What Is Open Interest (OI)? How Does It Differ from Volume?
Learn how Open Interest (OI) counts outstanding contracts while Volume counts matched trades, with examples and limits for interpreting derivatives data.
By Kru Kook (Krittiya Khunabrimas)
Open Interest (OI) and Volume are different measures reported alongside one another in derivatives markets. OI shows the number of contracts that remain open, while Volume shows the number of contracts matched during a specified period. Both describe past market activity; neither confirms whether prices will rise or fall.
Open Interest (OI) Counts Outstanding Contracts
Open Interest is the number of futures or options contracts that have been opened and have not yet been offset, closed, or expired under the contract terms. Each outstanding contract has one buyer and one seller, but OI counts it as one contract—not two contracts for the two sides.
See the TFEX Open Interest glossary (Thai), and check that the details apply to the product type and contract specifications you are studying.
Volume Counts Contracts Matched in Trading
Volume is the number of contracts matched during the reported period, such as a trading day. Each matched contract counts as one contract of Volume, whether it opens a position, closes a position, or pairs one party opening with another party closing.
TFEX explains the definitions and interpretation of both measures in its article What Do Volume and OI Tell Us? (Thai).
Comparing OI and Volume
| Measure | What it counts | What the figure represents |
|---|---|---|
| Open Interest (OI) | Contracts that remain open | Outstanding contracts at the reporting time |
| Volume | Contracts matched during the period | Trading activity over a specified period |
For example, end-of-day OI is the number of contracts still outstanding. The day's Volume is the total number of contracts matched throughout the trading period, according to the market's reporting method.
How Does One Matched Contract Change OI?
The change in OI depends on whether each party in a matched trade is opening or closing a position. Volume increases by the number of contracts matched. The examples below describe one matched contract, assuming the data is measured over the same period:
- Both parties open new positions: Volume increases by 1 contract and OI increases by 1 contract.
- One party opens a new position while the other closes an existing position: Volume increases by 1 contract and OI is unchanged.
- Both parties close existing positions: Volume increases by 1 contract and OI decreases by 1 contract.
These examples explain the counting method. They do not indicate the market's net Long or Short direction. Reporting details may also depend on the product and when the data is published.
What OI Can Show—and Its Limitations
Changes in OI show how the number of outstanding contracts for a product or series has changed. Considered together with price and Volume, OI can help describe the context of market activity during a period. However, aggregate OI does not reveal the identities of position holders, identify large market participants, or show whether newly opened positions are Long or Short.
An increase in OI is therefore not proof that the market is bullish or that institutional capital is flowing in. A decrease in OI does not confirm that either side is closing positions to take profits. Price patterns, Volume, and OI can be used as information for study, but no single measure should be treated as a trading signal or a guarantee of results.
How to Check OI Data from TFEX
- Select the product and contract series that match the contract you want to study.
- Check the date, time, and trading period, and compare figures from the same period.
- Record price, Volume, and OI together, keeping outstanding contracts distinct from trading volume for the period.
- Check the expiration date and any move to the next series. Data for different contracts should not be joined without identifying the change.
- Verify definitions and market data against an official source, such as the TFEX Market Statistics page.
Intraday figures may refer to different reporting intervals depending on the product, so always read the notes on the original data page. This article is for educational purposes and is not a recommendation to trade any contract.
Summary
Volume counts contracts matched during a period, while OI counts contracts that remain outstanding. The two measures provide different information and cannot, by themselves, identify market direction or position holders. If you use them for study, check the product, contract series, and reporting time with TFEX before interpreting the figures.