What is open interest on Polymarket?

What the number actually counts

Every Polymarket contract is a share that pays one dollar if its outcome happens and nothing if it does not. Open interest is the value of all such shares that somebody is still holding. When two participants transact and one of them is opening a new position that the other is not closing, open interest rises. When a holder exits to somebody who is also exiting, it falls. When the market resolves, every share settles at once and the figure for that market drops to zero.

The term is borrowed from futures markets, where it has meant the same thing for a century: the count of contracts that remain outstanding rather than the count of contracts that have changed hands. The research aggregator Predik put the definition plainly in its 2026 write-up, describing open interest as the total value of unsettled positions across all active markets. Nothing about the prediction-market version changes that logic. What changes is that the underlying ledger is public, so the figure can be reconstructed from chain data instead of taken on trust from an exchange.

The practical consequence is that open interest is a stock and volume is a flow. A stock can shrink. This is the single most common confusion in coverage of the platform, and it is why a market can carry an enormous volume headline while almost nobody holds a position in it any more.

Volume, open interest, and liquidity are three different numbers

Market pages across the prediction-market ecosystem show some combination of these three, often without saying which is which. They answer different questions and they move independently. The table below sets out what each one measures and where each one goes when a market settles. Time-sensitive rows carry their own date.

The volume and liquidity columns are the pair this desk already covers in detail in volume versus liquidity on Polymarket. Open interest is the third leg, and it is the one that most directly answers the question people are usually asking when they look at a volume figure: how much money is actually on this right now.

The World Cup case: volume up 300 percent, open interest flat

The 2026 FIFA World Cup produced an unusually clean demonstration of the divergence. The Block reported on June 24, 2026 that Polymarket soccer volume ran past $2 billion in the first ten days of the tournament, with daily average volume climbing from about $53 million to roughly $220 million, a rise of around 300 percent. Over the same stretch, Polymarket's overall open interest stayed relatively flat.

Both facts are true at once, and together they describe a behaviour rather than a contradiction. Heavy churn in short-dated match markets generates enormous volume because positions are opened and closed within hours, but very little of that capital stays committed overnight, so the stock of open positions barely moves. The contrast was visible across venues in the same week: Kalshi's open interest passed $1.16 billion for the first time, up about 350 percent year to date, which The Block read as longer holding periods on that venue rather than more trading.

This is the reason a volume number alone is a poor proxy for conviction. A market with $50 million of volume built from thousands of same-day round trips and a market with $50 million of volume built from a few hundred positions held to resolution look identical on a volume axis and nothing alike on an open interest axis. The same distinction underlies the maker and taker split described in makers versus takers on Polymarket.

Where the published numbers come from

There is no single official Polymarket open interest ticker, so the figures in circulation come from a handful of places and do not always agree. Polymarket's own API documentation exposes an open interest endpoint that returns a value per market condition hash, but it does not state the units or the calculation method on the same page, which makes it hard to reconcile against third-party series without assumptions. Dashboards from The Block, Blockworks, and CoinGlass each publish their own aggregate charts.

The dated milestones are the most useful anchors. Prediction-market open interest in aggregate crossed $1 billion for the first time on February 9, 2026, at $1.066 billion. By late March 2026 the aggregate hit $1.2 billion, split as $497 million on Kalshi and $457.5 million on Polymarket, with Polymarket also carrying $2.54 billion of weekly notional volume and $434.7 million of total value locked at that point. Predik noted that Polymarket's open interest had by then surpassed its 2024 US election peak, and that the composition had broadened across geopolitics, crypto, sports, and entertainment instead of sitting in one race. The record as of this writing is $1.48 billion in aggregate for the week ending June 15, 2026, attributed to a16z Crypto.

Treat cross-source comparisons carefully. Kalshi is a CFTC-regulated venue reporting in a different framework from an on-chain order book, and the two are not guaranteed to define an open contract identically. The broader contrast between the venues is set out in Polymarket versus Kalshi.

What our own snapshot can and cannot show

Cent Signals does not currently carry open interest, and it is worth being explicit about why. Our snapshot reads Polymarket's public Gamma API, which returns cumulative volume and resting liquidity per market but not the value of open positions. Every activity figure on this site is therefore one of the other two, and which is which is set out on our methodology page.

What the snapshot does show is how far apart volume and live capital can drift. As of the July 17, 2026 snapshot, the 400 markets we track carried about $1.86 billion of cumulative volume against roughly $176.9 million of resting liquidity, an aggregate ratio near eleven to one, with a median of about six to one across individual markets. The extremes are far wider. The market on whether Gedion Timothewos becomes the next Prime Minister of Ethiopia showed about $55.9 million of lifetime volume against roughly $23,700 of resting liquidity, with YES quoted at a cent. A volume figure in the tens of millions and a book worth a few thousand dollars describe the same market on the same day.

None of those three numbers is the whole picture on its own. The markets index carries the volume and liquidity we do collect, the glossary defines each term as this desk uses it, and what is a Polymarket order book explains where the resting quotes behind the liquidity column come from.

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