How to compare two books by their margins

Two books, one market, and a question worth five minutes: which of them keeps more of the pool? The answer is the overround, the amount by which the implied probabilities pass 100%, and you can compute it for each book from the same complete market. The smaller number is the cheaper book to bet at, all else equal.

Compute both overrounds

Take one market at both books, say a soccer match priced 2.10, 3.60 and 3.70 at your book, and 2.08, 3.55 and 3.60 at another. Convert each price to an implied probability, 1 divided by the price, and sum each market. The first sums to 102.4%, an overround of 2.4%; the second to 104.0%. The first book keeps 2.4% of the pool, the second 4.0%.

The margin removal guide walks through the arithmetic in both its forms, and the habit it teaches applies here directly: the sum of implied probabilities is the margin made visible, checked 2026-10-09.

Why the comparison belongs on the same market

Margins vary across markets at the same book, sometimes widely. Comparing one book's soccer margin against another's tennis margin tells you nothing about either. Same match, same market, same moment: only then does the smaller number mean the cheaper book.

One match is also not a verdict. A book can shade a televised match tighter than a Tuesday qualifier. Compare across a few matches before you draw the conclusion.

What a lower margin buys you

A smaller overround means the book's prices sit closer to the fair ones, so more of the pool is returned to the prices themselves. For a value bettor the practical effect is simple: prices at a lower-margin book beat the fair price more often. The fair benchmark for that test is the no-vig line, which the guide explains is the reference precisely because it strips the margin out.

A book that is consistently a full point of margin more expensive is a book whose prices have to work harder to clear the same bar.