Polymarket vs Kalshi: which is bigger by volume, open interest and valuation

Kalshi is bigger. In the 30 days to 14 October 2026, DeFi Rate’s dashboard credits it with about three quarters of tracked prediction-market volume at face value, and about 70% of the dollars actually paid. It also has more open interest and the higher last closed valuation. Polymarket led until the second half of 2025, and its international site still trades more politics than Kalshi on the same dashboard. Below is every size metric I could source, with the date, the unit and who measured it, because the numbers disagree more than the headlines admit.

This guide is about size only. For fees, KYC, payouts and resolution, round by round, see my Kalshi vs Polymarket comparison.

The scoreboard, metric by metric

Three venues matter here, not two. Kalshi is one exchange. Polymarket runs an international, crypto-settled site and a separate US exchange, Polymarket US, with its own order books. Dashboards count them separately, and so do I.

On every row Kalshi comes first. On the September face-value figures, Kalshi is 82% of the three venues combined. On dollars paid it is 74%. That gap between the two percentages is the first hint that the unit matters.

Trading volume: who led when

Polymarket was the larger venue through mid-2025. DeFi Rate’s history starts on 1 August 2025, and that month shows Polymarket at $1.1B against Kalshi’s $859M. In September 2025 The Block reported Kalshi ahead, $1.3B to $773M for the month to date. Kalshi said in a statement that it held 62.2% of global volume at that point.

The lead held, narrowly at first: DeFi Rate’s tables show October 2025 at $4.4B for Kalshi against $4.1B. Then it grew. The same tables give Kalshi $13.1B in March 2026 against $10.6B for Polymarket’s international site. In June, the World Cup month, Kalshi reached $32.7B. Polymarket international did $11.3B and the new US exchange $4.1B. By September 2026 Kalshi traded $60.7B. Polymarket’s two venues together traded $13.5B.

Weekly numbers say the same. DeFi Rate’s report for 5–11 October 2026 puts Kalshi at a record $19.03B in contract volume. That is $4.42B at execution prices. Polymarket US reported 2.82 billion in its own “Trade Volume” field. Polymarket’s international site did $498.81M in taker cash volume. The author warns that the three figures use different methods and should not be added together.

All-time volume is the one number I cannot give you. Neither operator publishes a lifetime figure I could verify. DeFi Rate’s series start on 1 August 2025: since then Kalshi shows $299.4B at face value, Polymarket international $90.6B and Polymarket US $27.7B since 27 May 2026.

Why the volume numbers never agree

Take August 2026. The Block’s dashboard has Kalshi at $37.17B. DeFi Rate’s table has $40.0B. Both put Kalshi at about 82% of the three venues, so the ranking survives even when the totals do not. Here is where the differences come from.

• Face value against dollars paid. Kalshi reports contracts at their $1 face value. A contract bought at 10¢ adds $1 to volume and 10¢ to money spent. DeFi Rate shows both, and Kalshi’s 30-day figure falls from $74.3B to $17.4B on the second.
• Double counting on-chain. In December 2025 Paradigm’s Storm Slivkoff showed that most Polymarket dashboards summed both the maker and the taker record of each fill. His example: a $4.13 trade reported as $8.26. Paradigm discloses that it invests in Kalshi.
• Short markets recycle money. DeFi Rate notes that Kalshi’s 15-minute markets settle 96 times a day, so the same dollars can trade again and again.
• Combos. Multi-leg parlays sell for a few cents per leg but count at face value. DeFi Rate puts Kalshi’s combos at 26% of its dollars paid over 30 days. In one recent week, 65% of Kalshi’s volume sat outside DeFi Rate’s category labels.
• Wash trading. A Columbia University study posted on SSRN in November 2025, not peer reviewed, estimated that about 25% of Polymarket’s volume was likely wash trading. In sports markets the estimate was 45%. The study looked at Polymarket only.
• Revisions. Two The Block articles give October 2025 as $4.4B against $3.02B and as $5.81B against $3.7B. Same publication, same month, different figures.

Open interest: the money still in the market

Open interest is the value of positions not yet settled. Allium describes it as money committed to unresolved positions. It is harder to inflate by trading back and forth, so I trust it more than volume. It is also where the data is messiest.

DeFi Rate’s all-venue table shows Kalshi at $965M on 14 October 2026. Its Kalshi page shows $3.09B for the same close, with a peak of $4.16B on 11 October. The page does not reconcile the two. In June 2026 The Block reported a Kalshi record of $1.16B, its first time above $1B. Polymarket’s international site sat at $346M–$348M on DeFi Rate on 14 October, and Polymarket US at $60M–$85M, again depending on which table you read.

Whichever Kalshi figure is right, it is above Polymarket’s two venues combined. Allium, using its own terminal data in July 2026, also says Kalshi holds more open interest than Polymarket.

Size by category: sports, crypto, politics

This is where “bigger” changes answer. DeFi Rate’s 30-day category tables, face value:

Three things stand out. First, Kalshi’s sports line excludes its combos, which DeFi Rate ranks separately. Polymarket US’s sports line is 98% of its total, so its parlay-style contracts sit inside it. The two sports figures are closer than they look, and not comparable as they stand.

Second, politics is the one category where Polymarket’s venues together, $324M, out-trade Kalshi. It is also a small slice of both businesses in a month without a national election.

Third, a single market can say the opposite. On 15 October 2026 I pulled the 2026 House control market from both public APIs. Kalshi showed 57.5 million contracts traded and 41 million open, about $14.6M at current prices by my arithmetic. Polymarket’s international market showed $17.5M of volume and $7.9M of open interest in its own units. On that election market Kalshi held more money.

Valuation, funding and revenue

Private valuations are what investors paid at one moment, not a measure of trading. Still, they track the volume story.

• Kalshi: $5B in October 2025 (Series D, over $300M), $11B in late 2025 ($1B), then $22B in a $1B Series F led by Coatue, announced on 7 May 2026. A Form D reported by The Block on 26 August 2026 shows $1.12B of equity sold since 3 April. On 30 September 2026 Reuters reported “advanced talks” for about $1B at $40B; I found no report that it closed.
• Polymarket: ICE agreed in October 2025 to invest up to $2B at about $8B pre-investment, per ICE’s release. Bloomberg reported a round of about $1B at $15B as closed by August 2026. On 31 August 2026 Bloomberg reported 1789 Capital leading a $1B round at a $21B post-money valuation, with no closing reported.

Revenue is thinner. Reuters reported on 26 June 2026, from one source, that Polymarket’s annualised revenue had passed $1B; Finance Magnates, citing Bloomberg, later put it above $1.2B. Kalshi does not publish revenue. InGame estimated from Kalshi’s trades and fee formula that it had earned $1.15B in fees since launch by late June 2026. That is an estimate, not a company figure.

Kalshi also said in May 2026 that its annualised volume had risen from $52B to $178B. Treat that as the operator’s own claim.

Regulatory footprint and where each one is open

Size depends partly on who is allowed in. I describe the registrations and the operators’ own country rules; I make no claim about what is permitted where you live.

• Kalshi: the CFTC designated KalshiEX LLC as a contract market on 4 November 2020. In October 2025 it said it was available in over 140 countries; its member agreement then listed 38 restricted jurisdictions, including Canada, France and the UK, per Cointelegraph. Its help centre, dated 20 March 2026, says only that you can trade “from many countries”.
• Polymarket international: settled with the CFTC in January 2022 for $1.4M under its earlier operator. Its geoblocking page lists 37 close-only jurisdictions, including the US, UK, France, Germany, Italy, Australia, Brazil and the Canadian provinces of British Columbia, Ontario, Alberta and Quebec.
• Polymarket US: built on QCX, a CFTC-licensed exchange and clearing house Polymarket bought for $112M in July 2025. The CFTC issued an amended order of designation on 25 November 2025. The app opened to its waitlist on 3 December 2025, sports first.
• Canada: Wealthsimple announced in June 2026 an app offering nearly 4,000 Kalshi contracts. CIRO authorised it in March 2026 for economic indicators, financial markets and climate only, with settlement 30 days or longer.

So Kalshi’s volume runs through one US-regulated exchange with its own restricted list. Polymarket’s is split between a sports-first US app and a global but heavily geoblocked site. The legality guide tracks regulators’ actions country by country.

What “bigger” means when you trade

Platform size is not your fill price. What helps you is depth and a tight spread on the market you want, plus access from where you are.

On 15 October 2026 I compared two markets on both public APIs. Seahawks vs Broncos, played that night: both quoted Seattle at 56¢ bid, 57¢ ask. Kalshi had traded 1.86 million contracts on the game, and Polymarket $552.6K on its moneyline. On the House market Kalshi quoted 90.2¢ / 90.3¢, a tenth of a cent wide, and Polymarket 90¢ / 91¢. In my comparison a week earlier, Polymarket was tighter on the October Fed decision. Size tilts the odds toward a tight book, but check the market you actually trade.

Three practical points follow:

1. Volume rankings are dominated by sports and short crypto markets. If you trade elections or niche events, look at open interest and the order book for that market, not the platform total.
2. The bigger venue is useless if it is closed to you. Check each operator’s restricted list for your physical location; the reviews of Kalshi and Polymarket quote them with dates.
3. Fees are a separate cost, highest at mid prices. The fee formulas and a worked example are in prediction markets vs sports betting.

The short version

By trading volume, open interest, trade count and last closed valuation, Kalshi is bigger than Polymarket in October 2026, with roughly three quarters of tracked face-value volume on DeFi Rate’s dashboard and about 70% of dollars paid. Polymarket led until mid-2025 and still edges Kalshi in politics over the last 30 days. Every figure depends on the measurer: face value or dollars paid, one side of a trade or both, with or without combos. If you are new to the mechanics, start with Polymarket for beginners, and see the prediction markets category for every platform I have reviewed.