Sportsbook odds add up to more than 100% because the book builds in a margin (the vig). This calculator strips it out. Enter the American odds for every outcome in a market, and you get the vig, the fair probability for each side by three methods, the fair American odds, and the equivalent price in cents so you can compare directly with Kalshi, Novig or Polymarket.
How to read the result
- Overround is the book's margin. −110/−110 gives 4.5%; a three-way soccer market is often 5–7%; player props can be 8–10%.
- Implied is what the posted odds say before removing the margin. The implied numbers always add up to more than 100%.
- Fair is the probability after removing the margin, by each method. The book's "true" estimate.
- Fair price in cents is the number to put next to an exchange price. A Kalshi Yes at 52¢ against a fair 54¢ means the exchange is 2¢ cheaper than the book's own opinion.
Why the method matters
Books don't spread their margin evenly. They usually put more of it on the underdog (the "favourite–longshot bias"), because casual bettors overpay for big payouts. Multiplicative devigging ignores that and scales everything proportionally. The power method raises each implied probability to a common exponent until they sum to 1, which pulls more margin off the longshot and is generally closer to what sharp bettors use. On a −110/−110 market all three methods agree; on −400/+300 they don't, and the power method's fair price for the underdog will be noticeably higher.
What this doesn't tell you
Removing the vig gives you the book's opinion, not the truth. A sharp book (Pinnacle, Circa) with low margin is a good proxy for true odds; a recreational book's line can be off by several points even after devigging. And an exchange price has its own small cost: the fee (see Kalshi's) and the spread. The comparison that matters is: fair price from a sharp book versus exchange price plus fee.