Pricing
What is overround in betting?
Overround is the percentage by which a bookmaker's prices on a market exceed 100%. It is the bookmaker's built-in profit margin — the cushion that lets them pay out winners and still make money over time.
Every fixed-odds market has an implied probability for each outcome. Convert decimal odds to implied probability with 1 / odds: a runner at $4.00 implies a 25% chance of winning. In a fair market with no margin, those probabilities would add up to exactly 100%. In a real bookmaker market they always add up to more than 100% — and the surplus is the overround.
Overround is also called the book percentage, the vig (short for vigorish), the juice, or simply the margin. The terms are interchangeable. A "tight" market sits a few percent over 100%; a "loose" or "soft" market can be 110-120% or worse, particularly in less competitive racing markets.
Two markets with the same outcomes can have very different overrounds, and the same operator can run very different overrounds across different sports — which is why comparing overrounds across operators is one of the most useful things a tracker can show you.
Worked example
Imagine a two-runner heat at the dogs. Bookmaker A prices Runner 1 at $1.80 and Runner 2 at $2.10.
Implied probabilities: 1 / 1.80 = 55.6% and 1 / 2.10 = 47.6%. Total: 103.2%. The overround on this market is 3.2% — a fairly tight book.
If a different operator priced the same heat at $1.70 / $2.00, the implied probabilities would be 58.8% and 50.0%, summing to 108.8% — an overround of 8.8% and a noticeably worse market for a punter.
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