# What is open interest in options?

Canonical: https://halfonadouble.com/learn/open-interest
Data through: 2026-10-02 close
Publisher: Half on a Double

**Open interest is the number of option contracts that exist right now**:
positions that have been opened and not yet closed, exercised or expired.
Every contract has a buyer and a seller, and one contract counts once.

## Open interest versus volume

Volume counts contracts _traded_ in a session. Open interest counts contracts
_outstanding_ at the end of it. A single trade can move them differently:

| Trade                                                             | Volume | Open interest |
| ----------------------------------------------------------------- | ------ | ------------- |
| A buyer opens a new position against a seller who is also opening | +1     | +1            |
| A holder sells to close, to a new buyer opening                   | +1     | unchanged     |
| A holder sells to close, to a seller who is also closing          | +1     | −1            |

So a busy day with flat open interest means positions changed hands; a day
where open interest jumps means new risk was created.

## Why it matters for the stock

Every open contract has a counterparty, and the largest counterparties are
market makers who hedge in the underlying. The bigger the open interest, the
larger the hedging book behind the stock and the more the stock's own
trading is shaped by that hedging. That is the whole premise of this site:
treat the entire open interest of a ticker as one portfolio, compute its
greeks, and find the prices where the hedging flips.

Two levels fall out of that portfolio: the [delta neutral price](/learn/delta-neutral-level),
where its summed delta is zero, and [gamma max](/learn/gamma-max), where its
summed gamma peaks.

## How this site reports it

Every ticker page shows **total open interest** across all listed
contracts, the change versus the previous session, and, for tickers with a
full history page, the one-year percentile and the 5- and 21-session
change. The [most open interest](/screens/most-open-interest) screen lists
the hundred largest chains each day, and [open interest surge](/screens/open-interest-surge)
lists where it grew fastest over five sessions.

The [unusual open interest accumulation](/screens/unusual-open-interest-accumulation)
screen standardizes today's absolute contract change within each ticker. It
subtracts the mean of up to 252 preceding daily changes and divides by their
sample standard deviation. Today is excluded from that baseline, and at least
200 prior changes are required. A `+2.00σ` reading therefore means today's
increase was two standard deviations above that ticker's prior-year norm; it
does not identify who opened the positions or whether they were bullish.

Open interest is reported once a day after the close, by the exchanges,
which is why all of this site's numbers are end-of-day.

## Related

[Put/call ratio](/learn/put-call-ratio) · [Gamma exposure](/learn/gamma-exposure) · [Methodology](/methodology)