How Bookmaker Margins Work

What the margin actually is

Decimal odds convert to an implied probability with 1 ÷ odds. A price of 2.00 implies 50%. In a fair, zero-margin market the implied probabilities of all outcomes would sum to exactly 100% (1.00). Bookmakers shorten prices slightly so the sum comes out above 100%. The excess is the overround, or margin.

The formula for a two-way market: margin = (1 / odds_A) + (1 / odds_B) − 1.

For a three-way market (home/draw/away in football) you add a third term. The higher the sum above 1.00, the more the book has taken out of the true price. Margin is the single most useful number for comparing value between bookmakers, because it strips out which team is favoured and isolates how much the operator is charging to price the event.

A worked overround example

Take a tennis match on the ATP Tour with two outcomes. Assume the genuine, fair probabilities are 60% and 40% — fair odds of 1.667 and 2.500.

Now price it at a bookmaker with a 5% margin. The book shortens both sides so the implied probabilities sum to 105%: - Player A: priced around 1.587 (implied 63.0%) - Player B: priced around 2.381 (implied 42.0%) - Sum of implied probabilities = 105.0% → margin = 5.0%

At a higher-margin book charging 8%, the same match is priced nearer: - Player A: 1.531 (implied 65.3%) - Player B: 2.294 (implied 43.6%) - Sum = 108.9%

Stake €100 on Player A at the two prices. The 5% book pays €158.70; the 8% book pays €153.10. Same selection, same result — a €5.60 difference on a single €100 bet purely from margin. Repeat that hundreds of times across a season and the gap compounds into the difference between a profitable and a losing staking record.

The practical takeaway: shopping the lower-margin book is not about one lucky price, it is a structural, repeatable edge.

Measured average margins across Irish bookmakers

Below are the average margins we recorded from sampled markets. Lower is better for the bettor. Sample size is shown because a margin from 30 prices is less reliable than one from 200+.

• Betfair — 4.29% avg margin (213 sampled)
• Unibet — 5.73% (191)
• Ladbrokes — 5.83% (193)
• Betway — 6.32% (195)
• Paddy Power — 6.93% (195)
• William Hill — 6.96% (219)
• 888sport — 7.13% (219)
• bet365 — 7.44% (30, small sample)
• BoyleSports — 7.51% (195)
• BetVictor — 7.51% (188)
• LiveScore Bet — 8.37% (193)

The spread between the tightest and widest is roughly 4 percentage points. On the €100 tennis example above, that is close to a €6 swing per bet. Hollywoodbets and Novibet are not included here because no odds sample was available for them.

Note bet365's figure sits on only 30 sampled prices, so treat it as indicative rather than settled — its odds rating of 8.8 reflects broader factors beyond this single margin snapshot.

Best-price share: who tops the market most often

Average margin tells you how much a book charges overall. Best-price share tells you how often a bookmaker posts the top available price on the exact selections we sample — a complementary metric that captures line-by-line competitiveness.

• Betfair — best price on 53.1% of sampled selections
• Ladbrokes — 22.3%
• Unibet — 17.3%
• William Hill — 17.4%
• Betway — 13.8%
• Paddy Power — 12.3%
• BoyleSports — 11.8%
• 888sport — 11.4%
• LiveScore Bet — 10.4%
• BetVictor — 6.4%

Betfair's dominance here is structural: its sportsbook sits alongside an exchange model where prices are set by backers and layers rather than a fixed retail margin, which is why both its margin (4.29%) and best-price share (53.1%) sit clear of the field. Among traditional fixed-odds books, Ladbrokes and Unibet lead on how frequently they beat the market, consistent with their low measured margins.

Why margins vary by sport and market

The overround is not fixed within a bookmaker. It widens on markets that are harder to price or less liquid, and tightens on high-turnover headline markets.

Expect the thinnest margins on match-result markets in the Premier League, Champions League and top ATP/WTA matches, where competition is fiercest. Margins widen on: - Outright and futures markets (many outcomes, each carrying a slice of overround) - Niche in-play props and same-game Bet Builder legs - Lower-tier football, GAA and smaller Rugby fixtures with less liquidity

When you build a multi-leg Bet Builder, each leg carries its own embedded margin, so the combined overround multiplies. That is why accumulators and same-game multis are the most expensive product on the shelf in margin terms, even at a low-margin book. Horse Racing prices also behave differently, with early prices and market drift meaning the effective margin you get depends heavily on when you take the price.

How to use margin data when choosing a book

Margin is the clearest single value signal, but it is not the only factor. A book with a marginally higher overround may still make sense if it offers a market the leaders do not, or a feature you use.

A practical approach: - Hold accounts at two or three low-margin books (the top of the table above) so you can take the best price on each selection. - Compare the actual prices before every bet — a bookmaker's average margin is a guide, not a guarantee on your specific market. - Treat best-price share as a shortlist tool: the operators topping that list are the ones most often worth checking first. - Weigh non-price factors separately — licensing, payment options such as PayPal, Apple Pay and Revolut Pay, withdrawal speed and features like Cash Out or Live Streaming.

Everything on this page is licensed for the Irish market. Several operators — Paddy Power, BoyleSports and Novibet — are noted as regulated by the Gambling Regulatory Authority of Ireland (GRAI). The minimum age to bet in Ireland is 18.