Overround

Also called book percentage · sum of prices

The amount by which a market's outcome prices sum to more than 100% — the margin built into the book.

In a one-winner event, the YES asks across every outcome would sum to exactly one dollar if the book were perfectly efficient and free to trade. They usually sum to more, and the excess is the overround. It is the visible price of liquidity: whoever is quoting those legs is charging for the risk of holding them.

The interesting case is the reverse. When the sum drops below a dollar the set is underround, and buying one YES of every leg costs less than the dollar that one of them is certain to pay. The mirror trade — one NO of every leg — pays n − 1 and locks when the summed NO asks plus fees fall below that. Both directions require the candidate list to be genuinely exhaustive, which is why the venue flags for that property carry so much weight.

Fees decide most of these opportunities. Rows are ranked on net edge, defined as one minus the summed prices and fees divided by the guaranteed payout, and negative rows are still listed rather than hidden — seeing how tight each book runs is useful even when nothing is tradeable. On a five-leg Kalshi set near the middle of the range, per-leg fees alone can eat several cents of gross overround.

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Definitions describe how Polymarket and Kalshi behave and how WhaleTracks models them; venue rules and fee schedules can change, so confirm anything you size a position on with the venue itself. WhaleTracks is informational analytics, not financial advice. Past performance does not guarantee future results.