Across 310 multi-outcome books live on both venues today, the median overround is 6 pts on Kalshi and 1.48 pts on Polymarket - about 4.1 times as much.
In a one-winner market the prices of every outcome should sum to 100. What they actually sum to, above 100, is the operator’s margin — the overround. It is the part of the price that is not a probability, and it is what the venue keeps. Measured in points, because points are the unit it is paid in.
| Venue | Median | Typical range | books |
|---|---|---|---|
| Kalshi | 6 pts | 2 pts to 18 pts | 242 |
| Polymarket | 1.48 pts | 0.5 pts to 3.15 pts | 68 |
An aggregate gap can be an artefact of what each venue happens to list: one venue carrying more of the naturally wide markets would produce the same headline without either of them charging more. So here is the same measurement, split two ways. A split is shown only where both venues have at least five books in it.
| Category | Kalshi | Polymarket | books |
|---|---|---|---|
| Politics | 4.8 pts | 0.9 pts | 40 / 14 |
| Sports | 8 pts | 2.4 pts | 151 / 23 |
| Outcomes in the book | Kalshi | Polymarket | books |
|---|---|---|---|
| 10+ outcomes | 18 pts | 2.17 pts | 120 / 46 |
| 4-9 outcomes | 4 pts | 0.48 pts | 73 / 20 |
Every book on the board with more than two outcomes, on either venue, read at the same moment from the same snapshot. The figure is the median, not the mean: one broken or untraded book at forty points would carry a mean on its own. Books this engine reads as incoherent are excluded by the same rule the board uses, so a stale quote cannot flatter or damn either venue. Two-outcome markets are left out entirely — their margin is a different and much smaller thing, and mixing them in would dilute the number until it said nothing.
It is not a fee schedule and it is not a recommendation. Overround is one cost among several: the venues differ in fees, in settlement rules, in liquidity, in who may lawfully use them, and in what they list at all — and a wider book on a market the other venue does not carry is not a worse price, it is the only price. This page reports one measurable difference between two order books. It does not say where anyone should go, and neither does Kanon.