"Kalshi vs. Polymarket" is really a three-way question on these markets, because Polymarket is two venues with two order books. Which one you're on decides what number your contract settles on. Here's the comparison, all read from each venue's own rules and public data on Sept. 28, 2026.
The three 15-minute Bitcoin contracts, side by side
| Kalshi | Polymarket US | Polymarket (international) | |
|---|---|---|---|
| Market | KXBTC15M | "BTC Up or Down: 15 min" | "Bitcoin Up or Down" (15-minute) |
| Who trades it | US exchange, CFTC-regulated | US exchange, CFTC-regulated | Offshore venue, closed to US persons |
| Settles on | CF Benchmarks BRTI, 60-print simple average | CF Benchmarks BRTI, 60-print simple average | Chainlink BTC/USD 60-second TWAP data stream |
| Tie | Resolves YES ("at least") | Settles Up ("greater than or equal to") | Resolves Up ("greater than or equal to") |
| Taker fee | 7% × contracts × price × (1 − price) | 0.0695 × contracts × price × (1 − price) | 0.07 × shares × price × (1 − price) |
| Fee on 100 at 50¢ | $1.75 | $1.74 | $1.75 |
| Makers | See Kalshi's fee schedule | Paid a rebate at the trade | Pay no fee |
| Volume per window, Sept. 21–27 | Median about 2.6 million contracts | Not published | Median about $24,800 traded |
Sources: the rules text on each market (Kalshi's public API, Polymarket US's public gateway, Polymarket's Gamma API), Kalshi's fee schedule, Polymarket US's fee schedule (effective Sept. 25, 2026) and Polymarket's fee docs. Volume units differ by venue: Kalshi counts contracts, and Polymarket's international venue reports dollars traded.
Same question, different referee
Kalshi's rule for each window: YES "if the simple average of the sixty seconds of CF Benchmarks' BRTI before" the close "is at least the simple average of the sixty seconds of CF Benchmarks' BRTI before" the open. Polymarket US words it differently but lands in the same place: "in the last minute before each of those two times, 60 BRTI prices are collected. The official value for that time is the simple average of these prices."
Polymarket's international venue uses a different referee: its markets resolve on "the time-weighted average price (TWAP) of Bitcoin, generated by Chainlink," from the BTC/USD TWAP data stream. Both are one-minute averages of Bitcoin across exchanges, so they almost always agree. Almost.
We checked 663 identical windows
For every Bitcoin 15-minute window over the week of Sept. 21–27, 2026, we matched Kalshi's result to the international venue's result for the same quarter-hour. Eight windows were missing a result on one side; 663 matched.
| Windows | |
|---|---|
| Compared | 663 |
| Settled the same way | 659 (99.4%) |
| Settled opposite ways | 4 |
| Decided by less than $10 on Kalshi's number | 59 |
| Decided by less than $5 | 30 |
All four disagreements came in windows Kalshi's settle decided by $5.10 or less: $2.53, $3.14, $3.76 and $5.10. Put differently, three of the 30 windows decided by less than $5 went opposite ways on the two venues, and four of the 59 decided by less than $10. On everything else, the two referees agreed every time.
That matters for anyone holding both sides. If you buy "up" on one venue and "down" on the other to lock in a gap, you are not hedged on exactly the windows most likely to be close. And close windows are common: over five weeks, 8.8% of Kalshi's Bitcoin 15-minute windows were decided by less than $10 (our settlement check).
Fees are a wash; depth is not
All three venues use the same shape of taker fee, rate × contracts × price × (1 − price), charged at the fill. It peaks at 50 cents, where it's $1.75 per 100 contracts on Kalshi and on the international venue, and $1.74 on Polymarket US. Polymarket US also pays resting orders a rebate, and the international venue charges makers nothing.
The bigger difference is who's trading. Over Sept. 21–27, Kalshi's Bitcoin 15-minute windows traded a median of about 2.6 million contracts each. The international venue's traded a median of about $24,800. Polymarket US publishes no volume on its public gateway at all, so there's no number to compare. A thin book means a wider spread, and on a 15-minute contract the spread usually costs more than the fee.
Which venue are you actually on?
- In the US, on Polymarket's app, you're on Polymarket US. Your contract settles on the same CF Benchmarks average as Kalshi's, but it trades on its own order book with its own prices.
- Prices you see quoted from Polymarket's public data feed (Gamma, which most dashboards read) are the international venue's, which settles on Chainlink. Don't compare them with Kalshi as if they were the same contract.
- On Kalshi, the rule, the traps and our running settle check are on the KXBTC15M settlement page, and every other quarter-hour series is on the 15-minute markets board.
More on the two Polymarket venues: Polymarket US, explained. How the 15-minute contract compares with Kalshi's hourly and daily ladders and the perp: which Kalshi clock fits your view.