What is Overround?
Overround is the amount by which the implied probabilities of all outcomes in a betting market add up to more than 100%, representing the bookmaker's built-in margin.
In a fair two-way market, both sides at even money would imply 50% each, totalling 100%. A bookmaker might instead price both at 1.91, each implying about 52.4%, for a total of about 104.7%. The 4.7% excess is the overround, and it means the bookmaker expects to profit whichever side wins if money is balanced.
Overrounds vary by market. Major football match results may have overrounds of a few percent, while less liquid markets, novelty bets and accumulators can be much higher. In the US the same concept is commonly called the vig or juice.
The overround is closely related to the margin, the share of stakes the bookmaker expects to keep. Comparing the overround of different markets shows how much each costs to bet on.