Open interest is the number of option contracts that are open at the end of a trading day: opened and not yet closed, exercised or expired. Volume is the number of contracts traded during the day, whether those trades open or close positions, and it starts again from zero each session.
How it works
Every option contract has a buyer and a seller, and each side is either opening a new position or closing an existing one. Volume rises with every trade. Open interest changes only when the trade creates or extinguishes a contract:
| Buyer | Seller | Effect on open interest |
|---|---|---|
| Opens | Opens | Up by the number of contracts |
| Opens | Closes | Unchanged (the position changes hands) |
| Closes | Opens | Unchanged (the position changes hands) |
| Closes | Closes | Down by the number of contracts |
In the US, the Options Clearing Corporation tallies open interest once a day. The figure published in the morning reflects positions as of the previous session's close; it does not update during the day. Volume, by contrast, accumulates live. A standard US equity option covers 100 shares, so 10,000 contracts of open interest represent options on one million shares.
How to read it
- Open interest shows where positioning sits. Strikes with the most open interest are where hedging is concentrated, which is the basis of the call wall and put wall and of gamma exposure.
- Volume against open interest shows fresh activity. When a contract trades several times its existing open interest, much of that volume is likely to be new positions. This is the core of unusual options activity.
- The next day's change confirms it. A rise in open interest the following morning confirms that positions were opened; a fall shows they were closed.
- Put/call ratios. Put open interest divided by call open interest summarizes standing positioning; the same ratio on volume reflects the day's activity.
- Liquidity. High open interest and steady volume usually mean tighter bid/ask spreads and easier exits.
Worked example (illustrative numbers)
Follow one newly listed contract over three days:
| Day | Trades | Volume | Open interest at close |
|---|---|---|---|
| 1 | Trader A buys 10 to open; Trader B sells 10 to open | 10 | 10 |
| 2 | A sells 4 to close; C buys 4 to open | 4 | 10 |
| 3 | A sells 6 to close; B buys 6 to close | 6 | 4 |
On day 2, volume was 4 but open interest did not move, because a position simply changed hands. On day 3, both sides closed, so open interest fell by 6. Total volume over three days was 20 contracts, yet only 4 remain open, held long by C and short by B. A screen showing only volume would overstate how much positioning was built.
Limitations and common mistakes
- Open interest has no direction. Every open contract has a long and a short. High call open interest is not by itself bullish.
- It lags by a day. Today's positioning is visible only tomorrow, and same-day options opened and closed within the session never appear.
- Expiry resets it. Open interest concentrates in monthly expiries and disappears when they expire, so comparisons across dates need care.
- Volume alone is ambiguous. Heavy volume can be opening, closing or the same contracts trading back and forth.
- Thin contracts mislead. On strikes with little open interest and no volume, quotes can be wide and stale.
The same principle, comparing today's activity with a normal baseline, applies to shares as well; see unusual volume.
In Clear Tape
On the Options page, the free SPY, QQQ and IWM cards show the put/call open interest ratio along with the walls derived from open interest. Pro members can analyze any stock with listed options: a chart of open interest by strike, put/call ratios on both volume and open interest, an unusual flow table with volume, open interest and volume ÷ open interest for each contract, and a strategy builder that shows each leg's open interest and flags legs with open interest under 100 or no volume as thinly traded. Members see each session's closing data once the next session has opened. See pricing.
For educational purposes only. Not investment advice.