Polymarket vs Kalshi 2026: Fees, Liquidity, Legality & Which Is Better?

There is no unconditional winner, because this is a three-surface comparison, not a two-platform one: Kalshi (one CFTC-regulated US stack), Polymarket US (a second CFTC-designated exchange, live since December 2, 2025), and Polymarket International (the original crypto venue, restricted to US users). For most US traders the practical choice is Kalshi vs Polymarket US: Kalshi brings the longer operating record (designated 2020, no CFTC action located in AltStreet's scan, a named custody bank, a steady fee formula) and generally deeper US books; Polymarket US brings lower taker fees at the same price point ($1.50 vs $1.75 per 100 contracts at 50 cents), maker rebates instead of maker fees, and a newer venue whose notice archive documents technical issues, halts, and compensation credits in its first nine months — plus a parent carrying a 2022 CFTC settlement and an active CFTC probe opened June 26, 2026 (pending, not adjudicated). Non-US users compare Kalshi's international access against Polymarket International's crypto rails and jurisdiction rules. For pure information — reading the odds — Polymarket's markets are among the most-cited event prices in the world and cost nothing to consult.

It depends on the venue, the market category, and which side of the trade you are on — all three surfaces use the same formula family, Fee = coefficient x contracts x price x (1 - price), which peaks at 50 cents. At 50 cents per 100 contracts (as of August 30, 2026): Kalshi's standard taker fee is $1.75 (0.07 coefficient, rounded up, with 156 series on non-standard schedules including maker fees); Polymarket US charges takers $1.50 (0.06, after three increases in seven months) and PAYS makers a $0.31 rebate, with volume-tiered taker rebates of 10-50% and an 'Accelerated Tier' that honors proof of volume on another prediction market; Polymarket International charges by category — crypto $1.75 (0.07, the same as Kalshi), sports and economics $1.25 (0.05), politics and finance $1.00 (0.04), geopolitics free — and never charges makers. So: a politics taker is cheapest on Polymarket International (if eligible), a US sports taker is cheaper on Polymarket US, a maker is paid on Polymarket US and never charged on Polymarket International, and Kalshi is the most stable schedule of the three. And on any venue, expect execution cost to matter at least as much as the posted fee: on the thinner Kalshi markets AltStreet measured, the spread often exceeded the explicit fee.

The honest answer depends on the metric and the date, so any one-word claim is wrong. By open interest (Allium, June 18, 2026): Kalshi ~$1.02B, Polymarket International ~$488.5M, Polymarket US ~$91.6M — Kalshi led. By monthly trading volume (The Block, July 2026): Kalshi also led — after periods in which Polymarket dominated volume, including the 2024 election cycle (its presidential market alone was reported around $3.6B). Depth and spread at your specific size in your specific market are what actually determine execution, and neither platform publishes executable-depth data; Polymarket's own risk disclosure states liquidity providers may be absent from some contracts, and AltStreet's Kalshi microstructure study measured wide spreads outside headline markets there. Practical rule: check the order book at your intended size on the venue you can use, and treat 'which has more liquidity' as a per-market question, not a platform question.

Federally, both US venues operate under CFTC designation: KalshiEX since 2020, QCX (Polymarket US) since July 9, 2025. At the state level, both are in the same unresolved war over sports contracts, with a federal circuit split (Third Circuit for Kalshi, April 2026; Ninth Circuit for the states, August 28, 2026) pointing toward the Supreme Court. The maps differ in detail: Nevada is currently off-limits on both (a January 29, 2026 restraining order against Polymarket; Kalshi's Nevada sports trading suspended following the Ninth Circuit ruling, per press reports); Rhode Island sued both platforms in one action (May 21, 2026); Massachusetts enjoined Kalshi (February 6, 2026) and was preemptively sued by Polymarket (February 9); Arizona filed criminal charges against Kalshi's exchange entity (March 2026); Kentucky sued both ecosystems. Polymarket International is separate: it remains restricted to US users under its terms — the legacy of its 2022 CFTC settlement — with an application pending (filed April 28, 2026) to open it to US traders. Everything here is pending and changes monthly; check your state's current posture before funding either platform.

Price gaps between the venues exist and traders pursue them, but a same-titled market is not the same contract, and most 'arbitrage' that ignores that is basis risk. Before treating a combined YES+NO price below $1 as riskless, verify: (1) exact question wording; (2) deadline and time zone — a few hours' difference makes two different bets; (3) the resolution source and mechanism — UMA bonded proposals and token-holder votes on Polymarket International, published contract rules with exchange emergency powers on Polymarket US, exchange rules on Kalshi; (4) edge-case and clarification language; (5) the bid and ask at your size, not the last trade; (6) both legs' fees (all three schedules differ); (7) halt and cancellation rules; and (8) whether you can lawfully hold both legs from your jurisdiction at all — US persons cannot use Polymarket International. Add one-leg-fill risk and settlement-timing mismatches (UMA disputes can take four to six days) and the honest description is a spread trade with rule risk, sized accordingly.

Kalshi's own tax documentation (June 2026) describes multiple forms depending on activity: 1099-INT for interest, 1099-MISC for credits and rewards, 1099-B for certain broker transactions, and 1099-DA via ZeroHash for crypto transfers, plus downloadable profit-and-loss statements. Polymarket US's participant agreement includes consent to electronic delivery of IRS Form 1099-B. Polymarket International sends no US tax forms at all, and because it trades in pUSD/USDC, crypto property-transaction treatment can layer on top of the trading results themselves. The open question on both regulated venues is character: whether event contracts qualify for Section 1256's 60/40 futures treatment or produce ordinary income is unsettled among practitioners. Export complete trade histories from either platform's API and resolve treatment with a tax adviser before trading at scale — especially before touching the international venue's crypto rails.

Three completely different custody models. Kalshi: member cash sits in its clearinghouse's omnibus settlement account at a named bank (Silicon Valley Bank, a division of First Citizens) with an insured cash sweep — and Kalshi's own agreement discloses the limitation verbatim: because the account is not in your name, 'FDIC insurance is not guaranteed to you.' Polymarket US: USD in a segregated Member Property Account at QC Clearing via a settlement bank no public document names, with no deposit-insurance treatment disclosed, a no-refund clause, and a clearing rulebook that reserves the right to invest member cash without notice and retain the profits. Polymarket International: your own wallet — self-custodied pUSD (a USDC-backed token on Polygon), verifiable onchain and protected by no bank, insurer, or regulator. None of the three offers insured, refundable custody; the differences are in what is disclosed (Kalshi names its bank and its limits), what is reserved (both US rulebooks permit investing member float), and who holds the keys (you, offshore).

By use case, as of August 2026. Sports: both US venues are sports-dominated and sports is exactly what the state litigation targets — costs favor Polymarket US ($1.50 vs $1.75 per 100 at 50 cents, plus maker rebates), while Kalshi's US books have generally carried more open interest; either way, check your state first (Nevada is off both). Elections and politics: Polymarket built its reputation here and its international politics books have historically been among the largest anywhere (0.04 taker fee, or US-restricted access — the practical US route is whichever venue lists the market you want); Kalshi won the US legal fight that made election markets possible and lists them under the steadier fee schedule. Economics and rates (CPI, Fed, recession) — the markets AltStreet's audience actually uses for hedging: Kalshi has the longer-established US lineup and was the subject of AltStreet's own microstructure study; Polymarket US's economics lineup is newer and thinner. Weather: both list them; Kalshi's are longer-running. For pure information, read Polymarket's prices regardless — no account required.