What does this trade actually cost?
Kalshi and Polymarket both charge on the way in and the way out, and the fee is largest in the middle of the range — exactly where most contracts trade. Enter a size and a price to see the real number.
How each venue charges
Kalshi charges 0.07 × contracts × P × (1−P) on each trade, where P is the price as a decimal. Kalshi documents rounding up to the next cent, but against a real fractional fill it charged the unrounded amount — this calculator follows the observed behaviour, because that is what your account statement will show.
Polymarket uses the same P × (1−P) shape but the rate depends on the category — 7% on crypto, 5% on sports and economics, 4% on politics and finance, and zero on geopolitical and world-event markets, which the venue states outright. Only takers pay; if you post a resting order and someone crosses to you, you are charged nothing.
The trap: per contract and per dollar point opposite ways
Both venues’ fees scale with P × (1−P), which peaks at 50¢. So per contract, a coin-flip is the most expensive thing you can trade and a 5¢ longshot is the cheapest.
Measured against the money you actually put at risk, it inverts. Divide by the price and the fee per dollar is 0.07 × (1−P) — which rises as the price falls, because a dollar buys ten times more contracts at 5¢ than at 50¢. On Kalshi, 100 contracts at 50¢ is a 7.0% round trip on your stake; the same 100 contracts at 5¢ is 13.3%. Longshots are cheap per contract and expensive per dollar. The percentage above is the one to trust, because it is measured against what you are risking.
Why a round trip is the honest number
A single-leg fee understates what a position costs, because you pay again to close. If you are comparing a prediction-market price against a sportsbook line, the round trip is the figure to compare — it is the equivalent of the book’s vig. On a 100-contract position at 50¢ the round trip is a meaningful fraction of a thin edge, which is precisely why a 2¢ cross-venue gap can be no edge at all once both sides are paid for.
Where this figure can be wrong
Kalshi’s 0.07 multiplier applies to most categories but is higher on some premium markets — awards, elections and major championships. This calculator applies a flat 0.07, so on those markets the number it shows is too low, not too high. We would rather name the direction of the error than quietly pick a higher rate and present a guess as precision.
ThinBlimp prices every position with this exact module — so the fee you see here is the fee subtracted from the edge you see in the terminal. Convert a contract price to sportsbook odds →
Informational only, not trading advice. Confirm rates against your own fills.
The terminal does this across every market on both exchanges at once, and subtracts it from the edge before showing you a number.