The microstructure tax: how Kalshi and Polymarket fees actually work in 2026
Polymarket ended its zero-fee era with category-based taker fees; Kalshi's quadratic formula never left. Here is the exact math — including the per-order cent rounding most tools ignore — and what fee-aware sizing looks like.
Trevor Sardis · Jun 11, 2026 · 6 minute read · sourced & dated (verification-log discipline)
Kalshi charges takers 0.07 × C × P × (1−P) per order, rounded UP to the next cent (general schedule; source: the fee schedule PDF and the 'Fee Rounding' page in Kalshi's API docs, both checked June 11, 2026). The quadratic shape means fees peak exactly at 50¢ — a 100-lot at 50¢ costs $1.75 per side — and fall toward zero at the tails. Two details most tools get wrong: the rounding is per ORDER (a 1-contract order at 50¢ pays $0.02, not $0.0175), and makers are NOT always free — most series carry no maker fee, but designated series charge 0.0175 × C × P × (1−P), also rounded up to the next cent, with rounding overcharges reimbursed monthly above $10.
Polymarket's current schedule (docs.polymarket.com/trading/fees, fetched Jun 11, 2026) applies the same functional form with category-based rates, taker-only: crypto 7%, economics 5%, culture/weather 5%, politics/finance/tech/mentions 4%, sports 3% — and geopolitics 0%. Makers never pay; fee pools fund daily maker rebates (20% of crypto fees, 25% elsewhere).
Three consequences. First, a 1¢ cross-venue gap at mid-prices is usually NOT an arbitrage: buying YES at 42¢ on one venue and NO at 57¢ on the other costs roughly 1.7¢ + 1.2¢ in combined fees at those prices on a crypto market — the Brierly scanner nets fees per leg AND checks live order-book depth before calling anything actionable, which is why its 'actionable' column is sparser than screenshot accounts suggest. Second, the maker/taker asymmetry is the whole game for small accounts: CEPR's study of 300k+ contracts found takers average −32% returns versus −10% for makers (averages, pre-fee). Third, fee-aware Kelly sizing shrinks recommended positions near 50¢ markets meaningfully — the Greeks toolkit computes the exact haircut.
Every price difference shown on this site links to its fee-adjusted economics, and the scanner's methodology — the per-category rate table, the rounding, and the depth check — is published on the Methodology page.