Volatility Explained: What It Is, How It's Measured & Why It Matters in Finance

Volatility is one of the most important – and most misunderstood – concepts in finance. It's the backbone of option pricing, a key input in portfolio construction, and a standard measure of risk. For S&T interviews, volatility knowledge is non-negotiable. For IB interviews, it demonstrates the kind of market fluency that separates top candidates.

What Is Volatility?

Volatility measures the degree of variation in a financial instrument's price over time. A stock that moves 3% per day is more volatile than one that moves 0.5% per day. Higher volatility = larger price swings = more uncertainty about future prices.

Crucially, volatility is not the same as direction. A stock can be highly volatile while trending upward, downward, or sideways. Volatility measures the magnitude of price movements, not their direction.

In finance, volatility is typically expressed as an annualised standard deviation of returns. If a stock has 20% annualised volatility, its returns are expected to fall within ±20% of the mean about 68% of the time (assuming a normal distribution, which is an approximation).

Historical vs Implied Volatility

Historical (Realised) Volatility: Measures how volatile the asset actually was over a past period. It's calculated as the standard deviation of historical returns, typically daily returns annualised by multiplying by √252 (the number of trading days in a year).

Implied Volatility: Measures how volatile the market expects the asset to be in the future. It's extracted from option prices using an option pricing model (like Black-Scholes). When option prices rise, implied volatility rises – and vice versa.

The distinction matters because the relationship between historical and implied volatility is the foundation of many trading strategies:

If implied volatility is higher than realised volatility, options are 'expensive' – you might sell options to capture the premium. If implied volatility is lower than realised volatility, options are 'cheap' – you might buy options to profit from future price movements.

The VIX

The VIX – sometimes called the 'fear index' – is the most widely watched volatility measure. It represents the 30-day implied volatility of the S&P 500 index, derived from option prices.

VIX below 15: Low volatility, market complacency. Equity markets are typically calm and trending upward.

VIX 15–25: Normal to elevated volatility. Some market uncertainty.

VIX 25–40: High volatility. Significant market stress, often during corrections or geopolitical events.

VIX above 40: Crisis-level volatility. Historically rare – seen during the 2008 financial crisis (VIX hit 80), COVID crash in 2020 (VIX hit 82), and periods of extreme uncertainty.

The equivalent measure in Europe is the VSTOXX (based on Euro Stoxx 50 options) and the VFTSE (based on FTSE 100 options).

Why Volatility Matters

Option Pricing: Volatility is the single most important input in option pricing. The Black-Scholes model takes five inputs: stock price, strike price, time to expiration, risk-free rate, and volatility. Of these, volatility is the only one that can't be directly observed – it must be estimated or implied from market prices.

Higher volatility = higher option prices (both calls and puts), because greater price variation increases the probability that the option ends up in the money.

Portfolio Construction: Modern portfolio theory uses volatility (and correlations between assets) to construct efficient portfolios. The goal is to maximise return for a given level of volatility, or minimise volatility for a given return target.

Risk Management: Value at Risk (VaR), the standard risk metric in banking, depends on volatility estimates. Higher estimated volatility = higher capital requirements and more conservative position sizing.

Trading Strategies: Volatility itself can be traded. Buying options is a long-volatility position (you profit if vol rises). Selling options is short-volatility (you profit if vol stays low or declines). More complex strategies like straddles and strangles are pure volatility plays.

Volatility Skew and the Smile

In theory (Black-Scholes), implied volatility should be the same across all strike prices for a given expiration. In reality, it isn't:

Volatility Skew: OTM puts typically have higher implied volatility than OTM calls for equity indices. This reflects the market's greater fear of sharp downside moves relative to upside moves. The skew became pronounced after the 1987 crash.

Volatility Smile: In currency and commodity markets, both OTM puts and OTM calls have higher implied volatility than ATM options, creating a U-shaped or 'smile' pattern.

Understanding the skew is important because it tells you about market sentiment and risk pricing. A steepening skew (OTM puts getting more expensive relative to calls) signals increasing fear of a market downturn.

Common Interview Questions

'What is volatility?' The annualised standard deviation of returns – a measure of how much an asset's price fluctuates over time. It captures the magnitude of price movements, not direction.

'If a stock has 30% volatility, what does that mean practically?' Over one year, you'd expect the stock's return to fall within ±30% of its expected return about two-thirds of the time. Over shorter periods, daily expected moves ≈ 30% / √252 ≈ 1.9% per day.

'Does high volatility mean the stock will go down?' No. Volatility measures uncertainty, not direction. A stock can be highly volatile and trend upward. However, high volatility is often associated with market stress (which tends to be negative), which is why the VIX is called the 'fear index.'

'Why is implied volatility typically higher than realised volatility?' Because investors demand a risk premium for selling options (which means bearing tail risk). This 'volatility risk premium' means option sellers are compensated for taking on the risk of extreme moves, even if those moves don't materialise most of the time.