Kalshi vs Polymarket: Rules, Fees, and Open Interest

The most common way people rank Kalshi against Polymarket is by trading volume, and that comparison points the wrong way. Polymarket usually posts the larger headline volume, but Kalshi holds more open interest, meaning more money is actually committed to unresolved positions at any given moment. The two platforms differ in what they are allowed to do, who can legally use them, and how you pay to trade, and those differences explain the split.

Both are prediction markets: venues where you buy contracts that pay out based on the outcome of a real-world event, like an election, an economic release, or a sports game. If you are right, each contract you hold settles at $1. If you are wrong, it settles at $0. The price you pay in between, say 60 cents, reads as the market's estimate of the probability, in this case roughly 60 percent. Think of it as a structured, tradable version of betting on an election with a friend, except there are thousands of counterparties and the odds move in real time.

The rest of this piece compares the two across the things a chooser actually needs, then uses onchain data to show where capital sits.

Kalshi vs Polymarket at a glance

Regulatory status and who can use each

This is the difference that drives everything else. Kalshi operates as a designated contract market registered with the Commodity Futures Trading Commission, the US federal agency that oversees derivatives markets. That registration is what lets eligible US residents trade event contracts on a regulated exchange (Kalshi's own site describes its markets and structure).

Polymarket sits outside that framework. It is not a US-regulated exchange, and after a 2022 CFTC settlement it agreed to block US users. In practice its user base has been non-US, and trades settle onchain rather than through a US regulator. Allium's onchain analysis has found that US-linked wallets nonetheless accounted for hundreds of millions of dollars in political betting on Polymarket despite that restriction, a gap covered in our analysis of US activity on Polymarket.

The regulatory picture is still moving. Enforcement posture, licensing, and the treatment of prediction markets as either regulated event contracts or something closer to gambling remain contested and vary by jurisdiction. None of this is legal advice; it describes the state of play as the operators and the CFTC have stated it.

How fees actually work

The platforms charge in structurally different ways, which is why a simple "which is cheaper" answer is misleading.

Kalshi charges an explicit per-contract fee that depends on the contract's price and the number of contracts. The fee is smallest at the extremes (near 1 cent or near 99 cents) and largest in the middle of the price range, peaking around a 50 cent contract. A trade on a coin-flip market costs more per contract than the same size trade on a near-certain outcome. The mechanics are worked through in our explainer on how Kalshi works.

Polymarket advertises no trading fee. That does not mean trading is free. Your cost is embedded in the spread between the buy and sell price and in slippage, the difference between the price you expect and the price you get when your order moves the market. On a thin market that indirect cost can exceed an explicit fee. How the platform earns without a headline trading fee is covered in how Polymarket makes money.

The practical takeaway: Kalshi's cost is visible and predictable but peaks on 50/50 markets, while Polymarket's cost is invisible on the fee schedule and shows up only when you compare the price you wanted to the price you got.

Contracts, liquidity, and settlement

Both list event contracts across politics, economics, sports, and other categories. Kalshi runs a central limit order book operated by the exchange, and it resolves markets under CFTC oversight with payouts in US dollars. Polymarket uses an order book where positions are outcome shares settled onchain, with resolution handled through an oracle process and payouts in USDC.

Resolution is one of the genuinely unsettled areas. Onchain resolution depends on the oracle reporting an outcome correctly, and disputes over how a market should settle have drawn scrutiny; Bloomberg cited Allium's onchain data in reporting on possible settlement manipulation on Polymarket. Kalshi's resolution runs through the exchange under a regulator, which changes the recourse available but does not make disputes impossible.

Where capital actually sits: the open interest read

Volume answers "how much trading happened." Open interest answers "how much money is still at stake in unresolved positions." They can diverge sharply, and that divergence is where the Kalshi versus Polymarket comparison gets interesting.

Think of two casinos. One has people constantly buying and selling chips all day. The other has fewer transactions, but much larger bets sitting on the table. The first wins on volume, the second on open interest. That is the relationship between Polymarket and Kalshi.

Using Allium Terminal's prediction markets view, which tracks Polymarket, Polymarket US, and Kalshi on one consistent methodology, two patterns hold up. Polymarket dominates raw notional and dollar volume. Kalshi holds the larger share of open interest, reflecting a market mix and a regulated, USD-collateralized user base that tends to hold positions to resolution rather than trade in and out. Polymarket US, a separate platform from Polymarket because of US regulatory constraints, remains a small sliver and should not be folded into Polymarket's totals, since doing so overstates Polymarket's footprint. A step-by-step walkthrough of pulling and charting this data lives in our guide to comparing open interest with onchain data.

Open interest is also event-driven. Across a full history the largest spike lines up with the 2024 US presidential election, with combined open interest peaking near $900M before dropping sharply after the result resolved. Kalshi's steadier baseline reflects a broader mix of financial and economic markets that do not resolve in one concentrated burst.

The data problem behind the comparison

Comparing these two on open interest is harder than it sounds because the venues do not describe activity the same way. Kalshi's contracts and dollar balances live in the exchange's own records. Polymarket's positions are outcome shares held in self-custodied wallets and settled onchain in USDC. To put them side by side, every position has to resolve to the same fields: venue, market, outcome, collateral asset, notional size, USD value, and whether the position is open or resolved. Polymarket and Polymarket US have to be kept distinct rather than merged. Allium normalizes those records across 150+ chains into a consistent schema, which is what lets open interest, volume, and category share be measured on one methodology instead of stitched from mismatched sources. The underlying prediction market datasets and Kalshi tables are documented for anyone who wants the field-level detail.

Frequently asked questions

Is Kalshi or Polymarket regulated?

Kalshi is registered with the CFTC as a designated contract market, a federally regulated US exchange. Polymarket is not a US-regulated exchange; it operates offshore and settles trades onchain. This is the core difference between the two platforms.

Can US residents legally use Polymarket?

Polymarket agreed to block US users following a 2022 CFTC settlement, and its user base is primarily non-US. Allium's onchain analysis has found US-linked wallets active despite that restriction. Kalshi, by contrast, is available to eligible US residents through a regulated exchange.

Which is cheaper to trade, Kalshi or Polymarket?

It depends on the market. Kalshi charges an explicit per-contract fee that peaks around 50 cent contracts and is smallest at the extremes. Polymarket charges no stated trading fee, but cost appears in the bid-ask spread and slippage, which can be significant on thin markets.

Does Polymarket have more open interest than Kalshi?

Not currently. According to Allium Terminal data, Kalshi holds more open interest, meaning more capital is committed to unresolved positions. Polymarket generally generates higher trading volume, so the two lead on different measures.

How are markets settled on each platform?

Kalshi resolves markets through the exchange under CFTC oversight, with payouts in US dollars. Polymarket resolves through an onchain oracle process, with payouts in USDC. Onchain resolution has drawn scrutiny over possible disputes, an area Bloomberg has cited Allium data on.

Should Polymarket US be combined with Polymarket?

No. Polymarket US and Polymarket operate under different regulatory frameworks and offer different sets of markets. Combining them overstates Polymarket's footprint and distorts any comparison with Kalshi.