Margin

Margin and overround describe the same thing from slightly different angles. Overround is how far past 100% the book totals; margin is that excess expressed as a share of turnover.

For a book totalling 105%:

• Overround = 5%
• Margin = 5 ÷ 105 = 4.76%

At low margins the two are nearly identical, which is why they are often used interchangeably. At high margins the gap widens, a 140% book has a 40% overround but a 28.6% margin, so it matters which convention a comparison is using.

Theoretical versus realised

Margin is theoretical. It assumes money lands proportionally across every outcome, which it does not. Hold percentage is what the operator actually kept, and it is usually higher, driven by parlay volume and predictable bettor biases toward favourites and overs.

What margins look like in practice

Market	Typical margin
Tennis, major match	2–4%
Football, top leagues	3–6%
NFL spreads	4–5%
Lower divisions	6–10%
Player props	6–15%
Outrights and novelties	15–40%

Exchange commission is the honest comparison point: 2% to 5% on net winnings only, charged after the fact rather than embedded in the price. For an active bettor the difference compounds heavily, which is the main structural argument for betting exchanges over traditional books.

The one habit worth building

Compare margins rather than prices. A bookmaker offering an eye-catching price on one selection while running a 110% book is not generous; it has simply moved the margin onto the other outcomes. Adding up the implied probabilities of a market takes thirty seconds and tells you more about an operator than any review.