Polymarket vs Kalshi

Kalshi operates under a CFTC order, requires U.S. account registration, lists USD-priced event contracts, and charges a fee per contract. Polymarket trades permissionless against USDC collateral on an order book and carries no trading fee, settling against a reference price rather than an exchange adjudicator.

The compliance trade-off is real: Kalshi is bankable and boring in the good way, while Polymarket is faster and deeper but depends on indices and arbitration for settlement. Both are worth understanding from the data side, because each leaves a different quality of record.

Kalshi quotes move on a fast but ordinary internal cadence and its public API exposes prices and liquidity snapshots, not a granular full-L2 archive. Polymarket's books are captured every 250ms on this site, which is the cadence required to reconstruct a settlement minute: 76.2% of final-minute snapshots in 5-minute markets have an empty bid or ask side.

For anyone studying settlement behavior, the cadence is the product. A one-minute grid hides the final-second single-sidedness (90% of final-second books across 298 resolved contracts had one side), and only stored frames preserve it.

Kalshi charges a trading fee (typically a small share of winnings or a per-contract basis depending on membership) and is explicitly retail-oriented with sub-$100 position conventions. Polymarket's fee is zero; the real cost is spread and slippage inside a book where 16.9% of 5-minute snapshots are one-sided and 3.24% are crossed.

Measuring "cheap" on a venue with zero fees means measuring the book: the executable side, the land to the bound, and the 42,254-share median rest at 0.99 that settlement leaves behind. Those numbers come from the archive, not from a fee schedule.

Kalshi exposes a documented public API for prices, event questions, and market activity. Polymarket exposes Gamma and CLOB endpoints that are free for live reads, plus 1-minute price history, with full-L2 recorded depth available through the archive. The join that matters is on the UTC second: the reference print, the bid/ask, and the trade all share one clock.

Strategies that cross the two venues — quoting Kalshi against Polymarket drift — need both histories timestamped to the same second. The Polymarket side of that merge is precisely what this archive serves.

Choose Kalshi when the compliance and U.S. payment rails matter more than cadence, and when the question can be answered with its price series. Choose Polymarket when you need depth, sub-second settlement visibility, and an archived 250ms book you can replay.

On the trading side the two rarely compete head to head: Kalshi is an order-flow participant in its own ecosystem, Polymarket is a fast order book. The data choice is what this page really settles.

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