Compare Polymarket and Kalshi by quoted spread alone and you get a decoy answer. In the overlapping MLB window we have published before, both venues showed a median top-of-book spread of 1 cent. The question a bettor or a bot builder actually has is different: at my trade size, what does it cost to cross? That depends on how much sits behind the quote, not on the headline penny. This piece measures Kalshi's quoted spread by the liquidity behind the quote, then sets it against what we already know about Polymarket.
Method
The data is our two-day Kalshi tryout (2026-07-18 and 2026-07-19 UTC): 21,520,801 depth rows across 40 market families, 746,890 deduplicated trade prints (MLB, WNBA and NFL), and a measured median poll interval of 43.133 seconds. Depth is capped at 20 levels per side, the capture is a top-150-by-volume subset rather than a census, and sizes captured before 2026-07-19 were stored as whole contracts.
For each snapshot, the top-of-book spread is best_ask_cents - best_bid_cents. The notional resting at the best level on each side is size × price / 100 dollars. Each snapshot is bucketed by the smaller of the two sides — the liquidity actually available at the quote:
- under $10
- $10 to $100
- $100 and up
Within each bucket we take the median quoted spread. This is a resting-book measure, not the full cost of crossing after fees.
Findings
From the 1,155,531 best-level rows in the tryout, after dropping rows with a missing side or a spread outside 0–99 cents, 553,843 ticker-snapshot observations remain:
| Liquidity at the quote | Median quoted spread | Snapshots |
|---|---|---|
| Under $10 | 2.0 cents | 61,424 |
| $10 to $100 | 1.0 cent | 89,369 |
| $100 and up | 1.0 cent | 403,050 |
Spreads tighten as liquidity grows. Thin quotes — under $10 resting on the thinner side — show a 2-cent median; once $10 or more sits behind the quote, the median drops to 1 cent and stays there. Most observations (403,050 of 553,843, about 73%) are already in the $100+ bucket, so the "1-cent spread" headline describes most of the sample. The difference shows up in the thinnest ~11%.
How Polymarket compares
This dataset has no Polymarket rows, so the Polymarket side of the table above is not measured here. What we can say comes from our earlier MLB depth study: the median quoted spread was 1 cent on both venues, but the depth behind it was very different — roughly $297,000 to move Kalshi's ask by one cent, versus roughly $15,000–$17,000 on Polymarket. That study also found Polymarket depth varies widely by sport, and gets very thin in some markets; its per-sport table has the details.
For scale, the same "dollars to move the ask one cent" measure across all 40 families in this two-day tryout has a median of $2,269.76. The two figures are not comparable: one is a single liquid MLB window, the other a median across 40 mostly thinner families on two summer days. Read them as a reminder that liquidity depends heavily on the market you trade, not as a venue ranking.
Limitations
- Two days of data. No seasonal or peak-sport conclusion follows from it.
- The depth archive is a top-150-by-volume subset, not a census. A missing snapshot never proves an unchanged book.
- Only 3 of the 40 depth families have trade prints in this sample, so the spread results for the other 37 can't be checked against executed trades.
- Snapshot cadence varies (43.133-second median here); per-ticker continuity isn't guaranteed.
- A resting quote is not the full cost of crossing: exchange fees and your own market impact come on top.
What this means for your trade size
Under $10. Spread rarely decides the trade; fees, funding method and market availability matter more. The thinnest quotes are the exception — that is where Kalshi's median spread widens to 2 cents.
$10 to $100. Quoted spreads can look identical while depth starts to matter, especially in thin sports. In a liquid MLB or NFL game, either venue is usually fine. In esports, tennis or low-volume markets, check the actual resting depth before assuming a 1-cent quote will fill.
$100 and up. Shop depth, not spread. A 1-cent quote means little if walking the book costs more than you expected. In the MLB window we measured, Kalshi's book was much deeper, but that is not universal — check the specific market before sizing up.
If you build bots. Don't build an execution model on quoted spread alone. Bucket by the liquidity behind the quote and measure your fill cost against it.
No trading edge is claimed here. The point is the opposite: a quoted spread is not liquidity, and the right venue for one trade size is not automatically the right venue for another.
Reproduce it
The $9 two-day Kalshi tryout is the exact dataset used above, and the free samples show every schema. The earlier cross-venue depth comparison is here.