Historical Volatility HV
A backward-looking number whose answer is decided by a choice you barely noticed making.
Quick answer: Historical Volatility is the annualised standard deviation of an asset's past close-to-close log returns over a chosen window — a purely backward-looking measurement of how much price actually moved, in which the length of the window silently determines the answer you get.
In simple words
Historical volatility is what volatility looks like after the fact. You take a run of past closing prices, measure how spread out the daily percentage changes were, and annualise it — and you have a number describing how bumpy the recent past actually was. Suppose you compute it on the last 20 NIFTY closes and get 12%. That 12% is not a forecast and not an opinion; it is a measurement of a period that has already finished, the way a thermometer reads a temperature that already happened. The subtle part hides in the phrase 'last 20 closes': change 20 to 10 or to 60 and you will get a different number from the very same market, and every one of those numbers is correct.
Think of it as the width of the recent wobble. A 20-day historical volatility answers the question 'how much did this market typically move, per day, over the last 20 trading days, expressed as an annual rate?'. Because it only ever looks at closing prices, it is blind to everything that happened during each day — the day NIFTY fell 300 points by lunch and clawed it all back by the close counts, to historical volatility, as a quiet day. That blindness is not a bug you can patch; it is the definition of a close-to-close estimator.
The same market at three window lengths
The window length is not a detail — it is the question
10-day, 20-day and 60-day historical volatility of the same NIFTY series, plotted together.
Professional explanation
The window length is not a parameter, it is the question you are asking
Beginners treat the lookback window as a technicality to be set and forgotten. It is the opposite: the window defines what 'volatility' even means for that calculation. A 10-day historical volatility asks how turbulent the last two weeks were; a 60-day asks how turbulent the last quarter was; and in a market that has just calmed down after a scare, those are genuinely different and genuinely correct answers. A short window is responsive and noisy — it reacts fast and jumps around. A long window is stable and sluggish — it smooths the noise but carries old news for a long time. There is no universally right choice, only a choice matched to a horizon. A trader comparing a 30-day implied volatility against a 10-day historical volatility, and concluding options are expensive, has not found an edge; they have compared two different questions and blamed the answer on the option market.
A shock stays in the reading for exactly one window length
This is the single most misunderstood mechanical feature of historical volatility. A large move enters the window the day it happens and remains one of the window's observations for exactly as many more days as the window is long. During that entire time it inflates the reading, holding volatility up long after the market itself has gone quiet. Then, on the day it finally rolls off the back of the window, the volatility drops — sometimes sharply — with no corresponding event in the market. Traders routinely misread that scheduled cliff as fresh information, as if the market suddenly calmed on that specific morning. It did not. The estimator's memory simply expired. The rolling-vol chart on this page is built to make that artefact impossible to miss: the step down happens on the calendar, not in response to price.
It is close-to-close, and therefore blind to the entire trading day
Historical volatility, in its standard form, uses one price per day — the close — and measures the return from one close to the next. Everything that happens between the closes is invisible to it. A day on which NIFTY opened flat, plunged 400 points by midday on a rumour, and recovered every point by the close is, to close-to-close historical volatility, an utterly calm day: the return from close to close was zero. This is why the concept has cousins — realised volatility using high-low or intraday data — that try to see what close-to-close cannot. The trade-off is honesty about scope: close-to-close is robust and unambiguous because everyone agrees on the closing price, but it measures only the drift between two daily snapshots, not the distance travelled in between.
It is a measurement, not a forecast — but it is used as one
Nothing in the arithmetic of historical volatility looks forward. It is a summary statistic of prices that have already printed, as purely descriptive as an average. And yet it is used, constantly and reasonably, as the naive forecast of future volatility, because volatility clusters: a market that has been turbulent recently tends to stay turbulent for a while. This is the uncomfortable middle ground the page has to be honest about. Historical volatility is a legitimate and often hard-to-beat baseline forecast, and it is simultaneously guaranteed to be wrong at every turning point, because by construction it can only ever describe the regime that has just ended and never the one about to begin. It sees the last crisis perfectly and the next one not at all.
Overlapping windows make the HV series look smoother than the volatility is
When you plot a rolling historical volatility day after day, consecutive readings share almost all of their data — a 20-day window and the next day's 20-day window differ by only one observation in and one out. That overlap induces strong autocorrelation in the historical volatility series itself, so the line looks smooth, trending and well-behaved even when the underlying volatility is jumping around. It is easy to look at a gently sloping historical volatility line and believe volatility is changing gradually, when in fact the smoothness is manufactured by the overlapping windows. The line is a heavily filtered view of the truth, and the filter is doing more of the work than most chart-readers realise.
A shock leaves on a schedule, not when the market calms
A rolling 20-day historical volatility around a single large down day.
Formula
Historical volatility — annualised standard deviation of close-to-close log returns
HV = √( (1 / (n − 1)) × Σ (r_t − r̄)² ) × √252, with r_t = ln(P_t / P_{t−1})
The sample standard deviation of the last n daily close-to-close log returns, annualised by √252. The n−1 denominator (Bessel's correction) makes the estimate unbiased on a finite window. Over short windows the mean return r̄ is often assumed to be zero, which barely changes the result and simplifies the arithmetic. The choice of n — the window — is not incidental to the formula; it is the most consequential decision in it.
- HVHistorical volatility — the annualised figure, expressed as a percentage (0.135 = 13.5%).
- nThe window length: the number of daily returns used, e.g. 10, 20 or 60. This choice determines the answer.
- r_tThe close-to-close log return on day t: the natural log of that day's close divided by the previous close.
- P_tThe closing price on day t; P_{t−1} is the previous close. Only closing prices enter — the day's high and low are ignored.
- r̄The mean of the r_t over the window; often taken as zero for short windows.
- ΣSummation over the n days in the window.
- √Square root — of the variance to get the standard deviation, and of 252 to annualise.
- 252Approximate number of trading days in an Indian market year — the annualisation factor.
How to compute a historical volatility on NIFTY
- Decide the window first, because it is the question. A 10-day window measures the last two weeks; a 60-day window measures the last quarter. Match it to the horizon you actually care about.
- Collect that many-plus-one consecutive daily closing prices for NIFTY. A 20-day historical volatility needs 21 closes to produce 20 returns.
- Compute each close-to-close log return: r_t = ln(P_t ÷ P_{t−1}).
- Take the standard deviation of those returns with an n−1 denominator. Over a short window you may assume the mean is zero and simply use the root-mean-square of the returns.
- Multiply the daily standard deviation by √252 ≈ 15.87 to annualise, and report it as a percentage.
- Label it with its window. '20-day HV = 13.5%' is a complete statement; 'HV = 13.5%' is not, because the reader cannot reproduce or compare it.
- Before reading anything into a change, check whether a large day just entered or just left the window — a jump in HV with no market event is usually the window rolling, not new information.
Practical example
NIFTY worked example
Take a 20-day window in which NIFTY was quiet — say every day's log return was about ±0.5% — and then a single 3% down day lands. Watch what each window length does with that one shock. Assuming the mean is roughly zero, historical volatility is √(Σr² ÷ (n−1)) × √252. For a 10-day window holding nine 0.5% days and the one 3% day: Σr² = 9 × (0.005)² + (0.03)² = 0.000225 + 0.0009 = 0.001125; divide by 9 to get 0.000125; the square root is 0.01118; annualise by 15.87 and you get 17.7%. For a 20-day window (nineteen quiet days plus the shock): Σr² = 0.000475 + 0.0009 = 0.001375; ÷19 = 0.00007237; root 0.008507; annualised 13.5%. For a 60-day window: Σr² = 0.001475 + 0.0009 = 0.002375; ÷59 = 0.00004025; root 0.006345; annualised 10.1%. One market, one shock, three readings — 17.7%, 13.5% and 10.1% — and all three are correct. The interpretation is the lesson: there is no such thing as 'the' historical volatility, only a historical volatility for a stated window, and the shorter the window the more violently that single day dominates it.
BANKNIFTY worked example
BANKNIFTY teaches the flip side — that the reading is a property of what the window happens to contain as much as of the market. Say BANKNIFTY normally posts ±0.7% days, giving a placid 20-day historical volatility of 0.7% × 15.87 ≈ 11.4%. Now suppose the window happens to straddle a scheduled event — an RBI policy decision or the Union Budget — on which BANKNIFTY moved 4% in a session. Recompute: Σr² = 19 × (0.007)² + (0.04)² = 0.000931 + 0.0016 = 0.002531; ÷19 = 0.0001332; root 0.011542; annualised 18.3%. The historical volatility has jumped from 11.4% to 18.3% not because the market's character changed but because the window now contains an event day it did not before. This is why comparing today's 20-day historical volatility against last month's can mislead: if one window caught the Budget and the other did not, you are comparing two windows with different event content, not two states of the same market. The reading answers 'what did the last 20 days contain', and calendars, not just markets, decide that.
Lot sizes used above (NIFTY 75, BANKNIFTY 30) are those in force at the time of writing; NSE revises them periodically. Figures exclude brokerage, STT, exchange charges, stamp duty and GST. Examples are teaching scenarios built on round numbers — they are not historical quotes, not backtests and not trade calls.
Advantages & limitations
What it is good for
- It is a measurement, not an opinion. Given the same prices and the same window, everyone computes the same number, so it is reproducible, auditable and free of anyone's forecast or bias.
- It needs only closing prices, which are public and free. Anyone can compute it for any instrument with a price history, which makes it the universal baseline against which richer estimators are judged.
- It is an honest, often hard-to-beat naive forecast. Because volatility clusters, recent historical volatility is a legitimate first estimate of near-term volatility and a benchmark that fancier models frequently fail to improve on.
- It is unambiguous about scope. Close-to-close historical volatility measures exactly one thing — the dispersion of daily closing returns — and does not pretend to capture intraday travel it cannot see.
- It is directly comparable across instruments and periods once the window is fixed, so a 20-day NIFTY historical volatility and a 20-day BANKNIFTY historical volatility sit on the same scale.
Where it breaks down
- It is entirely backward-looking. It describes a period that has finished and, at every turning point, describes the regime that has just ended rather than the one beginning — it sees the last crisis perfectly and the next one not at all.
- Its answer is decided by the window, which is often chosen thoughtlessly. The same series gives 17.7%, 13.5% or 10.1% depending on whether you picked 10, 20 or 60 days, so an unlabelled historical volatility is uninterpretable.
- It is close-to-close and blind to the trading day. A session that plunged and fully recovered registers as calm, so historical volatility can badly understate the risk actually experienced intraday.
- It steps discontinuously as shocks enter and leave the window. A large drop in the reading can occur with no market event, purely because an old shock rolled off the back — an artefact routinely misread as fresh calm.
- Overlapping windows make the plotted series artificially smooth. Consecutive readings share nearly all their data, inducing autocorrelation that disguises how abruptly the true volatility is actually changing.
- It cannot see a scheduled event before it happens. A window that has not yet reached the Budget or an RBI decision reads exactly as calm as one in a genuinely quiet market, offering no warning of the known event ahead.
Common mistakes
- Quoting 'NIFTY's historical volatility' without stating the window. The number is meaningless without its lookback, because 10-, 20- and 60-day figures on the same series can differ by seven percentage points and all be right.
- Reading the scheduled roll-off of a shock as a market event. When a big day leaves a 20-day window and the reading drops on the twenty-first day, that cliff is the estimator's memory expiring, not the market calming — trading it as news is trading an artefact.
- Comparing a 30-day implied volatility against a 10-day historical volatility and concluding options are expensive. You have compared two different horizons and blamed the gap on the option market. Compare like tenor with like tenor.
- Sizing positions from a low historical volatility just after a calm stretch. Volatility clusters, so the lowest readings sit right before the accumulated leverage unwinds, and the number cannot warn you until after the damage has printed.
- Assuming a smooth historical volatility line means volatility is changing gradually. The smoothness is manufactured by overlapping windows; the underlying volatility can be jumping while the filtered line glides.
- Trusting close-to-close historical volatility to capture a whipsaw day. A session that fell hard and recovered by the close reads as flat, so a book that took real intraday pain sees none of it in the historical volatility.
- Using too short a window and mistaking its noise for signal. A 5-day historical volatility reacts to everything and settles on nothing, so its swings are mostly estimator noise, not changes in the market's true volatility.
Professional usage
On a desk, historical volatility is the reference point against which the tradeable number — implied volatility — is judged. A volatility arbitrage trader compares implied volatility against a forecast anchored on recent realised and historical volatility, and the whole trade is a bet on the gap between the two, delta-hedged so the residual profit and loss depends on volatility rather than direction. Risk managers keep a suite of historical volatilities at several window lengths precisely because a single window hides the term structure of recent movement — a rising 10-day against a flat 60-day flags a fresh disturbance the long window has not yet absorbed. And every serious desk treats the choice of window as a modelling decision to be justified, not a default to be inherited, because they have all been burned by a shock rolling off a window at the wrong moment.
Quant researchers rarely use plain close-to-close historical volatility as an endpoint; they use it as the input to a forecasting model — an exponentially weighted moving average that down-weights old data, or a GARCH model that formalises clustering and mean reversion — precisely to fix the two defects the raw measure has: its equal weighting of a three-week-old shock and yesterday's move, and its cliff-edge roll-off. The honest admission the best of them make is that these refinements improve the forecast at the margin and still cannot see a regime change before it arrives, because no estimator built purely on past prices ever can. Historical volatility is where volatility forecasting starts, not where it ends.
Key takeaways
- Historical volatility is the annualised standard deviation of past close-to-close log returns over a chosen window — a backward-looking measurement of what the market already did.
- The window length is the question, not a setting. A 10-, 20- and 60-day historical volatility of the same series can read 17.7%, 13.5% and 10.1%, and all three are correct, so an unlabelled HV means nothing.
- A shock inflates the reading for exactly one window length and then drops out on a schedule, producing a cliff in the volatility line with no market event behind it. Do not trade that artefact as news.
- It is close-to-close and blind to intraday movement, so a day that whipsawed and recovered by the close registers as calm.
- It is a measurement used as a forecast: a hard-to-beat naive baseline because volatility clusters, and yet guaranteed to be wrong at every turning point because it can only describe the regime that just ended.
Historical volatility is the most honest and the most misleading number in the volatility family at the same time. Honest, because it is a pure measurement with no forecast smuggled inside it — everyone who has the prices gets the same answer. Misleading, because that answer is completely determined by a window most people choose without thinking, and because the number's every movement can be an artefact of a shock entering or leaving that window rather than anything the market did. Learn to always state the window, to check the roll-off before you read meaning into a change, and to remember that a low reading is a description of a calm that has ended, never a promise of one that will continue.
Frequently asked questions
What is historical volatility in simple terms?
How is historical volatility calculated?
What window should I use for historical volatility?
Why do 10-day and 60-day historical volatility differ so much?
Is historical volatility a forecast of future volatility?
Why does historical volatility drop suddenly with no news?
What is the difference between historical and implied volatility?
What is the difference between historical and realized volatility?
Why does historical volatility only use closing prices?
Can historical volatility be zero?
Does a low historical volatility mean the market is safe?
How many days of data do I need to compute historical volatility?
Why is historical volatility annualised?
Does historical volatility predict market direction?
Why does a rolling historical volatility line look so smooth?
Should I assume the mean return is zero when computing historical volatility?
How does historical volatility handle a scheduled event like the Budget?
Is a shorter window always more responsive and better?
How does historical volatility relate to India VIX?
Can two people get different historical volatility for the same stock?
Why does the same shock affect a short window more than a long one?
Is historical volatility useful if it is always backward-looking?
Voice search & related questions
Natural-language questions people ask about historical volatility.
What is historical volatility?
Which lookback should I pick for historical volatility?
Why did historical volatility fall today when nothing happened?
Does historical volatility see what happens during the day?
Can I use historical volatility to predict tomorrow?
Is low historical volatility a green light to add leverage?
Why do my broker and I get different historical volatility numbers?
Sources & references
- Zerodha Varsity — Volatility Calculation (Historical)
- NSE — India VIX methodology
- Cboe — VIX White Paper
- Zerodha Varsity — Volatility Basics
Last reviewed 10 July 2026. Educational content only — not investment advice.