Options traders talk about volatility constantly, but the word covers two quite different measurements. Historical volatility describes how much prices actually moved. Implied volatility describes how much movement option prices are paying for.
Comparing the two is one of the most useful habits in options trading. This article explains each, how the comparison is made, what it tends to show, and the mistakes people make with it.
Two kinds of volatility
Historical volatility — also called realised volatility — measures how much the underlying's price has actually fluctuated over a past period. It is calculated from price data and looks backwards.
Implied volatility is derived from option prices. It is the volatility figure that, put into a pricing model, produces the price the market is paying. It looks forward: it reflects what participants are collectively paying for over the life of the option.
One is a measurement of the past. The other is a price for the future. The difference between them is where much of options trading's logic sits.
How historical volatility is calculated
The usual method takes daily percentage changes in the underlying's closing price over a chosen window — for example the last twenty or thirty trading days — and measures how spread out those changes are, using the standard deviation. That daily figure is then scaled to an annual figure so it can be compared with implied volatility, which is also quoted on an annual basis.
In plain terms, if an index has been moving around one percent a day on average, its annualised historical volatility will be roughly sixteen percent. If it has been moving half a percent a day, roughly eight percent.
A handy shortcut follows from this, sometimes called the rule of sixteen: dividing an annualised volatility by about sixteen gives the typical daily move it implies. If an index option shows implied volatility of 15 percent, the market is pricing typical daily moves of a little under one percent. If the index's recent realised volatility is 11 percent, its actual recent daily moves have been closer to seven-tenths of a percent. That comparison — roughly 0.94 percent priced against roughly 0.69 percent delivered — is exactly the kind of gap traders look at.
The window matters. A ten-day window reacts quickly to recent moves; a ninety-day window is smoother but slower. Different windows can give quite different figures for the same underlying on the same day.
What the comparison usually shows
Over long periods, implied volatility on major indices has tended to sit somewhat above the volatility that is subsequently realised. In other words, options have more often been priced for slightly more movement than actually occurred.
This gap is often described as a volatility risk premium. It exists because option sellers take on the risk of large, sudden moves and demand compensation for it, and because buyers of protection are willing to pay for it.
But the gap is an average, not a rule. During sharp market falls, realised volatility can jump far above the implied volatility that was priced just before. Those episodes are precisely when option sellers suffer large losses. The premium is compensation for that risk, not a sign that the risk is absent.
Volatility also tends to cluster. Calm periods are usually followed by more calm, and turbulent periods by more turbulence, which is why recent realised volatility is a reasonable guide to the near future most of the time. The exception is the turning point: the first day of a new turbulent phase arrives with recent realised volatility still low, and that is precisely when a seller relying on the backward-looking figure is most exposed.
So a typical pattern is: implied above realised most of the time, by a modest amount, interrupted occasionally by periods where realised volatility explodes above implied.
How traders use the comparison
As context for buying or selling. If implied volatility is well above recent realised volatility, options are pricing in more movement than has lately occurred, which is more favourable to sellers, all else equal. If implied is unusually close to or below realised, options are relatively cheap for buyers.
Around events. Before results, policy decisions or budgets, implied volatility often rises well above recent realised volatility because a large move is possible. After the event, implied usually falls back. Knowing this helps explain why options lose value after an event even when the underlying moves.
For sizing. Rising realised volatility means positions swing more in value. Adjusting size to the current volatility keeps risk more stable.
For sanity checks. If a strategy depends on options being cheap or expensive, comparing implied with realised gives a concrete reason rather than a feeling.
None of these is a trading signal on its own. They are context for judging whether a particular option price is reasonable.
Pitfalls
Comparing mismatched horizons. A thirty-day implied volatility should be compared with a similar realised window, not a ninety-day or one-year figure.
Ignoring events inside the window. If the past period contained a major event, historical volatility will be inflated; if the coming period contains one, implied volatility will be inflated. Compare like with like.
Forgetting gaps. Historical volatility based on closing prices can understate the risk of overnight gaps, which matter enormously to option sellers.
Treating the premium as free money. The fact that implied has usually exceeded realised does not mean selling options always pays. The occasional period when realised volatility jumps far above implied can erase a long run of gains.
Using one number for all strikes. Implied volatility differs across strikes because of skew. Compare the strike you are actually trading, not just an index-level figure.
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Frequently Asked Questions
What is the difference between implied and historical volatility?
Historical volatility measures how much prices actually moved over a past period. Implied volatility is derived from option prices and reflects how much movement the market is paying for in the future.
How is historical volatility calculated?
From daily percentage changes in closing prices over a chosen window, measured by their standard deviation and scaled to an annual figure.
Is implied volatility usually higher than realised?
On major indices it has tended to be somewhat higher on average, a gap often called the volatility risk premium. During sharp falls, realised volatility can jump far above it.
Does high implied volatility mean I should sell options?
Not on its own. It means options are pricing in more movement than recently occurred, which can favour sellers, but sellers remain exposed to sudden large moves.
Which window should I use for historical volatility?
One that matches the horizon of the options you are comparing. Short windows react quickly; longer windows are smoother. Avoid comparing mismatched periods.
Related Reading
- The options course, for Raipur
- Where implied volatility comes from
- The index-wide volatility gauge
- Why one IV figure is not enough
- What the premium means for sellers
- Other ways to take a bearish view
Disclaimer: TheFinBaba provides educational content only - this is not investment advice. Trading involves risk of loss.
