Prediction market arbitrage calculator
Enter the YES ask on one venue and the NO ask on another. The calculator adds each venue's taker fee and shows whether the pair costs less than $1, and what that means for your budget.
Fee per share: rate × P² × (1 − P)
Fee per share: rate × P × (1 − P)
- Cost per pair
- 98.2¢
- Fees per pair
- 2.2¢
- Pairs you can buy
- 1018.0
- You spend
- $1,000.00
- Payout at resolution
- $1,018.04
- ROI
- 1.80%
- APR
- 21.94%
With these fees, NO on leg B must cost under 56.0¢ for the pair to be profitable.
Assumes both markets resolve the same way and your whole order fills at the ask. Check depth and both rule sets before trading.
How to use it
- 01
Find the same event on two venues
Use the scanner or compare by hand. The contracts must resolve on the same outcome, source and date.
- 02
Enter the asks you would pay
YES ask on one venue, NO ask on the other, in cents. Use the best ask, or a deeper level if your order is large.
- 03
Check the fee rates
Defaults match what the scanner uses for each venue. Polymarket's rate depends on the category: 0 for geopolitics, 3% for most sports, 5–7% elsewhere.
- 04
Read the result
If the cost per pair is under $1, the gap is your profit per pair. APR shows how good it is for the time your money is locked.
Formulas
- Cost per pair
- YES ask + NO ask + fee on each leg
- ROI
- ($1 − cost) ÷ cost
- APR
- ROI × 365 ÷ days until the later market resolves
- Profit
- pairs × ($1 − cost), where pairs = budget ÷ cost
Calculator questions
What counts as a prediction market arbitrage?
+
Buying YES on one venue and NO on another for a combined cost under $1, including fees. If both markets resolve the same way, exactly one side pays $1, so the gap is locked in.
Why include fees?
+
Fees are largest near 50¢, which is where most spreads appear. A 2¢ gap can disappear once Kalshi's 0.07 × P × (1 − P) fee and a Polymarket category fee are added.
What does APR tell me that ROI doesn't?
+
ROI ignores time. A 2% return in 3 days is about 243% APR; the same 2% over a year is 2% APR. Compare spreads by APR when your capital is the constraint.
Does the calculator check order book depth?
+
No. It assumes your whole order fills at the price you enter. The live scanner reads the books and caps profit at the size actually available.