What Was the Old Method?
Before CAS was introduced, the closing price of a stock was calculated using the Volume Weighted Average Price (VWAP). This method averaged trades from the last 30 minutes of continuous trading based on volume. While workable most of the day, it left room for manipulation — since closing prices affect derivative settlements, mutual fund NAVs, and index calculations, some traders placed large last-minute orders to push prices in their favor, a practice known as "marking the close."
Why Was CAS Introduced?
CAS pools all buy and sell orders from a specific window and calculates a single equilibrium price that clears the maximum volume of shares — unlike continuous trading where every order matches individually. This model is already used in major exchanges across the US and Europe, and it reduces the scope for last-minute price manipulation while improving price discovery.
Which Stocks Does CAS Apply To?
CAS applies only to stocks in the cash market segment that also have equity derivative contracts (futures and options) — typically higher market-cap, heavily traded stocks. Stocks without derivatives continue to use the existing pricing method.
Breaking Down the CAS Timeline
- 3:15 PM – 3:20 PM (Reference Price Calculation): Reference price is set using VWAP of trades between 3:00–3:15 PM; this anchors the auction band.
- 3:20 PM – 3:25 PM (Order Entry Period): Both limit and market orders accepted.
- 3:25 PM – 3:30 PM (Limit Order Modification Window): Only limit orders allowed; can modify/cancel. Closing time within the last 2 minutes is randomized to prevent gaming.
- 3:30 PM – 3:35 PM (Order Matching): Matching engine calculates the equilibrium price; trades confirmed at this uniform price — the official closing price.
- 3:35 PM – 3:50 PM (Post-Close Session): Additional order placement allowed at the newly set closing price.
Key Rules and Features You Should Know
- Reference price = VWAP of trades between 3:00–3:15 PM.
- Price band restriction: ±3% around the reference price.
- Only limit and market orders permitted — no stop-loss or iceberg orders.
Why This Matters for Everyday Traders and Investors
The closing price settles daily mark-to-market positions in futures, influences options pricing, and feeds into index calculations affecting index funds and structured products. If you trade CAS stocks in the last 30–35 minutes, know that orders are pooled and matched at one final price rather than executing instantly like normal market orders.
