Maximum drawdown (MDD) is the largest loss from a historical peak to the subsequent trough, before a new peak is reached:
MDD = max over time t of [(Peak_value − Trough_value) / Peak_value]
MDD is expressed as a positive percentage, representing the worst-case loss an investor would have experienced had they entered at the worst possible peak and exited at the worst possible trough.
Why it matters in practice
Unlike volatility-based metrics like the Sharpe Ratio, MDD captures the path-dependent experience of an investor. A strategy can have acceptable annualized volatility but an extreme drawdown that would cause most investors to redeem — or trigger fund-level risk limits — before recovery occurs.
The Calmar Ratio uses MDD as its risk denominator, providing a return-to-drawdown metric. For trend-following, option-selling, and other strategies where return distributions are significantly skewed, MDD is often more informative than volatility as a risk characterization.