What is open interest?

Open interest is the number of option contracts that exist right now — opened at some point and not yet closed, exercised or expired. Each contract binds one buyer to one seller, and the pair counts once. It is a stock, not a flow: volume counts the trades that happened today, while open interest counts the positions still standing after today. It is settled once per day, overnight, after the clearing house nets every account — so nobody has a live intraday figure, whatever a site may label it. Max pain, the call and put walls, and gamma exposure are all arithmetic performed on this one number.

That last sentence is why this page exists: open interest is the raw material every free number on this site is made of. If you understand how it is born and how it dies, every derived metric stops being mysterious.

The definition, and the picture behind it

Ten thousand open interest at a strike means ten thousand live agreements about that price are standing at this moment — not ten thousand trades that occurred, and not ten thousand people. One trader who opened a thousand contracts and one thousand traders who opened one each look exactly the same in the count.

The word doing the work is standing. Volume is what happened today; open interest is what remains after today. That makes it the closest thing options markets have to a map of commitments — which is exactly why positioning metrics are built on it.

How open interest is created and destroyed

A trade changes open interest only when it changes the number of live contracts:

Both sides open new positions → open interest rises by the traded amount.
Both sides close existing positions → open interest falls.
One opens, one closes → the contract changes hands; open interest is unchanged.

During the session nobody can tell which of the three just happened — the tape shows a trade, not an intention. Only the clearing house's overnight netting reveals the answer. That is the entire reason open interest settles once per trading day, and the reason every honest max pain or GEX figure carries a date rather than a time.

Reading a real chain: SPY, settled 2026-08-31

The SPY chain we hold carries 3,979 contracts within 45 days of expiry. The front expiration (2026-09-02) alone has 69,656 call open interest against 98,869 puts. The single heaviest strike in that expiration is $766.00, with 3,096 calls and 7,728 puts standing. None of those counts moved during today's session, no matter how much SPY traded — they are last night's settled positions, which is precisely what makes them comparable night over night. The full strike-by-strike picture is on the SPY max pain page.

What gets built on top of it

Three derived metrics, all from the same settled file, in increasing order of assumptions:

Walls — pure counting: the one strike with the most call open interest, and the one with the most put open interest.
Max pain — pure arithmetic: test each strike as a settlement price, sum every contract's payout, keep the smallest total. You can verify it in a spreadsheet with our calculator recipe.
Gamma exposure — an estimate: open interest multiplied by a Black-Scholes gamma, under the assumption that dealers are long every call and short every put. More informative if the assumption holds; wrong in sign where it does not.

Keep the ladder in mind when comparing sources: a wall can only be miscounted, max pain can only use a stale file, but a GEX number inherits a model and an assumption. The rawer the metric, the fewer the ways it can lie to you.

The two classic beginner mistakes

Reading open interest as direction. The count is side-blind: hedgers, income sellers and outright speculators all leave the same footprint. A put/call ratio of 1.3 says puts outnumber calls; it does not say the crowd is bearish, because you cannot see who is long those puts and who is short them.

Reading volume spikes as positioning changes. Expiration-week volume is enormous while much of it is rolling — closing one expiration, opening the next. The open interest file the following morning is what tells you whether positioning actually moved, or just changed address.

Common questions

What is open interest in options?

Open interest is the number of option contracts that currently exist and have not been closed, exercised or expired. Every contract has a buyer and a seller, and the pair counts as one unit of open interest. It is a stock, not a flow: it measures positions standing open right now, while volume measures trades that happened today.

What is the difference between volume and open interest?

Volume counts trades during the session; open interest counts contracts still alive after the session. A single contract day-traded a hundred times produces 100 volume and 0 new open interest. Volume can spike while positioning is unchanged; open interest can only change when new contracts are created or existing ones are closed — which is why positioning metrics are built on open interest, not volume.

Why does open interest only update once a day?

The clearing house nets every account's trades after the close and settles the official count overnight. During the session nobody — no broker, no data vendor — knows how much of today's volume opened new contracts versus closed old ones. Any site showing "live open interest" is showing last night's settled file. Every open-interest-derived number, max pain included, therefore updates at most once per trading day.

Is high open interest bullish or bearish?

Neither, on its own. Open interest counts contracts without saying who is long or short, or why. A large put position can be a hedge on a portfolio someone is happy to hold, income from a put seller, or an outright bearish bet — the count looks identical in all three cases. Open interest tells you where the crowd is standing, not which way it is facing.