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Expected Value Calculator

Compute the expected value of a prediction market trade — EV, ROI and edge from probability, odds, and stake. Platform fees included.

Your estimate that the event resolves YES.

2.00 = fair coin. Polymarket price 0.50 → odds 2.00. General: odds = 1 / price.

How much you put into the trade.

Polymarket 0.10 %. Kalshi variable (often 0–4 %). For pure education models: 0.

How it works

Expected value is the average money value of a trade if you played it infinitely often with the same edge. For prediction markets with binary outcomes (Yes/No):

EV = p × payout − (1 − p) × stake − fee

With p = your estimated probability, payout = stake × (odds − 1) (net winnings on a correct call), and fee = platform fee × payout (charged on profit only, not on the stake).

Edge: the sign, EV: the magnitude

The edge tells you whether the odds are in your favour (positive sign). The EV tells you how much money you expect to make per trade. A 0.5 % edge on a €100 stake = 50 cents EV — formally positive, but practically swallowed by estimation uncertainty. Rule of thumb: only trades with ≥ 3 % edge.

Fees change the trade

Polymarket charges 0.10 % of profit — negligible at large edges. Kalshi can charge up to 4 % — that turns a 5 % edge into a 1 % edge, which is no longer a tradeable setup. Always price in fees.

What the calculator doesn't model

Slippage (thin markets where your stake moves the price), resolution risk (the market doesn't resolve cleanly), correlation with other open positions. EV is enough for isolated trades — for portfolio logic you need Kelly + risk limits.

Related

Kelly Criterion Calculator← All toolsDirectional Alpha frameworkStrategy Factory

FAQ

What is the difference between edge and EV?
Edge is the advantage per unit staked (in percent). EV is the expected money value of your trade — edge × stake minus fees. Edge is odds-relative, EV is absolute.
What fees do Polymarket and Kalshi charge?
Polymarket: 0.10 % on profit at resolution (winning side only). Kalshi: varies per market, often 0–4 % on profit. Enter the percentage fee on expected winnings — not on the stake.
When is the trade worth it?
First rule: only when EV is positive after fees. Second rule: only when the edge is large enough that your estimation uncertainty doesn't eat it. An edge of 1–2 percentage points usually isn't enough — that's within estimation noise.
What if the odds move?
On prediction markets every trade shifts the price. What you see in the calculator applies to the current top-of-book odds. On thin markets a larger stake can move the average fill price itself — slippage. Realistic: start with smaller stakes.
Are my inputs stored?
No. The calculator runs entirely in the browser — no server, no tracking. Reload the page and all values are gone.