Volume and Open Interest Are Not the Same Number
One counts how much changed hands, the other counts how much is outstanding. They tell you different things and conflating them misreads a market badly.
Two liquidity numbers appear on almost every market page and they measure different things. Using the wrong one gives you a systematically wrong picture.
Definitions
Volume is how many contracts traded in a period. It is a flow: contracts per day, per hour, since listing.
Open interest is how many contracts are currently outstanding and not yet settled. It is a level: how much exposure exists right now.
A trade between two people opening new positions increases both. A trade where one person closes and another opens increases volume and leaves open interest unchanged. A trade where both close increases volume and reduces open interest.
What each tells you
Volume tells you about tradeability. Can you get in and out. High recent volume means someone is active and your order will probably find a counterparty. This is the number that matters for execution.
Open interest tells you about commitment. How much capital is actually committed to this question. A market with high open interest has people who have taken real positions and will hold them to settlement.
The four combinations
High volume, high open interest. A genuinely active market with committed participants. The best case, and the only one where the price deserves full weight as a forecast.
High volume, low open interest. Lots of trading, little accumulation. Positions opening and closing quickly — short-term activity rather than considered views. Fine for execution, less meaningful as a forecast.
Low volume, high open interest. Positions were established earlier and are being held. The price may be stale even though substantial capital is committed. This combination is common on long-dated markets and it is where stale quotes live.
Low volume, low open interest. Nothing is happening. The price is not a forecast. Exclude from any analysis. Thin market warning signs.
Common mistakes
Using cumulative volume since listing as a liquidity measure. A market listed six months ago with high total volume may have traded nothing this week. Always use recent volume.
Reading open interest as a directional signal. It counts contracts outstanding, not which side anyone is on. Every contract has a buyer and a seller. Rising open interest means more capital committed, not that anyone is bullish.
Ignoring both in cross-venue comparison. A price difference between a busy market and a dormant one is a difference between a forecast and a leftover. Filtering on recent volume on both sides removes most false findings. Gap size versus liquidity.
For building a filter
A reasonable liquidity screen uses several things together: recent volume, current depth within a couple of cents of the top, open interest, and the age of the last trade. Any single one of them can be fooled.
This site applies that kind of filter before comparing anything, which is why its tables are shorter than a naive comparison would produce and why the rows in them are more likely to mean something. The methodology.
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Educational content only. Nothing here is financial advice, a solicitation, or an offer to trade. Prediction markets can move against you and a position can lose its entire value. Where this guide describes a difference between venues it describes a theoretical cross-market edge in observed prices, which is not a profit: fees, spreads, capital lockup, transfer time and differing settlement rules all come out of it first.
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