Sportsbook margin, explained
How overround is computed, and why one market is not enough
Margin, or overround, is the amount by which a book's implied probabilities exceed 100%. It is the sportsbook equivalent of RTP, and unlike a bonus it applies to every bet you place.
To compare books fairly, take the same markets across football, basketball, tennis and esports, at the same time of day, and look at the median rather than one price. Books quote tighter on marquee events and wider on everything else, so a single-market comparison is misleading.
A book a point tighter costs you less over a season than any promotion returns. It is the least visible and most important number on a sportsbook.
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