Realized vs Implied Volatility - The Gap That Drives Volatility Trading

By EC Assets Research Team, Volatility Research · Published 20 May 2026 · Updated 28 May 2026

Realized vs Implied Volatility — Realized volatility measures how much an asset has actually moved, computed from past returns; implied volatility measures how much the options market expects it to move, backed out of option prices. The persistent gap between them - implied usually exceeding realized - is the variance risk premium that underpins most volatility strategies.

Two Volatilities, Looking in Opposite Directions

Volatility comes in two flavours that are constantly confused and must be kept apart. Realised volatility (also called historical or actual volatility) measures how much the underlying has moved, computed from past returns. Implied volatility measures how much the options market expects it will move, backed out of current option prices. One looks backward at what happened; the other looks forward at what is priced. The gap between them is where most volatility trading lives.

Measuring Realised Volatility

The standard close-to-close estimator annualises the standard deviation of daily log returns:

σ_RV = √(252 · (1/(n−1)) · Σ (r_t − r̄)²), where r_t = ln(S_t / S_{t−1})

The √252 scales a daily figure to annual terms (≈252 trading days). The choice of window matters enormously: a 10-day realised vol is jumpy and responsive; a 252-day realised vol is smooth and slow. More efficient estimators (Parkinson, Garman-Klass, Yang-Zhang) use the high, low, open, and close to extract more information from each day, but the close-to-close version is the reference everyone starts from.

Measuring Implied Volatility

Implied volatility is not computed from returns at all - it is the volatility input that makes a model price equal the option's market price. Invert Black-Scholes on a traded option and the σ that fits is its implied volatility. Because it is extracted from what people are willing to pay, it is forward-looking and embeds the market's collective expectation, including a risk premium. The VIX is simply a model-free aggregation of S&P 500 implied volatilities into a single 30-day number.

The Variance Risk Premium

On average, implied volatility sits above the volatility that subsequently realises. Option buyers pay for protection and certainty; option sellers demand compensation for bearing the risk of a large move. That persistent gap is the variance risk premium - one of the most studied and most harvested premia in markets.

Variance risk premium ≈ σ_IV² − σ_RV² (often quoted in vol points as IV − RV)

It is the economic engine behind covered calls, short straddles, iron condors, variance-swap selling, and most systematic options-income strategies. It is also why those strategies share a signature: steady gains punctuated by sharp losses when realised volatility finally overshoots what was implied.

Worked Example

Suppose the VIX is at 18, implying the S&P 500 options market expects roughly 18% annualised volatility - about ±1.13% per trading day (the Rule of 16). If the index then actually moves around ±0.7% per day, realised volatility comes in near 11%. Implied (18) exceeded realised (11) by about 7 vol points: option sellers were overpaid for risk that did not materialise, and they kept the difference. Run the same comparison into a crash, where realised volatility spikes to 50% against an implied 20%, and the sellers take the loss that the premium was compensating them for all along.

When Each One Matters

[!key] Use realised volatility to size risk and to judge what actually happened. Use implied volatility to price options and to read what the market expects. Trading the two against each other - selling rich implied to capture the premium, or buying cheap implied before a move - is the core of volatility arbitrage.

• Realised drives risk models, position sizing, and performance attribution. It is the σ in Sharpe ratios and VaR.
• Implied drives option prices, hedging costs, and sentiment reads. A spiking implied vol with flat realised vol signals fear being priced before it shows up in the tape.
• The spread (IV − RV) is itself a tradable signal and a regime indicator.

Why It Matters for Institutional Investors

• Strategy design. Knowing whether a strategy is long or short the variance risk premium tells you how it will behave in a crisis. Selling the premium pays steadily and then loses badly; buying it bleeds steadily and then pays off in a crash. Most "low-volatility, high-Sharpe" return streams are quietly short this premium.
• Hedging cost. The cost of a protective put is set by implied volatility. Buying protection when implied is already elevated is expensive insurance; the realised-versus-implied gap tells you how dear.
• Due diligence. A manager reporting smooth returns from "volatility strategies" should be asked the direct question: are you long or short the variance risk premium, and what is your loss in a realised-vol spike? The answer reveals the true risk.

[!warning] A high Sharpe ratio earned by systematically selling implied volatility is not free skill - it is rent collected for warehousing tail risk. The realised-versus-implied gap pays you most of the time and takes it back violently. Judge such strategies by their worst month, not their average.

References

1. Carr, P., & Wu, L. (2009). Variance Risk Premiums. Review of Financial Studies, 22(3), 1311-1341.
2. Bollerslev, T., Tauchen, G., & Zhou, H. (2009). Expected Stock Returns and Variance Risk Premia. Review of Financial Studies, 22(11).
3. Sinclair, E. (2013). Volatility Trading (2nd ed.). Wiley.
4. Cboe. VIX White Paper: The Cboe Volatility Index. Cboe Global Markets. (https://www.cboe.com/vix)