Portfolio Volatility
The standard deviation of a portfolio's returns, measuring the degree of fluctuation around the average return.
What is Portfolio Volatility?
Portfolio volatility is the statistical measure of the dispersion of a portfolio's returns over time, expressed as the annualized standard deviation of return observations. A higher portfolio volatility indicates that returns fluctuate widely from period to period; a lower volatility suggests more consistent, predictable returns. Portfolio volatility is not simply the weighted average of individual asset volatilities — it depends critically on the correlations between holdings. Adding assets with low or negative correlations to an existing portfolio can reduce total portfolio volatility even if those assets are themselves volatile, which is the mathematical foundation of diversification. Portfolio volatility is a key input into risk-adjusted performance metrics (Sharpe ratio, Sortino ratio) and portfolio optimization frameworks (mean-variance optimization). It is typically quoted on an annualized basis and can be estimated from historical returns, implied by options markets, or forecasted using factor models.
Example
A 100% US equity portfolio has had a historical annualized volatility of approximately 15–20%. Adding a 40% allocation to US bonds (historical volatility ~6%, correlation to equities −0.1 to −0.3) reduces portfolio volatility to roughly 10–13% — a meaningful improvement in risk per unit of return. The benefit comes not from bonds being 'safe' in isolation but from their low or negative correlation with equities.