One symbol, two lot sizes: how a lot-size revision reaches contracts that already exist

A lot size is a contract term, not a display setting

The lot size is the quantity of the underlying that one contract represents. It is written into the contract specification, so it determines what a position is worth. Change it on a contract somebody already holds and you have changed the size of their position without them trading.

Exchanges therefore do the only workable thing: the revised lot applies to contracts introduced after a stated date, and contracts that already exist keep the lot they were born with until they expire. The revision reaches the market gradually, as the old contracts run off.

The three dates, on a real revision

The October 2025 NSE index revision is a clean example, and it is the one behind the numbers on this site's lot-size table.

Read from the circular's own wording: The circular came into effect from 28 October 2025 EOD. Weekly and monthly contracts that already existed kept the old lot until the 30 December 2025 expiry, and existing quarterly and half-yearly contracts were revised at 30 December 2025 EOD — so every live contract carried the revised lot from 31 December 2025.

Between the effective date and the completion date, both lots are correct. "What is the lot size" is under-specified until you add "on which date, for which contract".

Why lots get revised at all

Index derivative contract sizes are kept inside a value band of ₹15–20 lakh. Lots are revised because an index level moves over time and the contract's value drifts out of that band — not because of anything that happened on the day of the circular.

The arithmetic makes that concrete without claiming where the index actually traded. At an index level of 27,000, one NIFTY contract at the old lot of 75 would be worth ₹20.25 lakh — above the ₹20 lakh ceiling of the band. At the revised lot of 65, the same level gives ₹17.55 lakh, back inside it.

What the window does to arithmetic done by hand

A premium is quoted per unit of the underlying. The rupee amount behind that quote is the premium multiplied by the lot size, so during a transition an identical quote on two contracts of the same index means two different amounts of money.

• Charges are levied on premium turnover, so they follow the quantity — a cost estimate computed with the wrong lot is wrong by the ratio of the two lots.
• The same applies to any per-lot figure computed by hand: each one inherits the lot that was assumed.
• The contract's own specification, not the newest circular, is what governs the contract in front of you.

When the notice names no completion date

SENSEX shows the other case. The notice revises the market lot for contracts introduced after 20 November 2024. Unexpired November 2024, December 2024 and January 2025 monthly contracts kept the old lot until their own expiry, and long-dated contracts changed at 27 December 2024 EOD.

Because the notice gives three different run-off dates rather than one, there is no single date on which every live contract carried the revised lot — so this site's table reports that the completion date is not stated, rather than picking the latest of the three and presenting it as fact.

The absence of a date is information. A reference table with a value in every cell is telling you things it does not know.

How to look one up correctly

1. Decide the date that matters — the date the contract was introduced, not today.
2. Take the record whose effective date is the latest one on or before that date. Not the newest record in the table.
3. Check whether that date sits inside a transition window; if it does, the contract in hand may still be on the older lot.
4. Confirm against the circular itself before anything depends on the number.

That is exactly what the lot-size page on this site does, and the same records are published as machine-readable JSON with their announced, effective and last-verified dates, so an automated consumer can apply the same rule.

What this site deliberately does not publish

• Expiry weekdays. Expiry assignments are set by their own circulars and were revised more than once during 2025. A wrong answer to "which day does this expire" is the answer that gets quoted, so none is published anywhere on this site.
• Single-stock F&O lot sizes. Roughly two hundred symbols on their own revision cycle; publishing them unverified is the mistake the lot-size page exists to avoid.