is sampled on only 30 markets, so treat its figure as indicative rather than settled.

Worked example: reading a match price start to finish

Suppose a Premier League match is priced across the three-way market as: - Home 2.10 - Draw 3.60 - Away 3.50

Step 1 — convert to implied probability: - Home: 1 ÷ 2.10 = 47.6% - Draw: 1 ÷ 3.60 = 27.8% - Away: 1 ÷ 3.50 = 28.6%

Step 2 — sum them: 47.6 + 27.8 + 28.6 = 104.0%. The margin is 4.0%.

Step 3 — strip the margin to estimate the 'true' price. Divide each implied probability by 1.04: - Home true probability ≈ 45.8% - Draw true probability ≈ 26.7% - Away true probability ≈ 27.5%

Step 4 — apply your own model. If your assessment gives the away side a 32% chance of winning but the fair (de-margined) price implies only 27.5%, the 3.50 on offer is value: 32% implied fair odds would be 1 ÷ 0.32 = 3.13, so 3.50 is longer than your estimate demands. Positive expected value = (0.32 × 3.50) − 1 = +0.12, i.e. +12% per unit staked in the long run if your 32% estimate is correct.

The entire discipline of value betting runs on this loop: convert, de-margin, compare to your own number, bet only when the price is longer than your estimate justifies.

Odds movement: opening line vs current price

Odds are not static. An opening line is the operator's first estimate; the current price reflects everything that happened since — money staked, team news, weather, and other bookmakers moving.

A price shortening (3.50 → 2.80) means implied probability rose (28.6% → 35.7%): the market now rates that outcome more likely. A price drifting (3.50 → 4.20) means confidence fell.

Sharp bettors watch for markets where one book is slow to move — that lag is where best-price share is won. Tracking these shifts is what a market movers feed is for; it shows where money and information are pushing prices before the whole market catches up.