Monday 2 Feb 2026 was the first full session after the Union Budget, which was presented on a Sunday. Nifty closed at 25,088.40. On the chain for the 30 Mar expiry, the biggest pile of put open interest sat at the 25,000 strike: 35,51,405 units, or 54,637 contracts. Chain readers have a name for a pile like that. They call it a "floor". On Monday 30 Mar 2026 the index settled at 22,331.40, 2,668.60 points under it. The pile was real. The index went through it. Before I read a pile, I ask who sits on each side of it. This lesson is about what the open interest columns count, and what they cannot tell you. Every figure comes from NSE end of day files, so nothing here is live and nothing is a view on where the index goes next.
3By the end you can
- Say what open interest counts, and how one trade moves it up, down or not at all.
- Read change in open interest next to volume, with both in contracts.
- Explain why the biggest pile on a chain is not a floor or a ceiling.
The idea, plainly
Open interest is a count of open contracts. Every open contract has one buyer and one seller, so the count is of pairs. It is not a count of traders, and it is not rupees. The buyer and seller module shows how the two results mirror each other, and the question page gives the short answer. A pair can only be born or die. That is why one trade moves the count by one, by zero, or by minus one.
| The buyer | The seller | Open interest |
|---|---|---|
| opens a position | opens a position | up by one contract |
| opens a position | closes an old one | no change |
| closes an old one | opens a position | no change |
| closes an old one | closes an old one | down by one contract |
Read 2 more paragraphs, 99 words
Volume counts all four trades. Change in open interest is the day's net: contracts opened, less contracts closed. A busy day can leave it near zero, so I never read the two columns as one.
Mind the unit. NSE's end of day file counts open interest in units, so a contract adds 65. Volume is in contracts. Convert open interest to contracts before you set one beside the other. Cumulative open interest is the same count added up across strikes or across expiries. It tells you how big the market is. It does not tell you where anyone stands.
Worked examples: two dated chains
The eve of the 30 Jun 2026 expiry
Take the Nifty chain for the 30 Jun 2026 expiry as it stood at the close of Monday 29 Jun, one session before it expired. The index closed at 23,946.25. Look at the 24,100 call.
| The 24,100 call, 29 Jun 2026 | Value |
|---|---|
| Open interest, units | 1,57,43,650 |
| Open interest, contracts | 2,42,210 |
| Open interest the day before, units | 78,25,935 |
| Change in open interest, units | 79,17,715 |
| Change in open interest, contracts | 1,21,811 |
| Volume, contracts | 94,99,236 |
| Settle price, rupees per unit | 32.35 |
Many contracts traded, few were added
Open interest piled up on the strikes above the index, in one session
- They show where contracts are open, and how many, for one expiry on one date.
- They show where contracts were added or closed since the day before.
- They do not show who is long, who is short, or who is hedging a share portfolio.
- They do not show where the index goes next.
Read 5 more paragraphs, 243 words
Read it in order. The pile grew by 101% in one session. Then look at volume: 94,99,236 contracts traded, which is 39.2 times the 2,42,210 left open. Only 1,21,811 contracts were added net. Almost every trade passed a contract from one hand to another, or opened and closed inside the day. Open interest keeps the small remainder.
Add the column down and you get the cumulative figure. All strikes of this chain held 22.8 crore units of call open interest, about 35.1 lakh contracts, and 17.6 crore units of put open interest, about 27.1 lakh contracts. Cumulative totals hide where the piles are. The put call ratio module turns the two totals into one number and asks what it is worth.
The next day the index settled at 23,865.75. Every call at 24,100 and above expired with no value. Whoever bought them lost the premium, and whoever wrote them kept it. Which of the two was the big player? The chain cannot say. A pile has two owners.
What can the columns support? The same four things hold on any chain.
Before any reading of a chain, on any screen, check the unit, the expiry, the date and the volume beside the open interest. The question page on what to look at in chain analysis keeps to the same list. Module F17 covers the columns. The open interest and build ups module covers futures, where the same count is read against price.
The biggest piles against what followed
Now test the floor from the opening. Here is the 30 Mar chain of 2 Feb, against what the index did next.
The index closed under the biggest put pile on 2 Mar and settled 2,668.60 points below it on 30 Mar
The index closed under the 25,000 pile for the first time on 2 Mar, at 24,865.70. No close reached the 27,000 call pile: the highest close in the window was 25,953.85. One expiry is a story, so here are four from the same evening.
| Expiry, 2 Feb chain | Biggest put pile | Biggest call pile | Settled at |
|---|---|---|---|
| 3 Feb 2026 | 24,000 | 26,000 | 25,727.55 |
| 10 Feb 2026 | 25,000 | 26,000 | 25,935.15 |
| 24 Feb 2026 | 25,000 | 25,500 | 25,424.65 |
| 30 Mar 2026 | 25,000 | 27,000 | 22,331.40 |
Read 2 more paragraphs, 144 words
Three expiries finished between the two piles. One finished 2,668.60 points under the put pile. Even the three hide a second story. For the 24 Feb expiry the index closed above the 25,500 call pile on most sessions, as high as 25,953.85 on 11 Feb, which is 453.85 points over it. It then settled 75.35 points under it. The same piles are consistent with a ceiling and with no ceiling at all, which is the point.
Piles also move. For the 30 Jun expiry the biggest put pile sat at 23,000 on 10 Jun, at 24,000 on 16 Jun and at 23,500 on 29 Jun, while the biggest call pile stayed at 25,000. On 16 Jun the top two put piles, 24,000 and 23,500, were only 6.5% apart. A title that changes hands on a gap that small is not a wall.
The common mistake
The claim: The strike with the biggest open interest is a floor or a ceiling for the index.
A big number looks like a big commitment, so it sounds like logic. The two examples above say otherwise. The put pile of 2 Feb sat 2,668.60 points above where the index settled. The call pile of the 24 Feb expiry was closed through on most sessions. The biggest put pile of the 30 Jun expiry changed strike twice in three dates. A pile has two owners, and the count cannot tell them apart. A hedger, a spread trader and a writer leave the same footprint. Module F17 shows one strike that held a big call pile and a big put pile at once. Read a pile as a place where contracts are open, and nothing more. The short answer is on the question page.
2 terms this section turns on
Key takeaways
- Open interest counts open contracts, one buyer and one seller each, not traders.
- Change in open interest is contracts opened less contracts closed on the day.
- Volume counts every trade. Open interest keeps only the net that stays open.
- NSE's file counts units, volume counts contracts. Convert before you compare.
- A pile shows where contracts sit, not who holds them or where the index stops.
- Biggest piles move from date to date, and the index can close through them.
Before you trade. Derivatives trading carries a high risk of loss. SEBI's study "Profitability of Individual Traders in the Equity Derivatives Segment (FY25 to FY26)" (August 2026) found that 87.7% of individual traders in equity F&O made net losses in FY26, with aggregate net losses of Rs 91,685 crore. Read the SEBI study