The 30 day at the money IV that sets the expected move changes from one session to the next
Start with the 30 day at the money implied volatility, the volatility the options market prices for the next month. The chart plots that series from the NSE end of day files. It moves each session, so the expected move moves with it.
The formula is spot, times implied volatility, times the square root of the time to expiry in years. Time in years is calendar days divided by the days in a year. Spot or the future both work, but use the same one that the volatility was measured against, and say which.
Read the result as a one standard deviation band under a normal model. Under that model the index closes inside the band a little more than two thirds of the time. Real returns have fatter tails than a normal curve, so the band is a rough guide, and moves beyond it are larger than the band suggests.
The at the money straddle gives a second estimate. Its price is roughly four fifths of the one standard deviation move, so the two come out close. When they disagree, check the expiry, the future used, and whether both strikes traded.
Used this way, the band is a check on prices and a measure of risk. A wide band is not a sign that the index will move far, and a narrow one does not say which way it will go. The examples here use dated chains and NSE series only.
Before you trade. Derivatives trading carries a high risk of loss. SEBI's study "Profitability of Individual Traders in the Equity Derivatives Segment (FY25 to FY26)" (August 2026) found that 87.7% of individual traders in equity F&O made net losses in FY26, with aggregate net losses of Rs 91,685 crore. Read the SEBI study