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In a market where exactly one outcome wins, fair prices add up to 100%. Anything above 100% is the overround — the built-in margin you pay for buying every outcome; sportsbooks set it deliberately, while on an exchange like Polymarket it comes from the bid-ask spread. Divide each price by the sum to get fair probabilities that add up to 100%.
Sum of the listed outcome prices on the busiest one-winner markets, October 5, 2026:
| Market | Outcomes | Sum | Over 100% (pts) |
|---|---|---|---|
| Super Bowl LXI Winner (2027 NFL Champion) | 32 | 102.8% | +2.8 |
| UEFA Champions League: 2027 Champion | 36 | 101.4% | +1.4 |
| Democratic Presidential Nominee 2028 | 53 | 91.5% | -8.6 |
| Presidential Election Winner 2028 | 54 | 91.9% | -8.1 |
| NBA: 2027 Champion | 30 | 104.7% | +4.7 |
| Lima Mayoral Election Winner | 23 | 99.1% | -0.9 |
| Bahia Governor Election Winner | 7 | 100.3% | +0.3 |
| Worlds 2026: Winner | 18 | 105.8% | +5.8 |
Prices are midpoints or last trades, so small gaps either way are normal; a sum well below 100% usually means some outcomes are not listed or barely trade.
If a book prices four outcomes at 52, 31, 14 and 6, the sum is 103%: buying all four costs $1.03 to win $1, a 3-point overround. Normalizing (52 / 103 = 50.5%, and so on) removes it. On Polymarket each outcome trades on its own, so the sum of midpoints hovers around 100% and the real cost shows up as the spread you cross — see how to read the order book. To compare a sportsbook's odds with Polymarket, use the odds converter; the term itself is in the glossary.
Put real prices into the calculator: every page below shows current odds, 24-hour moves and the market rules.
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The amount by which the prices of all outcomes in a one-winner market add up to more than 100%: the bookmaker's margin, also called the vig.
Divide each outcome's price by the sum of all prices. The results add up to 100% and are the fair probabilities the prices imply.
Roughly: the midpoints of a one-winner market usually sum close to 100%, and the gap you pay shows up as the bid-ask spread instead of a built-in margin. A sum far below 100% usually means not every outcome is listed.
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