A trader reads that the 23,000 put carries 1.24 crore of open interest, and a friend says it is 1.9 lakh. Both are right. One is counting units and the other lots, and the number that turns one into the other is the lot size. It sounds like a footnote, and it is the source of more confused arguments about open interest than any other single thing, because the exchange publishes in units, brokers quote in lots, and charts built from either look nothing like each other across a revision. This guide explains what a lot size is and who sets it, how to convert open interest between units and lots, what a revision does to a series, and how to keep three sources on the same footing.
What a lot size is
A derivatives contract is not written on one share or one index point. It is written on a fixed number of units, and that number is the lot size. One NIFTY option contract covers a lot’s worth of the index, so its value is the index level times the lot size, and its premium in rupees is the quoted price times the lot size. The exchange sets the lot size for each instrument and revises it from time to time so that the value of one contract stays inside the band the regulator specifies; when the index rises a long way the lot shrinks, and when it falls the lot grows. That is why the NIFTY lot has changed several times over the years, and why the number printed on a chain today is a fact about this month rather than a constant.
Stocks have their own lot sizes, set per stock by the same logic on the stock’s price, so two stocks at very different prices can have very different lots and a stock’s lot changes when its price moves far enough. The F&O glossary covers the related terms.
Units and lots: the conversion
The exchange counts open interest in units, the number of underlying units across all open contracts. Open interest in lots is that figure divided by the lot size. On the chain in the screenshot the 23,000 put shows 1,24,13,660 units against a lot size of 65, which is about 1.91 lakh lots. Multiply back and you have the units again. The same conversion applies to volume and to the change in open interest, and it applies to futures as well as options.
Both units are correct; they answer different questions. Units are what the exchange publishes and are stable across a lot-size revision, so they are the right unit for comparing a strike with itself over months. Lots are what you actually trade, so they are the right unit for thinking about how many contracts stand behind a wall and what it would take to move it. The mistake is not choosing one; it is comparing a figure in one unit with a figure in the other without knowing it.
Why a revision breaks a chart
Suppose the lot size is cut from 75 to 65. A position of 100 lots is now worth 6,500 units instead of 7,500, and existing contracts are handled under the exchange’s transition rules, but the open interest in units at a strike is unchanged the moment after the revision: the same underlying exposure is open. Open interest in lots, however, jumps, because the same units now divide into more lots. A chart of open interest in lots shows a step that never happened in the market, and a chart in units shows nothing at all.
So a series that must survive a revision should be kept in units and converted for display, and any comparison across a revision date should be done in units. This is also why an old screenshot of a chain in lots cannot be compared with today’s without knowing both lot sizes, and why the tools that show lots take the current lot size from the instrument rather than from a fixed number.
Three sources, three conventions
The confusion has a shape. The exchange’s chain and its end-of-day reports are in units. Most broker terminals show open interest in lots, because that is how their clients trade. Analytics tools do one or the other, and the better ones let you switch. Add two more differences and the numbers can look irreconcilable: some sources sum open interest across all expiries while others show one, and intraday open interest is a provisional feed that the official end-of-day figure, settled after clearing, typically lands slightly below.
The Futures OI page also measures the change bar against the previous bar for the current-month contract, while the exchange’s change-in-OI figure compares against the previous close. None of these are data disagreements; they are lenses. Knowing which lens each source uses, and converting to a common one before comparing, dissolves most arguments about whether “the OI is right”.
Reading a wall in lots
Where lots earn their keep is scale. A wall of 1.21 crore units at 23,200 is a large number without a comparison; the same wall as 1.86 lakh lots can be set against how many lots trade in a session, how many a single large writer might hold, and what it would take, in contracts, for that wall to be covered. The option chain shows the lot size beside its unit toggle for this reason, and hovering a converted cell shows the raw quantity so the two views can be reconciled on the spot. For open interest read as a chart, the OI Stats page draws the ladder in whichever unit is selected.
A worked reading
Two reports disagree: one says the 24,000 call added 20 lakh of open interest today and the other says it added 30,000. Divide 20 lakh by a lot size of 65 and you get about 30,769 lots, so the two are the same fact in different units, with the small residual coming from one source counting the provisional intraday feed and the other the settled figure. A third report shows a jump in open interest in lots across last month’s revision date that does not appear in units, and that jump is the lot size changing, not the market.
Where to read open interest in lots
The option chain page in OIData carries a Qty and Lots toggle with the instrument’s current lot size shown beside it, converts every open-interest, change and volume cell, and keeps the raw quantity on hover. The same toggle runs the Futures OI page, including its session totals, and the Trending OI page. Stock lot sizes come from the instrument itself, so a stock chain converts at its own lot, and the CSV export writes whichever unit is showing.
Lot size FAQ
What is lot size in options? The number of units of the underlying that one contract covers. A contract’s value is the price times the lot size, and the exchange revises the lot size so that value stays inside the regulator’s band.
How do I convert open interest to lots? Divide the open interest in units by the lot size. Multiply lots by the lot size to get units. The same applies to volume and change in open interest.
Why does open interest in lots jump on some dates? Because the lot size was revised. The units did not change; they now divide into a different number of lots. Compare across a revision in units.
Why does my broker’s open interest differ from the exchange’s? Usually the unit: brokers quote lots, the exchange counts units. Aggregation across expiries and the provisional intraday feed against the settled figure account for the rest.
Takeaways
- The lot size is the units in one contract, set and revised by the exchange to keep the contract’s value in a band.
- Open interest in lots is units divided by the lot size; both are correct, for different questions.
- Keep series in units across a revision and convert for display; a jump in lots on a revision date is not the market.
- Most “the OI is wrong” arguments are two sources using two lenses.