Why we report open interest in shares, not contracts
Halve the lot size and the contract count doubles overnight, with no economic change at all.
StockLearn reports futures open interest in shares, not in contracts. Many sources do the opposite. The reason is a quirk of how NSE manages contract sizes, and it makes contract counts quietly unreliable across time.
Lot sizes are not fixed
A futures contract on an Indian stock covers a fixed number of shares — the lot size. NSE sets that number per symbol, with the aim of keeping the notional value of one contract inside a target range. Since share prices move, the exchange periodically revises lot sizes to keep contract values in band.
A stock that has doubled will tend to see its lot size cut; one that has halved will tend to see it raised. These revisions are announced and take effect on a specified date.
Why that breaks contract counts
Suppose a symbol has 10,000 contracts of open interest at a lot size of 500 shares. That's 5,000,000 shares of exposure. NSE then halves the lot size to 250. Nothing about anyone's position changes economically — but the same exposure is now expressed as 20,000 contracts.
A chart of contract counts across that date shows open interest doubling overnight. Nothing happened. The unit changed.
Same economic position, two lot-size regimes:
| Measure | Before revision | After revision |
|---|---|---|
| Lot size (shares) | 500 | 250 |
| Open interest (contracts) | 10,000 | 20,000 |
| Open interest (shares) | 5,000,000 | 5,000,000 |
Multiply that across a history and any measure built on contract counts inherits a series of discontinuities that have nothing to do with market activity. Day-over-day change, percentage change, buildup classification — all of them misfire on revision dates.
Shares are comparable across symbols too
The second benefit is cross-sectional. Lot sizes differ enormously between symbols, because they're set from share price. A high-priced stock may have a lot size in the tens; a low-priced one in the thousands.
So "50,000 contracts" means wildly different exposure depending on which symbol you're looking at. Converting to shares makes the number mean the same thing everywhere, which is a precondition for ranking or comparing symbols at all.
What we actually do
Method
The NSE F&O bhavcopy reports open interest in contracts alongside the contract's lot size. We multiply the two to get shares, aggregate across expiries, and store the share figure. Day-over-day change and buildup classification are computed on shares.
One consequence worth stating: on a lot-size revision date the share figure stays continuous, which is the point — but it also means our numbers will disagree with sources that report contracts. That disagreement is expected, and the share figure is the one that means something stable.
Where shares still mislead
Converting to shares fixes the unit problem. It does not make open interest comparable across time in any deeper sense, because the free float and total shares outstanding of a company also change — through issuance, buybacks and corporate actions.
Ten million shares of open interest means something different for a company with a hundred million shares outstanding than for one with ten billion. We don't currently normalise for that, and any cross-symbol comparison of raw share open interest should be read with it in mind.
That's a limitation we'd rather state than paper over. Shares beat contracts. Neither is a complete answer.
Key takeaways
- NSE revises lot sizes periodically to keep a contract's notional value inside a target band.
- A revision makes contract-count open interest jump with no underlying change in exposure.
- Lot sizes also differ hugely across symbols, so contract counts aren't comparable cross-sectionally.
- We multiply contracts by lot size and store shares; buildup classification runs on the share figure.
- Shares still don't normalise for float or shares outstanding — a remaining limitation.
See these ideas on real stocks
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