How to size a Polymarket position with EV and Kelly
Having a probability is half the trade; sizing it is the other half. Turn your judgment into a number at executable prices.
Steps
- Expand the market row and open the position calculator.
- Type your own probability for Yes.
- Read the output: which side is +EV at current executable prices (best ask for Yes, 1 − best bid for No), the expected value per dollar, and the Kelly fraction.
- Size at half-Kelly or less — full Kelly assumes your probability is exactly right, which it never is.
Why this works
The calculator prices from the live order book rather than the midpoint, so the EV you see is the EV you can actually execute.
FAQ
Why is my "sure thing" showing negative EV?
Because the executable price already contains the crowd's probability plus spread. If your edge doesn't clear the spread, the calculator honestly says so.