Finding Options Edge:
Historical vs Implied Vol
The gap between what the market has done and what options are pricing is one of the most reliable signals in options trading — if you know how to read it.
Two views of the same question
Every options trade involves a bet on future volatility, whether the trader knows it or not. Two measurements of volatility sit at the heart of this question:
- Historical volatility (HV) — also called realised volatility or statistical volatility. It is calculated from actual past price returns, typically over 10, 20, 30, or 90 days. HV answers: "How much has the underlying actually moved?"
- Implied volatility (IV) — extracted from live option prices by reverse-solving an options pricing model (usually Black-Scholes). IV answers: "How much is the market pricing in for future moves?"
One looks backward. The other looks forward. Their relationship is the core of volatility trading.
How historical volatility is calculated
HV is the annualised standard deviation of daily log returns over a rolling window. The steps:
- Take the closing prices for the past N days (e.g. 20 days)
- Calculate daily log returns: ln(today's close ÷ yesterday's close)
- Find the standard deviation of those returns
- Multiply by √252 to annualise
The result is a percentage. For NIFTY, 20-day HV might typically sit between 10% and 20% in normal conditions, with spikes above 30% during high-stress periods. You do not need to calculate this manually — most options data platforms and TradePulse's IV screen display it alongside the current IV.
How implied volatility is extracted
IV works in the opposite direction. You start with the observed market price of an option and ask: what level of future volatility, if plugged into the pricing model, would produce this price? The answer is the implied volatility. Every option on the chain has its own IV — typically displayed in the IV column of the NIFTY option chain.
The India VIX is a weighted average of near-month NIFTY option IVs, giving a single number that represents the market-wide expected 30-day volatility for NIFTY. Watching India VIX is the quickest way to track the aggregate IV of NIFTY options.
The volatility premium: IV vs HV gap
Historically on most liquid indices — including NIFTY — IV has tended to run slightly above HV most of the time. This difference is called the volatility premium (or "variance risk premium"). The reason: option sellers demand compensation for the risk of being short an option during unexpected volatility spikes. Over time, that premium has on average been positive, which is a structural reason why mechanical options selling strategies have worked in many markets, though past performance does not guarantee future results.
Understanding the sign and magnitude of the gap is a starting point for deciding whether options are overpriced or underpriced:
- IV significantly above HV: options carry elevated premium. Selling strategies (short straddles, iron condors) may offer statistical edge. Buyers are paying up for insurance.
- IV near or below HV: options are cheap relative to recent moves. Buying strategies may offer better value. This is comparatively rare on NIFTY but occurs after a prolonged calm following a spike.
A worked NIFTY example
Suppose NIFTY is near 22,500 and you are checking the market before placing a trade (hypothetical numbers throughout):
- 20-day HV: 12% — the market has been relatively calm recently
- Current ATM IV (from the option chain): 18% — the near-month ATM call/put are pricing 18% volatility
- India VIX: 17.5 — consistent with 18% IV on ATM options
The IV-HV gap is +6 percentage points. Options are pricing in significantly more volatility than has been realised recently. A trader who believes this gap is excessive might sell an iron condor, collecting the inflated premium while defining risk with OTM wings. Conversely, a trader who expects an imminent catalytic event (say, RBI meeting in 3 days) might believe the elevated IV is justified and avoid selling.
Event-driven IV spikes and post-event IV crush
One of the most predictable volatility patterns on NSE is the event-driven IV spike. Before a scheduled catalyst — RBI policy, union budget, quarterly results, Lok Sabha election results — IV rises as traders buy options to position for the unknown outcome. After the event is announced, uncertainty collapses and IV drops sharply. This is called an IV crush.
IV crush is the bane of options buyers who are late to the trade. Even if NIFTY moves in the direction they expected, the fall in IV can wipe out premium gains. Sellers, on the other hand, benefit from the crush — but only if the underlying move is smaller than what was priced in.
Comparing IV to HV in the lead-up to events is essential: if IV has already priced a 2.5% move but 20-day HV is only 0.9%, options are very expensive and buyers need a larger-than-average move to profit.
Choosing the right HV window
There is no single "correct" HV window. Common practice:
- 5-day HV: useful for weekly NIFTY options (Thursday expiry) — reflects only the most recent trading conditions.
- 20-day HV: the most widely used; closely matches the typical 30-day option term.
- 30-day HV: aligned to monthly expiry cycles on NSE.
- 90-day HV: smooths out short-term spikes; shows structural volatility level; useful baseline for longer-dated positions.
Comparing current IV against all three windows simultaneously gives a richer picture. If IV is above HV across all windows, the elevation is structural, not just a recent-data artifact.
Common mistakes
- Comparing IV to only one HV window and drawing premature conclusions — short HV windows can be misleadingly low or high after brief calm or volatile patches.
- Assuming IV always mean-reverts quickly — before major events, IV can stay elevated for weeks.
- Ignoring the direction of the IV-HV gap: a large gap alone is not a trade signal; you need a view on whether the catalyst justifies the premium.
- Forgetting that HV is backward-looking — a calm 20-day HV right before an RBI meeting is not a reason to sell volatility; the relevant comparison is expected future realised vol.
Frequently asked questions
What is the difference between historical and implied volatility?
Historical volatility (HV) measures how much the underlying has actually moved over a past period, calculated from real price data. Implied volatility (IV) is the market's forward-looking expectation of volatility, extracted from current option prices. HV looks backward; IV looks forward.
What does it mean when IV is higher than HV?
When implied volatility exceeds recent historical volatility, options are carrying a volatility premium — the market expects more turbulence ahead than it has seen recently. This is a common setup before scheduled events like RBI policy, budget, or elections. Sellers often prefer this environment because they are collecting elevated premium.
When is HV higher than IV?
HV can exceed IV in a fast-trending market where the underlying is moving sharply but without uncertainty — for example, a sustained one-directional rally. In such cases options may be relatively cheap versus realised moves, favouring buyers. However, this setup is less common because markets usually price in uncertainty around directional moves.
Which time window for HV is most useful to compare with IV?
The most common practice is to compare IV with 20-day or 30-day HV, since that roughly matches the time-to-expiry of near-month options. For weekly NIFTY options you might also look at 5-day HV. Using a longer HV window (e.g. 90 days) smooths out short-term spikes and shows the structural volatility level.
See live IV and HV side by side
TradePulse's volatility dashboard displays current IV, India VIX and rolling HV in one view — no manual calculation needed.