Open interest vs volume comes down to one simple split: volume counts how many contracts changed hands during a period, while open interest counts how many contracts are still open right now. Volume measures activity, and open interest measures positions that have not been closed or settled yet. This guide on volume vs open interest explains how each number is calculated, walks through one worked prediction market example, and shows what each figure can and cannot tell you.

Quick answer: volume vs open interest

  • Volume is the number of contracts traded in a period. It measures activity.
  • Open interest is the number of contracts still open. It measures positions outstanding.
  • Volume can be higher than open interest. Over time, cumulative volume only adds up, while open interest goes up and down.
  • Neither number alone tells you how easy it is to trade. That is liquidity.
  • On a prediction market, check both numbers plus the bid/ask spread.

Open interest vs volume: the short answer

Both numbers appear on almost every trading screen, from futures and options to prediction markets, and they are easy to mix up because they are both counted in contracts. The difference is about time. Volume looks back over a window, such as one day, and adds up every trade in that window. Open interest is a snapshot: it tells you how many contracts exist at a given moment and have not yet been closed out or paid out.

Here is an analogy, and only an analogy: think of a theater. Volume is like the number of tickets sold during the day, including tickets that were resold. Open interest is like the number of seats occupied right now.

VolumeOpen interest
What it countsContracts traded during a periodContracts still open at a point in time
Can it go down?Daily volume can be lower than the day before, but cumulative volume never fallsYes, when positions are closed or the market settles
When does it reset or end?Resets each period (for example, each day)Drops to zero when the contract expires or settles
What it hints atHow active a market is right nowHow much money is still committed to positions
Common misreading"High volume means lots of open positions""High open interest means the market is easy to trade"

What is trading volume and how is it calculated?

If you are asking what is trading volume, the answer is short: it is the number of contracts that traded during a set period. CME Group, one of the largest futures exchanges, describes volume as the number of contracts traded on a given date, with each trade counted once. Kalshi's glossary uses the same idea for event contracts: volume is the number of contracts traded during a period.

So how is volume calculated? Every time a buyer and a seller agree on a trade, the number of contracts in that trade is added to the running total for the period. If 40 contracts trade at 10 a.m. and 60 more trade at noon, volume for the day so far is 100. It does not matter whether the trade opened a new position or closed an old one. A trade is a trade.

The "one side vs both sides" caveat matters. Every trade has a buyer and a seller, but the trade is counted once, not twice. A trade of 100 contracts adds 100 to volume, not 200. On a prediction market, there is an extra twist: a Yes buyer and a No buyer can be matched together. If a data source adds up Yes trades and No trades separately, it can count the same match twice. Dune, a third-party data provider, warns about this for Polymarket data and says that summing Yes and No price candles would double-count each Yes-versus-No match.

Volume can also be reported in different units. Some platforms show the number of contracts, and others show dollar value. As of October 2026, Kalshi's API documentation describes volume and open interest as one-sided figures and returns both contract counts and dollar values. A dollar figure depends on the prices at which contracts traded, so two markets with the same contract volume can show very different dollar volume.

Finally, volume resets. Daily volume starts at zero each day. Cumulative volume, the total since a market opened, keeps growing for as long as people trade. For a short glossary definition, see our entry on volume.

What is open interest?

What is open interest? It is the number of contracts that have been opened and not yet closed, offset or settled. CME Group defines it for futures as the total number of contracts held by market participants at the end of the trading day. In plain terms, the open interest meaning is "how many contracts are still out there."

Like volume, open interest is counted on one side only. Every open contract has a holder on each side, but CME counts it once. A market with 300 open contracts has an open interest of 300, not 600.

Open interest changes only when a trade creates or removes contracts:

  • It goes up when both traders are opening new positions. A new contract is created.
  • It goes down when both traders are closing existing positions. A contract disappears.
  • It stays the same when one trader opens and the other closes. The contract simply changes hands.

That is why a busy day of trading can leave open interest unchanged, or even lower. A useful open interest definition to remember: it tracks positions, not trades. CME Group also publishes the day-over-day change in open interest, which shows how many contracts were added or removed. Our glossary has a short entry on open interest.

What is open interest in options?

Open interest options data works the same way. The Options Industry Council explains that open interest is the number of option contracts still open, calculated daily by the Options Clearing Corporation from trade reports sent by the exchanges. Volume, by contrast, captures activity within one trading session.

The same three rules apply. When both sides of a trade are "to open," open interest rises. When both are "to close," it falls. When one side opens and the other closes, it stays the same. Because the clearing house calculates it from the day's trades, options and futures open interest is reported once a day, not tick by tick. So when people compare open interest vs volume options figures, they are often comparing a live intraday count with a number from the previous close.

On prediction markets, the idea carries over cleanly: open interest is the number of contracts still outstanding until the market settles. When the market resolves, every remaining contract pays out and open interest falls to zero.

A worked example: one prediction market, three days

Hypothetical example, numbers are made up for illustration. Imagine a market called "Will X happen by year-end?" A Yes contract pays $1 if X happens, and a No contract pays $1 if it does not. We count contracts on one side only, the way Kalshi's API describes it.

DayTradeWhat happenedVolume that dayOpen interest at end of day
1Ana buys 100 Yes, Ben buys 100 No100 new contracts created1000 + 100 = 100
1Ana sells 50 Yes to CaraPosition changes hands, nothing created100 + 50 = 150100 + 0 = 100
2Dan buys 200 Yes, Eli buys 200 No200 new contracts created200100 + 200 = 300
3Dan sells 100 Yes, Eli sells 100 No100 contracts closed out100300 − 100 = 200
3Cara sells 30 Yes to FayPosition changes hands, nothing created100 + 30 = 130200 + 0 = 200
Total150 + 200 + 130 = 480Settlement: 200 → 0

Day 1. Ana and Ben take opposite sides, so 100 brand-new contracts exist. Volume is 100 and open interest is 100. Later, Ana sells half her Yes contracts to Cara. That adds 50 to volume, but no contract is created or closed: Cara simply takes Ana's place. The day ends with volume of 150 and open interest of 100.

Day 2. Dan and Eli both open new positions, creating 200 more contracts. Volume for the day is 200, and open interest climbs from 100 to 300.

Day 3. Dan and Eli both want out. Their orders match each other, so 100 contracts are closed and removed. That trade adds 100 to volume but cuts open interest from 300 to 200. Then Cara sells 30 Yes to Fay, a simple transfer: volume rises to 130 for the day, and open interest stays at 200.

Settlement. When the market resolves, the 200 remaining contracts pay out. Open interest drops to zero. The 480 contracts of cumulative volume stay in the market's history.

Three lessons come out of this example:

  1. Volume can be higher than open interest. On Day 1, volume was 150 while open interest was 100.
  2. Open interest can fall on a day with plenty of volume. Day 3 had 130 contracts of volume, yet open interest dropped by 100.
  3. Cumulative volume keeps growing, while open interest returns to zero after settlement. The history shows 480 traded, but nothing is left open.

What each one tells you (and what it does not)

VolumeOpen interest
Tells youHow active a market is and how much attention it gets right nowHow much money is still committed to open positions
Does not tell youWhether those trades opened or closed positionsHow easy it is to buy or sell at a fair price

Read together, the two numbers give a little more context. Rising volume along with rising open interest generally suggests new money is entering a market, because trades are creating contracts. Falling open interest suggests positions are being closed. CME Group notes that, in futures, rising open interest is commonly read as traders entering the market to support a trend, and declining open interest as weakening conviction. That is a common interpretation, not a rule.

Neither number predicts the outcome. A market with huge volume can still resolve against the side most people bought, and open interest says nothing about which side is right. Neither one is a signal to trade. To understand what a price itself suggests, see our guide on implied probability.

What is liquidity in trading? Why volume and open interest are not the same thing

What is liquidity in trading? It is how easily you can buy or sell close to the current price, without moving that price much. Liquidity is related to volume and open interest, but it is a separate idea, and neither number measures it directly.

To judge liquidity, look at two more things:

  • The bid/ask spread is the gap between the best price someone is willing to pay and the best price someone is willing to sell at. Kalshi's glossary notes that a narrower spread generally means the two sides are closer to agreement. See our glossary entry on the spread.
  • The order book is the list of open buy and sell orders at each price, showing how many contracts are waiting on each side. Our order book entry has a short definition.

These measures do not always line up. A market can have big volume and still a wide spread, for example when a few large trades happened earlier but few orders are resting now. A market can also have small open interest and a tight spread, if traders are actively quoting prices close together. That is why our glossary keeps liquidity as its own term.

How to read volume and open interest on Kalshi and Polymarket

Both major US-facing prediction market platforms publish activity figures, but they do not always define them the same way. As of October 2026, definitions and units can differ between platforms, so treat any cross-platform comparison with care.

Kalshi volume. Kalshi's glossary defines volume as contracts traded during a period and open interest as contracts that remain open. Its API documentation describes both as one-sided figures and returns contract counts as well as dollar values. A Kalshi contract settles at $1.00 if its condition is true and $0.00 if false, and a 63-cent Yes price corresponds to a $0.63 contract price before fees.

Polymarket volume. On Polymarket, each market has a Yes token and a No token traded on a central limit order book, and each token can be redeemed for $1 if its side wins. Third-party data providers rebuild these figures in their own way. Dune, for example, measures open interest from the supply of outcome tokens on the Polygon blockchain, per side, at the end of each hour, and measures volume as outcome tokens traded in the hour, with each match counted once.

When you look at a market page, a practical routine is:

  1. Find the volume figure and check whether it is daily or cumulative, and whether it is in contracts or dollars.
  2. Look for open interest, if shown, to see how many positions are still outstanding.
  3. Check the spread and the depth of the order book before deciding anything about liquidity.
  4. Read the platform's own help pages for exact definitions.

If fees matter for your planning, our fee calculator can estimate them.

Common mistakes to avoid

  • Treating volume as open interest. Volume counts trades; open interest counts open positions. A market can trade a lot while very few contracts stay open.
  • Assuming high volume means high liquidity. Past trades do not guarantee that orders are waiting near the current price right now.
  • Reading open interest as a prediction. A large number of open contracts says how much is committed, not which side will win.
  • Comparing numbers across platforms without checking units. Contracts vs dollars, and one-sided vs two-sided counts, can change a figure a lot.
  • Ignoring the spread. The gap between bid and ask is often the most direct clue to how costly it is to trade.
  • Using one day of data. A single day can be distorted by one large trade. Look at a trend over several days.

Want to see these numbers on a real market?

Looking at a live market page is a calm way to spot volume, open interest and the spread side by side, and to practice reading them. You do not need to trade to learn from it. If you decide to go further, start with our beginner guide on how to trade on Polymarket.

Key terms

  • Volume: the number of contracts traded during a period, with each trade counted once.
  • Open interest: the number of contracts opened and not yet closed or settled.
  • Liquidity: how easily you can buy or sell near the current price.
  • Bid/ask spread: the gap between the best bid and the best ask.
  • Order book: the list of resting buy and sell orders at each price.

Sources