Expected value calculator for prediction markets
Is a trade worth it? Enter the price, your probability and the fee to see what a Kalshi or Polymarket contract is worth on average, and the probability you need to break even.
The ask for the side you buy, Yes or No
Your honest estimate for that side
Fee per contract = rate × price × (1 − price). Rates as of October 2026.
Optional
Positive EV: your 50.0% is above the 41.7% you need to break even after fees.
| Your probability | 40.0% | 45.0% | 50.0% | 55.0% | 60.0% |
|---|---|---|---|---|---|
| EV per contract | −1.7¢ | +3.3¢ | +8.3¢ | +13.3¢ | +18.3¢ |
How to use the expected value calculator
- Price you pay: the price of one contract on the side you are buying, in cents. Use the ask (what you actually pay), not the midpoint shown as the market's probability. For a "No" contract, enter the No price.
- Your probability: your honest estimate, from 0% to 100%, that the contract pays $1. For "No", your probability of No.
- Platform and fee: the fee schedule for where and how you trade. A market order on Kalshi pays the taker fee; a resting limit order pays the lower maker fee; Polymarket charges takers a rate that depends on the category, and makers nothing.
- Contracts and days: how many contracts you plan to buy, and optionally how many days until the market resolves, to see the return as a yearly rate.
"Check the other side" flips the inputs to the opposite contract, so you can see whether "No" has the edge instead.
The formula
A contract pays $1 if it happens and $0 if not. If it costs price + fee and you think it happens with probability p:
- EV per contract = p − price − fee
- Fee per contract = rate × price × (1 − price), the shape both Kalshi and Polymarket use
- Break-even probability = price + fee
- Return on money at risk = EV ÷ (price + fee)
- Annualised return = (1 + return)^(365 ÷ days) − 1
A worked example
A contract trades at 40¢ on Kalshi and you think the true chance is 50%. A market order pays the taker fee: 7% × 0.40 × 0.60 = 1.68¢. Your all-in cost is 41.68¢, so:
- EV = 0.50 − 0.4168 = +8.32¢ per contract
- 100 contracts: +$8.32 expected, on $41.68 at risk, a return of about 20%
- You need the outcome to happen more than 41.68% of the time to break even
If your estimate were 41% instead, the same trade would lose about 0.7¢ per contract: fees turn small edges into losses.
What the result does not tell you
- Expected value is an average. A +8¢ trade at 40¢ still loses its whole stake about half the time. How much to bet so that losing streaks do not knock you out: Kelly criterion calculator.
- Your probability is the weak point. If you cannot say why you know better than the market, assume the market is right. Track your forecasts with the Brier score.
- Bigger orders cost more than the quoted price. Large market orders walk up the order book (slippage explained), and Kalshi rounds its fee up to the next cent on each order.
- Fees change. The rates here are the published schedules as of October 2026; check the platform's fee page before you trade (Polymarket fees vs Kalshi fees).
Expected value: frequently asked questions
What is expected value in betting?
The average profit or loss of a bet if you could make it many times: each outcome's result times its probability, added up. On a $1 prediction market contract it is your probability minus the price minus fees.
How do you calculate EV on Kalshi?
Subtract the price and Kalshi's fee from your probability. At 40¢ with a 50% estimate and a taker fee of 1.68¢: 0.50 − 0.40 − 0.0168 = +8.32¢ per contract.
What is a good expected value?
Any positive EV after fees is an edge. Compare the return with how long your money is tied up: 5% in a week is excellent, 5% over two years is worse than cash.
Does the calculator include fees?
Yes. Pick the platform and order type; choose "No fee" for Polymarket limit orders that rest in the book or for fee-free world-events markets.
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