Overround / Vig

The overround is why all a market’s implied probabilities sum to, say, 105%: the extra 5 points are the book’s structural margin (the vig), independent of picking winners.

Where margins actually sit

Competitive football match markets run 2–6%; obscure leagues, player props and multi-leg accumulators run far higher — margin hides where comparison shopping doesn’t reach. Accumulators compound margin per leg, which is why books promote them so warmly. Explaining margin honestly is the sports-content trust play: the reader who learns to see the vig credits the site that taught them.