Lot Size (F&O)
The fixed number of units in one F&O contract. NSE revises lot sizes by circular, and both old and new sizes trade simultaneously during a revision.
Also called: nifty lot size · bank nifty lot size · contract size · market lot
What it means
Derivatives trade in standardised lots: one NIFTY contract currently carries 65 units, BANKNIFTY 30. Every premium, margin and profit figure scales by this number, so an error in it is an error in the entire position size.
The part that catches people out is that a lot size belongs to the CONTRACT, not to the symbol and not to the trade date. During an NSE revision, existing contracts run off at their original lot while newly introduced ones carry the revised one — so on a single day in December 2025, a NIFTY weekly expiring on the 23rd was 75 units and one expiring on 6 January 2026 was 65.
Quarterly and half-yearly contracts are the exception: the exchange converts those in place at a stated end of day, so for them the trade date decides and the expiry does not. And "at EOD" applies from the next session — trading on the stated day is still at the old lot.
How OptionLab Pro’s engine uses it
In the live engine
Our lookup is keyed on (symbol, contract type, expiry, as-of date), not on symbol alone, and the coarse date-only variant deliberately REFUSES rather than guessing inside a revision window. Symbols whose per-symbol boundaries have not been read from the circular carry an explicit unresolved span and refuse too — a stale but plausible lot size is more dangerous than an error, because it is silent. Both the Go and TypeScript tables pin a same-trade-date / different-expiry case and the BANKNIFTY 30 → 35 → 30 sequence, either of which fails instantly under a date-only model.
go/internal/orders/lots.go (GetLotSizeForContract) · frontend/src/lib/instruments.tsThe common mistake
Where traders get this wrong
Storing one lot size per symbol. It is a contract attribute, and the single-number model is wrong on every revision boundary — in both directions, since values return to earlier numbers (NIFTY 25 → 75 → 65) and a model keyed on the value rather than the boundary silently merges two different regimes.
Related terms
Educational and analytical content only. OptionLab Pro is not a SEBI-registered investment adviser or research analyst and does not give buy or sell recommendations. Derivatives trading carries a high risk of loss.