What "fair odds" actually means
Every price you see at a sportsbook or on an exchange includes a margin. Add up the implied probabilities of both sides of a two-way market and you get more than 100% — that extra slice is the vig (sportsbooks) or the spread and fees (exchanges). Devigging removes it, rescaling the prices so the fair probabilities sum to exactly 100%. The result is the market's best estimate of the true probability, with the house edge taken out.
Fair odds are the yardstick for everything else: whether a price at another book is +EV, whether an exchange contract is cheaper all-in, and how much to stake. This tool computes them four different ways because no single devig method is right for every market.
The four devig methods, briefly
Multiplicative scales each price down in proportion to its size — the industry-standard default, simple and fast. Additive subtracts the same amount of probability from every outcome, which treats the margin as a flat charge. Shin's method assumes some of the margin exists to protect the book against informed bettors and shades favourites and longshots accordingly; for a two-way market it reduces exactly to the additive method. Power solves for an exponent that makes the prices sum to 100%, which handles high-hold and multi-way markets the most gracefully — so it is our default there.
On a tight market the methods agree to within a rounding error. On a high-hold prop or a longshot they can differ by several points, and that spread is real uncertainty. The method comparison table above bolds the worst-case (most conservative) number for each outcome so you never talk yourself into an edge that only one method sees. We do not ship probit or odds-ratio devigging: there is no verified backtest showing they beat the power method on these markets, and an unvalidated method has no business being a headline number.
Kalshi and Polymarket vs. the sportsbook
An exchange contract price can be read as a market-implied probability, but the executable bid or ask, spread, fees, and payout rules determine the actual break-even price. A midpoint is a market reference, not an independent fair probability. Kalshi says transaction fees can vary by market and maker fees can apply to executed resting orders.[2] This tool compares quoted and modeled prices; check live depth and venue rules before treating a displayed opportunity as executable.
Fee schedules can change. Check the venue's current schedule and the calculator's displayed verification date before using a fee-adjusted comparison.
Keep going
Once you know the fair price, the next questions are sizing and tracking. Use the CLV calculator to grade the price you actually got against the close, and the parlay calculator for multi-leg fair odds and payout. For the theory, our guides on positive-EV betting and the Kelly criterion go deeper.
Sources
- Fees — Kalshi Help Center. Reviewed .