POLYMARKET CALCULATOR
Polymarket arbitrage calculator
An arbitrage calculator for a sportsbook against a prediction market: the Polymarket or Kalshi price in cents and the venue fee charged on top of it are already built in. For example, a sportsbook at 2.20 against Polymarket at 52¢ on a 1,000 bankroll gives an ROI of 1.31% instead of 2.61% without the fee: 460.50 at the sportsbook, 1,013.10 shares for 539.46 and a profit of 13.14.
On a bookmaker or lay leg, the field next to the odds is the commission on net winnings, % (as at Betfair); empty means none. On an exchange ¢ leg the fee is charged on top of the price: the venue rate is already filled in and can be changed.
Σ 1/k′ = 0.9870surebet
ROI 1.31 % · profit 13.14 whatever the outcome
| Leg | Odds | Stake | Payout | Threshold |
|---|---|---|---|---|
| 1 | 2.20 | 460.50 | 1,013.10 | 2.139 |
| 2 | 52.00¢ | 1,013.10 shares · 539.46 (fee 12.64) | 1,013.10 | 53.30¢ |
| Total staked | 999.96 | |||
If 50-50: -32.91 Cancellation, draw or postponement: the exchange pays $0.50 per share and the bookmaker refunds the stake.
The threshold is the value up to which the surebet holds: bookmaker odds no lower than it, an exchange price and lay odds no higher.
Settings
Real amounts are capped by bookmaker limits, and odds last only seconds. The rounding step is set in the settings: the calculator picks the best set of rounded stakes and works out the actual profit and ROI from the amounts shown.
How to calculate a Polymarket arbitrage
- Enter the sportsbook leg as decimal odds. For the "Exchange ¢" leg, choose the venue: Polymarket with its market category (sports, crypto, politics, geopolitics), Polymarket US, Kalshi or a custom fee rate.
- Enter the share price in cents, as in the order book: 52, 52¢ or 0.52. For NO, use the NO price from the order book, not 100 minus the YES price: there is a spread between them.
- If the legs settle in different currencies, turn on "Different currencies" and enter the rate. If you buy and withdraw at different rates, enter both with a slash: 1.09/1.07.
- Choose what to calculate from: the bankroll, the stake on one leg (shares for an exchange) or the limits. The rounding step is in the settings; Polymarket shares always round to 0.01 and Kalshi contracts to whole numbers.
- Check the "If 50-50" line: if the match is cancelled, the exchange pays $0.50 per share while the sportsbook refunds the stake, so such a surebet does not end at zero.
How Polymarket and Kalshi fees are calculated
The taker pays the fee on top of the cost of the shares: fee = r × C × p × (1 − p), where C is the number of shares, p the price and r the venue rate. So the net odds of a share are k′ = 1 / (p × (1 + r × (1 − p))): 52¢ in Polymarket sports gives k′ = 1.878 instead of 1 / 0.52 = 1.923, and against a sportsbook at 2.20 that is an ROI of 1.31% instead of 2.61% without the fee. A Polymarket handicap "MOUZ (−1.5)" at 46¢ against "G2 (+1.5)" at 2.05 on a $500 bankroll: 520.71 shares for $245.99 (fee $6.47), $254.00 at the sportsbook, a profit of about $20.71, ROI 4.14%. Two exchanges: YES on Polymarket at 45¢ and NO on Kalshi at 50¢ for 1,000 shares cost $979.88 with fees and return $20.13, ROI 2.05% (5.26% without fees). The mechanics and numbers are in the article "Polymarket, Kalshi and Bookmaker Arbitrage".
Venue fees
| Venue | Rate r | Rounding and notes |
|---|---|---|
| Polymarket · sports, economics, culture, weather, other | 0.05 | to $0.00001; makers pay nothing |
| Polymarket · crypto | 0.07 | to $0.00001; makers pay nothing |
| Polymarket · finance, politics, mentions, tech | 0.04 | to $0.00001; makers pay nothing |
| Polymarket · geopolitics | 0 | fee-free |
| Polymarket US | 0.0695 | since October 28, 2026; to the cent, half to even; table tennis 0.10 |
| Kalshi | 0.07 | up; the calculator rounds to the cent per order, direct members to $0.0001 |
The taker pays r × C × p × (1 − p) on top of the cost of the shares. Rates checked on November 5, 2026 against the venue docs: Polymarket, Polymarket US, Kalshi. If a market has its own rate, enter it on the leg as a percentage: 0.05 is 5%.
Where Polymarket and Kalshi are available
Checked on November 5, 2026. By its own list, Polymarket is closed to the United States (Polymarket US is a separate venue there), the United Kingdom, France, Germany, Italy, Brazil, Australia and about thirty more countries: see the Polymarket help center. By its Member Agreement, Kalshi is closed to the United Kingdom, Canada, Australia, France, Italy, Poland and more than 40 other jurisdictions: see Section VI of the Kalshi Member Agreement. The calculator works out the surebet, not whether you may trade from your country: that is decided by each venue's list and local law.
What the calculator does not account for
- The order book: the price of N shares is the average across book levels. The calculator takes one price; the fee at the average price is slightly higher than the real one, which is the safe side.
- The Polymarket minimum order is usually 5 shares, but each market sets its own: check the market page. Kalshi sells whole contracts on most markets.
- Fee rounding: Polymarket rounds to $0.00001, Kalshi rounds up, Polymarket US rounds to the cent (half to even). The calculator rounds the Kalshi fee up to the cent per order and does not round Polymarket US: the gap is at most 0.5¢ per order.
- Exchange rates: the rate can move between the bet and the withdrawal, and buying and selling currency go at different rates. That is the risk of a surebet in two currencies.
- Settlement rules: Polymarket resolves an esports market 50-50 if the match is cancelled, ends in a draw, is postponed by more than 14 days or a forfeit is declared before the start. A sportsbook usually refunds in the same cases, but that depends on the book.
Finding such pairs by hand takes too long: the Dozor scanner compares sportsbook lines with Polymarket and Kalshi and shows the surebet with the venue fee already included. What you lose on withdrawal is in the article "Polymarket Fees and Withdrawal".
FAQ
What is the Polymarket fee in sports?
The taker pays 0.05 × C × p × (1 − p): on 100 shares at 50¢ that is $1.25 on top of the $50 cost. Makers pay nothing. Crypto is 0.07, finance and politics 0.04, geopolitics is fee-free (checked on November 5, 2026).
How does Kalshi round its fee?
The Kalshi model fee is 0.07 × C × P × (1 − P), rounded up. Rounded up to the cent per order, 1 contract at 50¢ carries a $0.02 fee instead of $0.0175, that is 4% of the cost instead of 3.5%. The calculator works the same way, so small Kalshi orders do not pay.
What happens to the surebet if the match is cancelled?
Polymarket resolves the market 50-50 and pays $0.50 per share, while the sportsbook usually refunds the stake. For 1,013.10 shares at 52¢ against $460.50 at the sportsbook the result is −$32.91: the calculator shows it in the "If 50-50" line.
Does Polymarket have handicaps and totals?
Yes, in esports: "Map Handicap: MOUZ (−1.5) vs G2 (+1.5)", "Games Total: O/U 2.5", map round handicaps and totals. All lines are half lines with no push, so it is a regular two-way surebet. Whole and quarter sportsbook lines are handled by the "Handicaps and totals" tab.
Can I build a surebet between Polymarket and Kalshi?
Yes: YES on one venue and NO on the other, with the same number of shares. There is a surebet if the sum of prices with fees is below 1: c = p × (1 + r × (1 − p)). YES at 45¢ on Polymarket and NO at 50¢ on Kalshi: 0.4624 + 0.5175 = 0.9799, ROI 2.05%.
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