Answered with numbers you can check — including ours.
The overround is the amount by which a market's implied probabilities exceed 100%. Add up 1 divided by each outcome's decimal odds; anything above 1.00 is the bookmaker's built-in edge.
Typical sizes vary a lot by market and by book. Major-league match-result markets are often in the 2-6% range at sharper books and wider elsewhere. Obscure leagues, exotic props and long-shot markets carry considerably more. Some exchanges effectively charge commission on winnings instead.
This matters more than most people realise, because the margin is the hurdle every strategy has to clear before it makes a penny. A model with a genuine 3% edge is a losing strategy in a market with a 5% overround, and a profitable one in a market with 2%.
It is also why the choice of bookmaker changes results more than most tipping strategies do. Comparing prices across books, and preferring low-margin books, is a larger and far more reliable effect than most selection systems.
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