How to read it
The idea
The fair price of a contract that pays the realised variance of the index over a period can be built from out of the money options of every strike. Each strike counts for its price over the square of its strike, times the gap between strikes. Taking the square root of the annualised variance gives a volatility.
The steps
Find the forward from the strike where the call and put prices are closest. Take K0, the highest strike at or below the forward. Use puts below K0, calls above and the average at K0, and stop each wing after two strikes in a row that did not trade. Add up, subtract a small correction for the forward not sitting on K0, and divide by the time.
The 30 day blend
There is one number for each expiry. The near and the next expiry are weighted by how far 30 days lies from each, in total variance, then scaled to a yearly figure. The weights are shown; they add to one when 30 days lies between the two expiries.
Why it does not match exactly
NSE uses bid and ask quotes through the day; this page uses one settlement price per strike. The gap on the six dates is the chart. The module on India VIX rebuilt goes through the arithmetic and the variance swaps module explains why the strip replicates variance.
What it leaves out
This is a teaching reconstruction. The wing rule is an adaptation because an end of day file has no bid prices, the rate is the T-bill of the date and the chains hold strikes within 25% of the forward that traded or carried open interest. The published index depends on NSE's own inputs and timing, so the gap to this page is expected and small but not zero. India VIX is a measure of implied volatility over the next 30 days, not a forecast of the index.