What Is a Bookmaker's Margin (Overround)?
Every market is priced so the bookmaker profits whatever happens. This built-in edge — the overround — is the single biggest reason bettors lose. Here's how to spot and beat it.
If you've ever wondered how bookmakers stay in business, the answer is the margin — also called the overround or 'vig'. It's a small edge baked into every price so that, across all outcomes, the book is tilted in the bookmaker's favour. Understanding it explains why casual betting is a losing game and why finding value is the only route to profit. This guide shows how the margin works and how to keep it as small as possible, extending how betting odds work.
How the margin is built in
In a fair coin-toss market both outcomes would be priced at 2.0 (50% each), adding to exactly 100%. But a bookmaker might offer 1.9 and 1.9 instead. Convert those to probabilities (1 ÷ 1.9 = 52.6% each) and they sum to about 105% — that extra 5% is the overround, the bookmaker's expected profit.
The bigger the overround, the worse the value for you. On competitive markets it might be 2–4%; on obscure ones or big multiples it can balloon to 15% or more.
Why the margin makes betting hard
Because every price is shaded against you, backing selections at their 'true' chance still loses money over time — you're effectively paying a tax on every bet. This is the mathematical reason most bettors lose, explored further in why do most bettors lose?.
It's also why a genuine, sustained edge is rare and valuable. A tipster who consistently beats the margin over a large sample is doing something genuinely difficult — the sort of record we help you verify in the evaluation hub.
How to keep the margin small
Shop around. Different bookmakers price the same event differently, and taking the best available odds shrinks the effective margin you pay. Betting exchanges often have the lowest margins of all because you bet against other people, not the house.
Avoid the highest-margin bets: long multiples, exotic specials and 'bet builders' stack the overround on every leg. Simpler markets at competitive prices give you the best chance — a theme in our bet types explained guide.
Margin and value go hand in hand
Beating the overround means finding prices bigger than the true probability — value betting. The margin is the hurdle; value is how you clear it. A skilled bettor or tipster wins by finding enough value to overcome the built-in edge across thousands of bets.
Whenever you assess a tipster, remember they must beat this margin just to break even, which is why we stress large samples and honest pricing throughout how to evaluate a tipster and rank services on their Trust Score.
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Last reviewed 17 August 2026 · Written and fact-checked against our review policy and scoring methodology.
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