Two numbers sit next to almost every prediction market, and people mix them up constantly. One is trading volume. The other is open interest. They look like they measure the same thing, the size of a market, and they don't. Conflating them is probably the most common error in prediction-market reporting, and it's an easy one to make, because on a busy day the two figures can point in opposite directions.
Here's the whole distinction in one line: volume is how many contracts traded over a period; open interest is how many contracts are open right now. Volume is a flow that only ever accumulates. Open interest is a level that rises and falls as positions open and close. A single trade always adds to volume, but it can raise open interest, lower it, or leave it untouched. Get that difference and most of the confusion dissolves.
What each number actually measures
Trading volume is the total number of contracts that changed hands over a window, a day, a week, since the market opened. It answers a question about activity: how much trading, attention, and money a question is drawing. Volume is cumulative. Once a contract has traded, it counts toward volume forever. The number only goes up.
Open interest is the number of contracts currently held open, positions that have been entered but not yet closed out or settled. It answers a question about commitment: how much is actually at stake in the market at this moment. Open interest is a running balance, not a total. It climbs when people take on new positions and falls when they close them or when the market settles.
The plainest way to feel the difference is a storefront. Volume is the turnstile at the door, every person who walks through, counted once on the way in. Open interest is how many people are inside the store right now. Someone who steps in and immediately walks back out adds two clicks to the turnstile and nothing to the headcount inside. A store can log heavy foot traffic all day and still be nearly empty at any given moment, or hold a steady, quiet crowd that barely touches the door.
If you'd rather keep it in market terms, open interest is to a prediction-market contract what shares outstanding are to a stock. Volume is the trading churn on top; open interest is the stock of positions that actually exist. The two are related, but they are not the same measurement, and neither one can be read off the other.
How a single trade moves them (the part people get wrong)
This is where the mechanics get precise, and precision matters, because "does volume mean open interest" has an exact answer: no, and here is exactly why.
Every trade involves two sides. Each side is either opening a new position or closing one it already holds. (On a Yes/No venue, "opening" just means taking on a new position, whether that's buying Yes or buying No.) Volume always ticks up by the size of the trade. What happens to open interest depends entirely on what the two sides are doing.
| The trade (one contract) | Volume | Open interest |
|---|---|---|
| New buyer meets new seller — both opening | +1 | +1 (a contract is created) |
| Two existing holders offset — both closing | +1 | −1 (a contract is retired) |
| A new entrant takes over from someone exiting — one opens, one closes | +1 | unchanged (the position just transfers) |
So a fresh contract only comes into existence when both sides are new to the market. When one participant simply hands a position to another, open interest doesn't budge, even though volume just recorded a trade. And when two people who are both leaving trade with each other, open interest actually falls while volume rises. That third and second row are the whole reason the two numbers drift apart. Volume counts trades. Open interest counts positions that still exist.
Two markets, same lesson
The clearest way to see that volume and open interest move independently is to watch them disagree. Both examples below use illustrative numbers, not a specific real market.
Huge volume, flat open interest
A market attached to a breaking news event, say a court ruling expected within the hour, draws a wave of short-term traders flipping contracts in and out as headlines cross. Over the day, 500,000 contracts change hands. But open interest starts the day around 40,000 and ends it around 42,000. Almost all of that half-million in volume was the same positions being opened and closed over and over, one entrant handing off to the next. The market is loud. The amount of money actually committed and held is modest and barely changed. Read volume here and you'd wildly overstate how much conviction is parked in the market.
Modest volume, deep open interest
An election market six months out works the opposite way. Participants take a view, buy, and hold. On a given quiet day only 20,000 contracts trade, low volume, not much happening. Yet open interest sits at 3 million: an enormous, patient pile of positions that people opened weeks ago and are simply holding through to resolution. Read volume alone and you'd dismiss the market as sleepy, when in fact it carries more committed money than the frantic news market above. The activity is low; the stakes are high.
Which number answers which question
Once you stop treating them as interchangeable, each number has a clear job.
- "How busy is this market? How much attention is it drawing?" That's volume. It's the best single gauge of activity and interest over a period, which is exactly why TickerTracker ranks and compares markets by volume: the busiest questions are usually the most informative. Browse the markets index and the ordering you see is a volume ordering.
- "How much is actually riding on this right now?" That's open interest. It's the size of the standing pile of live positions, the money on the table at this moment rather than the money that has passed across it.
- "How big is this in dollar terms?" That's a scale question, and it's usually answered in notional value, the face value of the contracts counted at $1 each. Notional can describe either volume or open interest; it just puts a dollar figure on whichever one you're measuring.
Both volume and open interest are, in a sense, measures of "size." They just size different things: volume sizes the flow, open interest sizes the standing balance. The reporting mistakes happen when someone quotes one as if it were the other, most often citing a big lifetime volume to suggest a market is heavily committed today.
The prediction-market wrinkle: open interest is the better conviction gauge
Prediction markets add a twist that ordinary equity markets don't have, and it makes open interest especially revealing.
Every prediction-market contract has a built-in expiry. It settles to $1 or $0 on a fixed date, when the event is decided. That means open interest has a natural endpoint: as a market approaches settlement and passes it, open interest drains toward zero, because the standing positions all get resolved and closed out. Volume, being cumulative, never falls. So a settled market routinely shows an enormous lifetime volume and zero open interest. Everything happened in the past; nothing is at stake now. That is the single sharpest case where the two numbers tell opposite stories, and treating the leftover volume as if it were live commitment is the classic error.
The deeper point is what open interest filters out. Volume includes every in-and-out flip, every trade that was closed minutes later, all the churn and short-term speculation. Open interest ignores all of that. It counts only the positions people have chosen to hold and are still holding, the money committed to a view rather than the money that merely passed through. So when the question is "how much genuine conviction is sitting behind this price right now?", open interest is the better answer. Volume tells you how much a market is being talked about. Open interest tells you how much is being believed.
None of this makes one number superior. A careful reader uses both, and reads them against the market's liquidity too. For the practical habit of reading a price alongside these figures, see how to read prediction market odds; for the full field guide to every metric a market emits, see prediction market data: where it comes from and how to read it.
Common questions
Does volume mean open interest?
No. Volume is the number of contracts traded over a period, a cumulative measure of activity. Open interest is the number of contracts currently held open, a running measure of what's at stake right now. A single trade always adds to volume but may raise open interest, lower it, or leave it unchanged, depending on whether the two sides are opening or closing positions.
What is open interest in a prediction market?
Open interest is the total number of contracts currently held open, positions that have been entered but not yet closed out or settled. Because every prediction-market contract eventually settles to $1 or $0, open interest drains toward zero as a market resolves. It's the best single gauge of how much money is committed to a market at this moment.
Can a market have high volume but low open interest?
Yes, and it's common. In-and-out trading pumps volume without adding lasting positions, so a market churning heavily on a news event can show huge volume and nearly flat open interest. It also happens after settlement: a resolved market keeps its full lifetime volume but its open interest falls to zero because no positions remain open.
Which matters more, volume or open interest?
It depends on the question. For "how busy and well-traded is this market?" volume is the right gauge, which is why it's the primary ranking signal on TickerTracker. For "how much conviction is committed right now?" open interest is better, because it counts positions people are still holding rather than trades they've already exited.
Does open interest go up every time someone trades?
No. Open interest only rises when both sides of a trade are opening new positions, creating a contract that didn't exist before. If one side is closing while the other opens, the position simply transfers and open interest is unchanged. If both sides are closing, a contract is retired and open interest falls, even as volume rises.