Kalshi & Polymarket fee calculator
Enter a price and a size to see what Kalshi, Polymarket US and international Polymarket charge, side by side. Everything runs in your browser from the exchanges' published formulas.
The formulas
Kalshi fees
fee = round up to the next cent (0.07 × C × P × (1 − P)), where C is contracts and P is the price in dollars. Series that charge maker fees use 0.0175 for resting orders that fill. Some special-event markets use different coefficients. (Kalshi fee schedule PDF, Kalshi help: Fees)
Polymarket US fees
fee = Θ × C × p × (1 − p), rounded to the cent with banker's rounding (half to even). Takers pay Θ = 0.0695 (0.10 in table tennis). Makers pay nothing, and makers above $10M in monthly volume earn a rebate at Θ = −0.0035, −0.0070 or −0.0104. Combos use C × p × [0.0695 × (1 − p) + 0.06 × (1 − p)⁴] with no maker rebate. Takers can also earn back 3% to 30% of fees through volume tiers, paid weekly; the calculator doesn't subtract that. (Polymarket US fee docs)
International Polymarket fees
fee = C × feeRate × p × (1 − p) in USDC, charged to takers only and rounded to 5 decimal places (anything under 0.00001 rounds to zero). The rate depends on category: Crypto 0.07; Sports, Economics, Culture, Weather and Other 0.05; Finance, Politics, Mentions and Tech 0.04; Geopolitics 0; Combos 0.05. (Polymarket fee docs)
Bot builders: don't hard-code the table. Each international market carries its own feeSchedule, and in our CLOB API Starter tests some live rates (such as 0.03) didn't match the category table. Read the market's rate before every order.
Worked examples from the official docs
The calculator reproduces each of these.
| Venue | Trade | Math | Fee |
|---|---|---|---|
| Kalshi (taker) | 100 contracts at $0.50 | 0.07 × 100 × 0.50 × 0.50 = 1.75 | $1.75 |
| Kalshi (taker) | 100 contracts at $0.05 | 0.07 × 100 × 0.05 × 0.95 = 0.3325, rounded up | $0.34 |
| Polymarket US (taker) | 1,000 contracts at $0.10 | 0.0695 × 1,000 × 0.10 × 0.90 = 6.255, banker's rounding | $6.26 |
| Polymarket US (taker) | 1,000 contracts at $0.65 | 0.0695 × 1,000 × 0.65 × 0.35 = 15.81125 | $15.81 |
| Polymarket US (combo) | 1,000 combo contracts at $0.10 | 1,000 × 0.10 × [0.0695 × 0.90 + 0.06 × 0.90⁴] = 10.1916 | $10.19 |
| Polymarket international | 100 shares at $0.50, 0.05 rate | 100 × 0.05 × 0.50 × 0.50 = 1.25 | $1.25 |
Fees and your bot
- An "edge" smaller than the round-trip fee isn't an edge. Our arbitrage alert monitor only alerts when a pair costs under $1.00 after fees.
- Fees peak at 50¢. Cheap and expensive contracts pay far less per contract, but a small fee on a 5¢ contract can still be a big share of the cost.
- Resting orders are cheaper on every venue here. Passive (maker) orders pay no fee on Polymarket and only on some Kalshi series.
- Model the fee in paper mode first. Every BotCookery recipe starts there. Start with the Kalshi API Starter, the Polymarket US API Starter or the Polymarket CLOB API Starter.
FAQ
How are Kalshi fees calculated?
Kalshi charges a trading fee of 0.07 × contracts × price × (1 − price), rounded up to the next cent. The fee is highest at a 50¢ price: 100 contracts at $0.50 cost $1.75 in fees. Some series also charge maker fees on resting orders (0.0175 × C × P × (1 − P)), and some special markets use different coefficients, so check the market before you trade.
How much are Polymarket fees?
It depends which Polymarket. Polymarket US charges takers 0.0695 × C × p × (1 − p) (0.10 for table tennis), rounded to the cent with banker's rounding, and pays high-volume makers a rebate. International Polymarket charges takers C × rate × p × (1 − p), where the rate depends on the market category (0 to 0.07), and makers pay nothing.
Do makers pay fees?
On international Polymarket, never. On Polymarket US, makers pay no fee and Tier 2 to 4 makers earn a rebate. On Kalshi, makers pay a fee only on series that list maker fees.
Why is the fee highest at 50¢?
All three formulas multiply by p × (1 − p), which peaks at p = 0.50 and shrinks toward 1¢ and 99¢. A trade at 30¢ pays the same fee as a trade at 70¢.
Should my bot use these numbers?
Use them for planning and paper trading. For live orders, read the fee from the exchange: on international Polymarket each market's feeSchedule is the source of truth, and we have seen live rates that differ from the category table.
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