Open Interest and Volume: What They Tell You

Every options chain shows two columns that look similar but measure completely different things: open interest and volume. Confusing the two, or only looking at one, is a common mistake that can lead to trading contracts that are harder to get in and out of than they appear.

What Volume Measures

Volume is the number of contracts traded on a given day for a specific strike and expiration. It resets to zero at the start of every trading session. High volume means a lot of trading activity happened in that contract today specifically.

What Open Interest Measures

Open interest is the total number of contracts currently open, meaning contracts that have been bought or sold to open a position and not yet closed. Unlike volume, open interest doesn't reset daily. It accumulates over time and only changes when positions are opened or closed, not simply traded.

An easy way to separate the two: volume answers "how much traded today," while open interest answers "how many contracts currently exist."

How a Single Trade Affects Each One

A single transaction can move volume and open interest differently depending on what the two parties are doing:

If a buyer opens a new long position and a seller opens a new short position, both volume and open interest increase by that trade size, since a brand new contract now exists.

If a buyer closes an existing short position and a seller closes an existing long position, volume increases but open interest decreases, since a contract that existed before has now been closed out.

If one side opens a new position while the other closes an existing one, volume increases but open interest stays flat, since one contract was simply transferred rather than created or destroyed.

This is why open interest changes don't always match volume on a given day. The relationship between the two tells you something about what kind of activity is happening, not just how much.

Why Both Matter for Liquidity

Both figures matter for assessing how easily you can get in and out of a position. High volume on a given day suggests active trading interest right now. High open interest suggests a contract has broad, sustained participation, which generally correlates with tighter bid-ask spreads and more reliable fills.

A contract with high open interest but low volume today might still be reasonably liquid, since there's a large existing base of participants who could become active again. A contract with high volume but very low open interest might be seeing a short-term spike in interest, often around news or an earnings date, without the deeper liquidity that comes from sustained open interest.

A Practical Way to Use This When Screening Trades

Before entering a position, especially in a less commonly traded underlying, checking both numbers gives a fuller liquidity picture than either alone. Low open interest combined with low volume is generally a sign to expect wider spreads and more difficulty exiting the position at a fair price, which matters more the larger the position size.

Frequently Asked Questions

Does open interest reset every day like volume does?

No. Volume resets to zero at the start of each trading session. Open interest carries forward and only changes based on positions opening or closing.

Is high open interest always a good sign?

Generally, higher open interest correlates with better liquidity and tighter spreads, but it's worth looking at both open interest and volume together rather than relying on just one.

Why would open interest go down on a high volume day?

That typically means more existing positions were closed than new positions were opened, even though total trading activity (volume) was high.