{ "schemaVersion": "vultax-public-research-data-v1", "articleUrl": "https://vultax.com/research/kalshi-polymarket-open-interest-by-category", "datasets": [ { "title": "Kalshi versus Polymarket, the venues", "columns": [ { "name": "", "unit": null }, { "name": "Polymarket", "unit": null }, { "name": "Kalshi", "unit": null } ], "rows": [ [ "Structure", "On-chain order book on Polygon; USDC", "CFTC-regulated designated contract market; USD" ], [ "Who can use it", "The international exchange is closed to U.S. residents; Polymarket US, launched on the QCEX designation, is sports-first", "U.S. residents, subject to state restrictions" ], [ "Taker fee", "C × feeRate × p × (1 − p); feeRate 0.07 crypto, 0.05 sports/economics/culture/weather, 0.04 finance/politics/mentions/tech, 0 on geopolitical markets", "0.07 × C × p × (1 − p) for takers, capped at $1.75 per 100 contracts at 50 cents, per the published schedule; some markets carry different fees" ], [ "Maker fee", "\"Makers are never charged fees\"", "Charged on resting orders that fill, at a lower rate than takers" ], [ "Fee on 100 contracts at 50 cents", "$1.75 crypto, $1.25 sports, $1.00 politics, $0 geopolitics", "$1.75" ], [ "Public data", "CLOB, Gamma and Data APIs with OpenAPI specs; every fill is on-chain", "Trading API with documented fee rounding" ], [ "Leaderboard", "Official board by profit and volume, no method stated", "No public wallet leaderboard" ] ], "measurementContext": "Polymarket rates from its fee documentation, read 5 September 2026. Kalshi's kalshi.com pages rate-limited automated reading on that date; the taker formula and cap are from its help centre and fee schedule as quoted by Dropstab's May 2026 comparison, and should be re-read from the schedule before relying on them.", "sources": [ { "label": "Polymarket trading fees", "url": "https://docs.polymarket.com/trading/fees.md", "note": "Formula, per-category rates, maker exemption; read 2026-09-05" }, { "label": "Kalshi fee schedule", "url": "https://kalshi.com/fee-schedule", "note": "Effective July 2026; rate-limited automated reading on 2026-09-05" }, { "label": "Kalshi help centre: Fees", "url": "https://help.kalshi.com/en/articles/13823805-fees", "note": "Maker and taker definitions; some markets carry different fees; read 2026-09-05" }, { "label": "Dropstab: Kalshi vs Polymarket arbitrage opportunities 2026", "url": "https://news.dropstab.com/research/kalshi-polymarket-open-interest-by-category", "note": "Quotes the Kalshi taker formula and the $1.75 per 100 contracts cap; updated August 2026" }, { "label": "Polymarket API documentation", "url": "https://docs.polymarket.com/", "note": "CLOB, Gamma, Data and real-time APIs" }, { "label": "Vultax: Why Bitcoin's price differs between exchanges", "url": "https://vultax.com/research/why-bitcoin-price-differs-between-exchanges", "note": "The same mechanics on crypto exchanges" }, { "label": "Vultax: Best Polymarket Tools in 2026", "url": "https://vultax.com/research/best-polymarket-tools-2026", "note": "Cross-venue tools in context" }, { "label": "DefiRate — Prediction markets top $11B in contract volume as sports, combos drive Kalshi growth (week of 24–30 Aug 2026)", "url": "https://defirate.com/news/volume-report-top-11b-contracts-sports-combos/", "note": "Kalshi at about 90% of combined contract volume in late August, driven by sports and multi-leg combos; the share is contract volume, not dollars at risk." }, { "label": "Pew Research Center — Trading volume on prediction markets has soared in recent months (27 May 2026)", "url": "https://www.pewresearch.org/short-reads/2026/05/27/trading-volume-on-prediction-markets-has-soared-in-recent-months/", "note": "Sports made up about 80% of Kalshi's volume and 39% of Polymarket's since July 2024." }, { "label": "Finance Magnates — Polymarket rolls out U.S. app after CFTC green light, starting with sports", "url": "https://www.financemagnates.com/forex/polymarket-rolls-out-us-mobile-app-after-cftc-green-light-starting-with-sports-events/", "note": "Polymarket US launched 2 December 2025 on the QCEX designation acquired for $112 million; the invite waitlist was removed in May 2026; iOS only at the time of writing." }, { "label": "The Hill — Appeals court rules states can regulate prediction market platforms like gambling", "url": "https://thehill.com/policy/technology/6058357-ninth-circuit-prediction-markets-kalshi-states-cftc/", "note": "The Ninth Circuit held that federal law does not pre-empt state regulation of prediction markets, contradicting other rulings and setting up a Supreme Court question; Kalshi is blocked in Washington, Michigan and Nevada." }, { "label": "ESPN — CFTC proposes rules limiting prediction markets: no injuries, no props", "url": "https://www.espn.com/espn/betting/story/_/id/49019930/cftc-proposes-rules-limiting-prediction-markets-kalshi-sports", "note": "Proposed rules would bar event contracts on officiating outcomes and player injuries, with a 45-day comment period." }, { "label": "Kalshi Research — Prediction market calibration: how accurate is Kalshi? (20 Aug 2026)", "url": "https://news.kalshi.com/p/prediction-market-calibration-how-accurate-is-kalshi", "note": "The venue's own study over more than 2.2 million data points: forecasts improve with time, participation and volume; markets with about $10,000 or twenty traders were already informative; the comparison offered is superforecaster Brier scores of 0.10–0.15." }, { "label": "Krause — Calibration and Forecast Accuracy of Macroeconomic Prediction Markets: Evidence from Kalshi, 2025–2026 (SSRN)", "url": "https://papers.ssrn.com/sol3/papers.cfm?abstract_id=7117919", "note": "594 Kalshi macro markets, 1,860 observations, nine series: overall Brier 0.0987; federal-funds-rate markets near-perfectly calibrated (0.0001); unemployment-rate markets the weakest (0.1302); one-hour horizon 0.0812 against 0.1587 at thirty days, as summarised in the abstract." }, { "label": "Polymarket Gamma API — open events", "url": "https://gamma-api.polymarket.com/events/keyset?closed=false", "note": "Every open event with tags and open interest, 23:22–23:24 UTC 13 September 2026" }, { "label": "Kalshi public trade API — open events", "url": "https://api.elections.kalshi.com/trade-api/v2/events?status=open&with_nested_markets=true", "note": "Every open event with category and market open interest, 23:21–23:22 UTC 13 September 2026" }, { "label": "Vultax: Is Polymarket legal in the US?", "url": "https://vultax.com/research/is-polymarket-legal-in-the-us-2026", "note": "Kalshi's 24-hour trading by category, 12 September 2026" }, { "label": "Vultax: Polymarket sports and esports markets", "url": "https://vultax.com/research/polymarket-sports-and-esports-markets", "note": "Esports share of money traded" }, { "label": "Vultax: Polymarket's up-or-down crypto markets", "url": "https://vultax.com/research/polymarket-bitcoin-up-or-down-5-minute-markets", "note": "Share of trades in short crypto markets" }, { "label": "Vultax: Kalshi–Polymarket arbitrage after fees", "url": "https://vultax.com/research/kalshi-polymarket-arbitrage-gaps-after-fees", "note": "Whether a cross-venue gap survives both venues' fees at depth" }, { "label": "Vultax: Polymarket vs Kalshi midterm odds", "url": "https://vultax.com/research/polymarket-vs-kalshi-midterms-odds-2026", "note": "The same comparison across 65 matched midterm contracts" } ], "measurement": { "kind": null, "temporalCoverage": null, "population": null, "methodology": null }, "id": "venues-table", "kind": "published-table", "description": "Vultax compared Kalshi and Polymarket on structure, fees, holdings and prices. On 13 September 2026, 55.3% of Polymarket's $341.3 million open interest sat on politics and 43.6% of Kalshi's 1.04 billion open contracts on sports; on a matched Fed pair in July, prices sat a median 5.7 cents apart.", "articleUrl": "https://vultax.com/research/kalshi-polymarket-open-interest-by-category", "sectionUrl": "https://vultax.com/research/kalshi-polymarket-open-interest-by-category#venues", "publishedAt": "2026-09-13", "modifiedAt": "2026-09-16", "version": "7408c3bbc04a8be54c1ad6f6a8ed954183c2db6cabedafad7b29d1cfd8f66d88", "isLive": false, "limitations": "Published research snapshots and calculations; these exports do not contain current quotes or live account data. Observation windows and populations are stated in the source captions and methodology. Missing metadata is unknown, not zero. Figures describe quoted prices observed by Vultax's cross-venue record over a single 24-hour window on eight matched contracts and are not a statement about either venue's conduct or a market-wide rate of arbitrage. Fee figures are as published on the stated date. Nothing here is financial advice. Open-interest figures are a single reading on 13 September 2026; Polymarket reports dollars and Kalshi reports contracts, so shares compare within a venue, not totals across venues.", "downloads": { "json": "https://vultax.com/research/kalshi-polymarket-open-interest-by-category/data.json?dataset=venues-table", "csv": "https://vultax.com/research/kalshi-polymarket-open-interest-by-category/data.csv?dataset=venues-table" } } ] }