Open interest

A count of contracts still outstanding at the end of a session. Every contract has a long and a short, and open interest counts the pair once, so the figure describes how much of the market remains committed rather than how many people are involved in it. Exchanges publish it per contract month, usually with the following morning's data.

It changes only according to what the two sides of a trade are doing. It rises when a new buyer meets a new seller, because a contract that did not exist now does. It falls when a closing buyer meets a closing seller, because a contract has been extinguished. It does not move when one party is opening and the other is closing, because the contract has merely changed hands. That is the whole difference from volume, which counts everything that traded in the period and resets each session, while open interest carries forward.

A widely repeated convention reads rising open interest alongside a rising price as new commitment behind the move, and falling open interest as an existing position being unwound. The convention is bounded and contested, and the exchanges that publish the data make no claim of the kind. Two limits matter more than the convention does. The data cover exchange traded contracts only, so they describe listed futures and options rather than the far larger market dealt over the counter. And a contract for difference has no open interest at all, since there is no central register of contracts and each one exists only between a client and the firm that wrote it.