Kalshi vs Polymarket US arbitrage, after fees
When does a price gap between the two exchanges survive their fees?
Kalshi and Polymarket US both list sports contracts that pay $1 if a team wins. When one venue prices a team’s win and the other prices the other team’s win for less than $1 between them, buying both looks like a locked profit. Both venues charge a taker fee on every order, though, and the fee is largest on contracts near 50 cents, which is exactly where most game prices sit.
What the fees take
Kalshi charges 0.07 × contracts × price × (1 − price), rounded up to the cent, and on baseball its game series carry a multiplier of 0.5. Polymarket US charges 0.0695 × contracts × price × (1 − price), rounded half to even. On 100 contracts a side, worked by the engine Bookbreaker uses:
| Kalshi, team A | Polymarket US, team B | Gap | Kalshi fee | Polymarket US fee | Left after fees |
|---|---|---|---|---|---|
| 47c | 50c | $3.00 | $1.75 | $1.74 | $-0.49 |
| 44c | 52c | $4.00 | $1.73 | $1.73 | $0.54 |
| 45c | 50c | $5.00 | $1.74 | $1.74 | $1.52 |
| 30c | 65c | $5.00 | $1.47 | $1.58 | $1.95 |
A three-cent gap near even money loses money once both fees are paid. Near 50 cents the two fees together take about three and a half cents a pair, so a gap has to be wider than that before anything is left, and a gap between a favourite and an underdog keeps more of itself because the fee shrinks away from 50 cents. On a baseball game Kalshi’s fee on 100 contracts at 47c is $0.88 instead of $1.75.
Fee formulas read 2026-10-09 from each venue’s own fee page: source, source.
What else can break it
Both venues settle on the official winner, but they can differ on a game that is postponed or not finished, and a price can move between your two orders. Depth matters too: the price on the screen may cover only a few contracts. The app checks how many contracts are resting at each venue’s price before it calls a pair an arbitrage, sizes the pair to the smaller of the two and the money you said you would bet, and says what it leaves after both fees.
Whether these sports contracts are legal is disputed state by state. As of 2026-10-17, courts had stopped Kalshi offering them in Massachusetts and Nevada, a Michigan judge had ordered a temporary shutdown, and Maryland and Ohio had won early rounds; federal courts had blocked state enforcement in New Jersey, Tennessee, Arizona and Minnesota, and Connecticut, Rhode Island and Wisconsin, among others, have sued or sent cease-and-desist letters. Appeals are open and the answer differs by state, so check your own state before using one. Read 2026-10-17, source; the New Jersey ruling, source.