Introductory Context
"India VIX is calculated using a model-free methodology based on the prices of a wide range of Nifty options across multiple strikes and two expiry months. It weights OTM options by their contribution to the market's volatility expectation. The model-free approach makes it independent of Black-Scholes assumptions — a more robust measure than any single option's IV. "
Why a Model-Free Approach
Earlier VIX calculations (including the original CBOE VIX before 2003) used at-the-money options and the Black-Scholes model to extract IV. The problem: this approach gives only the ATM IV — it ignores the information embedded in OTM options prices about tail risk expectations. The modern methodology, used by both CBOE and NSE, is 'model-free' — it incorporates options across a wide range of strikes and does not depend on Black-Scholes assumptions.
The model-free approach is more robust because it uses the market's actual pricing of all available options, not just ATM contracts. This gives VIX a richer information set — it captures both the general level of uncertainty (from ATM options) and the specific fears about tail events (from OTM options, particularly OTM puts).
The Conceptual Steps in VIX Calculation
Step 1: Identify the Two Relevant Expiry Dates
India VIX uses options from two expiry months to construct a 30-day constant maturity volatility measure. Specifically: the near-month contract (with at least 1 week to expiry) and the next-month contract. By weighting these two contracts appropriately, VIX produces a measure that always corresponds to exactly 30 calendar days — regardless of where the actual expiry dates fall.
Step 2: Select Eligible Strikes
For each expiry month, NSE uses all OTM options with non-zero bids (or within a specified distance of ATM). This typically includes 15–25 call strikes and 15–25 put strikes per expiry month. Including both calls and puts across this range ensures that demand for both upside and downside protection is captured.
Step 3: Weight by Strike Interval
Each strike's contribution to VIX is weighted by its distance from adjacent strikes (the strike interval). This ensures that tightly spaced strikes do not disproportionately influence the result. The weighting scheme ensures that strikes where OI is concentrated — typically ATM and nearby OTM — have appropriate but not exclusive influence.
Step 4: Calculate the Variance Contribution of Each Strike
Each option's price contributes a variance measure to VIX proportional to its price divided by the square of its strike. This transformation ensures that OTM options (which are more sensitive to tail volatility) receive appropriate weight — they contribute more per rupee of premium than ATM options.
Step 5: Sum Across All Strikes
The variance contributions of all eligible strikes are summed for each expiry month. This gives the total variance implied by the options market for each expiry horizon.
Step 6: Interpolate to 30 Days
The two monthly variance contributions are combined using time-based interpolation to produce a single 30-day constant maturity variance. The square root of this variance, multiplied by 100 and annualised, gives India VIX.
Why VIX Uses a Wide Strike Range
If VIX used only ATM options, it would miss the information in OTM puts about tail risk fears. In reality, the market's fear of a sharp decline is most visible in OTM put prices — these options are expensive precisely because participants are willing to pay a premium for downside protection. By including OTM puts in the VIX calculation, the index captures not just central volatility expectations but also tail risk fears. This makes VIX a more complete measure of market anxiety than any single option's IV.
What the Calculation Explains About VIX Behaviour
Why VIX Can Rise Even on Quiet Nifty Days
If demand for OTM puts increases (portfolio managers hedging before an event), those put prices rise, increasing their variance contribution to VIX. Nifty itself may not have moved — the underlying is flat — but VIX rises because the market is paying more for protection. This explains the often-observed phenomenon of VIX rising while Nifty is flat: it is capturing increasing hedging demand before an event.
Why VIX Falls Sharply After Events
After a Budget or RBI announcement, the OTM options that were expensive (because of uncertainty) are quickly unwound. Demand for protection falls. Prices of OTM puts collapse. Their variance contribution to VIX falls. VIX drops rapidly — the IV crush — even if Nifty is still moving. The cause is the removal of hedging demand from the VIX calculation.
Why Very Low VIX Can Be Fragile
When VIX is very low, the market is using narrow OTM options prices that reflect minimal uncertainty. Any sudden demand for protection — a surprise global event — can cause dramatic VIX spikes because the starting point is so low and the options are thinly priced. A jump from VIX 9 to VIX 18 is a 100% increase — more disruptive to option portfolios than a jump from 18 to 25.
Understanding how VIX is calculated transforms it from a 'fear gauge' label into an analytically tractable instrument. When VIX is rising while Nifty is flat, you know why: increasing demand for OTM puts. When VIX collapses after an event, you know why: the OTM protection demand that inflated it has been unwound. When VIX is unusually low, you know the fragility: a small external shock can cause a disproportionate spike because the starting conditions are compressed. This mechanistic understanding makes you a better interpreter of VIX signals.