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A sportsbook price converts to a Polymarket price through its implied probability. −150 implies 60.0% and +130 implies 43.5%; together that is 103.5%, so the book keeps a 3.5% margin. Remove it and the fair chances are 58.0% and 42.0% — a Polymarket YES share at 58.0¢ is the same bet at no margin.
American odds: for a favourite (−150) the implied probability is 150 ÷ (150 + 100) = 60%; for an underdog (+130) it is 100 ÷ (130 + 100) = 43.5%. Decimal odds: 1 ÷ odds. Fractional odds n/d: d ÷ (n + d).
A sportsbook's two sides add up to more than 100% — the extra is its margin (the “vig”). Dividing each side by the total removes it and gives the fair probability. A Polymarket share that pays $1 is priced directly in that probability: 60% ≈ 60¢, before Polymarket's own taker fee.
| American | Decimal | Implied probability | Polymarket price |
|---|---|---|---|
| -400 | 1.25 | 80.0% | 80.0¢ |
| -300 | 1.33 | 75.0% | 75.0¢ |
| -200 | 1.50 | 66.7% | 66.7¢ |
| -150 | 1.67 | 60.0% | 60.0¢ |
| -120 | 1.83 | 54.5% | 54.5¢ |
| -110 | 1.91 | 52.4% | 52.4¢ |
| +100 | 2.00 | 50.0% | 50.0¢ |
| +110 | 2.10 | 47.6% | 47.6¢ |
| +150 | 2.50 | 40.0% | 40.0¢ |
| +200 | 3.00 | 33.3% | 33.3¢ |
| +300 | 4.00 | 25.0% | 25.0¢ |
| +500 | 6.00 | 16.7% | 16.7¢ |
Turn the odds into an implied probability — favourites: odds ÷ (odds + 100); underdogs: 100 ÷ (odds + 100) — and read it in cents. −150 is 60%, so the same outcome would trade near 60¢ on Polymarket before fees and the sportsbook margin.
The vig (or margin) is the bookmaker's built-in edge: both sides of a line add up to more than 100%. Removing it gives the fair probability, which is what to compare with a Polymarket price.
Often closer to fair, because Polymarket charges a small taker fee instead of a margin on both sides — but it varies by market and liquidity. Compare the no-margin probability here with the Polymarket price.
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