Open interest is the number of futures contracts that remain outstanding in a specified contract month or market: contracts that have been entered into but have not yet been offset, fulfilled by delivery, cash settled or otherwise terminated under the applicable contract rules. Every open futures contract has one long and one short, so open interest counts the contract only once rather than adding both sides. The aggregate number of open longs therefore equals the aggregate number of open shorts.
This makes open interest fundamentally different from trading volume. Volume counts contracts traded during a period; open interest is a stock of contractual positions that survives beyond the transactions that created or transferred them. A heavily traded contract can therefore show little change in open interest if trading mainly transfers existing exposure from one participant to another.
What actually changes the count
Open interest rises when a transaction creates additional outstanding exposure on both sides—for example, when a buyer opening a new long trades with a seller opening a new short. It falls when both sides of an existing contract are closed. If one participant opens while the counterparty closes an existing position, the exposure is effectively transferred and total open interest is unchanged. A trade by itself therefore does not reveal the change in open interest: the position effect of both counterparties matters.
The same logic explains why rising open interest is not inherently bullish and falling open interest is not inherently bearish. An increase means more contracts remain open, but every new contract contains both a long and a short. Price, participant positioning, term structure, volume and other evidence are needed before attaching a directional interpretation. High open interest may also accompany a liquid market, but it is not a complete liquidity measure because executable depth, bid-ask spreads and concentration by contract month still matter.
The timestamp is part of the data
Open interest is a position statistic that is not necessarily synchronized with the latest trade. Publication conventions therefore matter. CME Group, for example, notes that its settlement pages display the previous trading day's futures open interest, while its daily volume-and-open-interest report is preliminary at the end of the trading day and official data are released the following morning. For historical or systematic research, the practical consequence is important: record the observation date and data vintage rather than assuming that open interest shown beside today's price and volume describes the same moment.