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Closing auction session, decoded.

Reading time · 4 min · Updated 3 Sep 2026

From 3 August 2026, F&O-eligible cash stocks no longer grind into a 15:30 VWAP close. They stop continuous trading at 15:15 and clear in a call auction. Options and futures keep trading until 15:40. This guide is the clock and the two prices that matter.

What CAS is

The Closing Auction Session is a stock-level mechanism. Every NSE/BSE name that has listed F&O goes into a call auction at the end of the cash session. Buy and sell orders build a book; the exchange finds the price that maximises matched volume — the equilibrium price. That print is the stock's closing price for the day (or the reference price if no equilibrium is found).

While the book is still open, both exchanges publish an indicative equilibrium price (IEP) — where the auction would clear if it ended now. That number moves with orders. It is not a trade until matching finishes.

Live board: NIFTY / BANKNIFTY / FINNIFTY / MIDCPNIFTY / SENSEX indicative close →

The clock (IST)

Continuous cash in F&O names stops at 15:15. Then:

15:15–15:20 — transition. Reference price is fixed. No CAS order entry.
15:20–15:25 — limit and market orders can be placed, modified, cancelled.
15:25–15:30 — market orders freeze; limits still change. Random close between 15:28 and 15:30.
15:30–15:35 — matching. Closing price set.
15:40 — equity futures and options session ends. Not 15:30.

Non-F&O cash stocks are unchanged. Index options stay open through the cash auction — that is why a live IEP next to a still-moving options book is useful, if you treat it as a forming print, not a fill.

Reference price vs IEP

Two different numbers, often mashed together:

Reference price — fixed at the start of CAS. It is the VWAP of that stock from 15:00–15:15 (or last traded / previous close if no trades). Auction orders must stay inside a ±3% band around it. It does not tick.
IEP — the live indicative equilibrium while the book is open: where the auction would clear if it ended now. It moves with orders. It is not a trade until matching finishes.

When IEP shows up

There is nothing to show until the auction book has a price. On NIFTY and the other NSE indices, that is usually around 15:20 — when order entry opens — not at 15:15. SENSEX often appears a few minutes earlier. After matching, the indicative number drops away; last traded keeps moving until 15:40 because F&O is still open.

If the live board says "—" at 15:12, the auction simply has not started printing yet.

The live board

Closing auction shows last traded all session, and IEP only while the auction is publishing it. Treat IEP as a forming close, not a fill — the cut-off is randomised, so the last indicative print may not be the final one.