Open Interest vs Volume: Activity Is Not Commitment

Volume shows how much changed hands. Open interest shows how much is still held. The two numbers answer different questions.

Published 2026-10-23 · Market & data intelligence · Educational, not advice.

Two numbers that look alike and measure different things

Open any prediction market on Kalshi, Polymarket or a broker that offers event contracts and you will usually see a few numbers next to the price. Two of them get mixed up all the time: volume and open interest.

They sound like they measure the same thing, how big a market is. They don't. Volume measures activity. Open interest measures commitment. Once you can tell them apart, a market's numbers start to show you how it is actually being used.

First, a reminder of the setup. Every contract has a buyer on one side and a seller on the other, and the venue's job is to match them. The price is just where the latest buyer and seller agreed. Both numbers below come straight from that structure.

What volume measures

Volume is the number of contracts that changed hands over a period of time. If one person buys 100 YES contracts from someone selling them, that adds 100 to volume. If those same 100 contracts change hands ten more times that afternoon, volume goes up by another 1,000.

Venues usually show volume over a window, such as the last 24 hours, or as a lifetime total since the market opened. Some count it in contracts and some in dollars. Check which one you are looking at before you compare two markets.

Volume tells you how busy a market is. It does not tell you how many positions are still open. A market with a small group of active traders passing the same contracts back and forth can show big volume without growing at all.

What open interest measures

Open interest is the number of contracts that are outstanding right now: opened and not yet closed or settled. Each one is a live agreement between two counterparties. One holds YES, the other holds NO, and they stay paired until one of them exits or the market resolves.

Think of it as a snapshot, not a running total. Volume adds up over time. Open interest rises and falls.

How a single trade moves open interest

This is what makes it click. Every trade moves volume by the same amount, but it can move open interest in three different ways, depending on who is trading:

• Both sides are opening new positions. A new buyer of YES meets a new buyer of NO. A new contract is created. Open interest goes up.
• Both sides are closing existing positions. Someone holding YES sells to someone holding NO who wants out. The contract disappears. Open interest goes down.
• One side opens, the other closes. A new trader buys YES from an existing YES holder who is leaving. The position just moves to someone else. Open interest stays the same.

All three trades add the same amount to volume. Open interest is where the difference shows up, though it is a net figure: opening and closing trades in the same period can offset each other.

Reading them together

Neither number means much alone. Here is how the combinations are usually read. These are ways to interpret a market's structure, not signals about what will happen:

• Volume high, open interest rising. New positions are being opened. Money is coming in, and more exposure is now held that settles when the outcome is known.
• Volume high, open interest flat. Mostly turnover. Positions are being handed from one trader to another, often around a news event, without the market getting any bigger.
• Volume high, open interest falling. People are closing out. Traders may be locking in gains or cutting exposure before resolution.
• Volume low, open interest high. Many positions are open, but few are being traded. Holders are sitting tight.

Why this matters for price

A price move backed by rising open interest reflects new commitments being made at that price. A move on flat or falling open interest may only reflect a few existing holders repositioning or leaving. Neither is right or wrong. They are different events, and the price alone cannot tell them apart.

The end of a contract's life

Prediction markets have a feature that most stock charts don't: a fixed end. When the outcome is known and the market settles, every outstanding contract pays out or expires worthless. Open interest drops to zero. Lifetime volume stays on the record.

So when you compare markets, think about where each one is in its life. A market that settles next week and one that settles next year can have similar volume and very different open interest, simply because of timing.

Common mistakes

• Treating volume as size. High volume can come from a handful of active accounts. Open interest is closer to how much exposure is actually being held.
• Comparing different units. Contracts on one venue and dollars on another are not the same measure.
• Ignoring the time window. 24-hour volume and lifetime volume answer different questions.
• Reading one snapshot as a trend. One open interest reading tells you little. How it changes over days tells you more.

Next time on The Line: liquidity and the order book, and why a market's last traded price is not always a price you could actually trade at.

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Sources

This explainer quotes no live prices. These are the regulator and platform pages that define how event contracts work.

References for this topic

• CFTC: Prediction markets (Learn and Protect)cftc.gov
• Kalshi Help Centerhelp.kalshi.com
• Polymarket documentation: Polymarket 101docs.polymarket.com

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