Expected Value Calculator for Prediction Markets
Check whether a contract is worth buying at its current price, given what you think the real odds are.
What you think the real chance is.
What the contract costs right now.
Positive expected value: you are paying 52.0¢ for something you price at 60.0¢. The edge only exists if your probability is better than the market's — and the market is right more often than most people assume. Fees are not included here.
How it works
Expected value on a prediction market is unusually simple, because a contract settles at exactly $1. Buy at price c and you make (1 − c) if you are right and lose c if you are wrong, so the expected value per contract is p(1 − c) − (1 − p)c. That reduces to p − c: the gap between your probability and the market's price, and nothing else.
Which means the whole question is whether your probability is better than the market's. It usually is not. Polymarket's markets have posted Brier scores around 0.09, which is roughly 94% accuracy — a bar most opinions do not clear. A positive number in this calculator is a statement about your own forecasting, not a discovery about the market.
Fees are not included, and they matter: a one-cent edge does not survive Kalshi's fee on a coin-flip contract. Run the same trade through the market fees calculator before deciding anything.
Frequently asked questions
How do you calculate expected value on a prediction market?
Subtract the price from your probability. A 52¢ contract you rate at 60% carries 8¢ of edge per contract, roughly 15% on the money staked. It is that simple because contracts settle at $1.
What is a good expected value on a trade?
Anything genuinely positive after fees is worth taking — the trouble is that the number depends entirely on your estimate beating a well-calibrated market. Polymarket's Brier scores sit around 0.09.
Does positive EV mean I will make money?
Not on one trade. EV is the average over many repetitions — a positive-EV bet at 60% still loses four times in ten, and a run of those is perfectly normal.
Other calculators
For live prices instead of hypotheticals, see the Polymarket vs Kalshi arbitrage scanner, or compare the venues in our head-to-head breakdown.
