When lot sizes change, your history changes meaning
NSE trimmed index lots for the January 2026 series. Any per-lot statistic that spans that date is comparing two units.
Exchanges revise contract sizes so that notional value stays inside the band the regulator expects as index levels move. For the January 2026 series, NSE cut Nifty from 75 to 65 units and Bank Nifty from 35 to 30, with Fin Nifty and Midcap Select also reduced.
For a journal, that is a break in the measuring stick.
A break in the series, not a change in skill
A ₹10,000 day on the old lot and a ₹10,000 day on the new lot represent different exposure. Any "average P&L per lot" chart crossing that boundary is quietly mixing units.
Units per contract. Nifty Next 50 was left unchanged.
Source: Economic Times — revised index F&O lot sizes, January 2026 series
Two acceptable fixes
Either normalise historic P&L to notional exposure, or split the series at the effective date and label both halves. What is not acceptable is a migration that rewrites old quantities to the new specification — that destroys your risk and excursion history.
Match on the contract code
Friendly names change, symbol strings change, and expiry conventions changed in 2025 as well. Matching on the broker’s contract identifier for that day is the only stable approach.
- Keep historic quantities exactly as they were filled
- Store the lot size that applied on the trade date
- Break or normalise per-lot charts at the effective date
- Match trades on the broker contract code, not the display name
- Annotate the change on any chart that crosses it
Key takeaways
- Split or normalise per-lot series at a revision date.
- Never rewrite historic quantities to a new lot size.
- Match trades on the contract code from the source file.
Where these numbers come from
- Economic Times — revised index F&O lot sizes, January 2026 series
- NSE India — circulars and contract specs
Not advice, not a call. NiyataBook is a personal trading journal, not a SEBI-registered investment adviser or research analyst. Everything above looks backwards at trades that already closed. What happened in a book does not promise what happens next, and F&O can take a year’s savings in a week.