What it measures
Kurtosis measures how heavy the tails of a return distribution are, in other words how often very large moves happen compared with a bell curve. Most tools report excess kurtosis, where a normal bell curve scores 0.
Fat tails in markets
Daily stock market returns have high excess kurtosis. On a bell curve with the S&P 500's volatility, a one-day fall of 20% like October 1987 would essentially never happen. It did. Days of 5% moves occur many times more often than a normal curve predicts.
Why it matters
- Risk measures built on Standard deviation and volatility understate the chance of disasters.
- Leveraged strategies are especially exposed: one extreme day can wipe out years of gains. See Leveraged ETFs explained.
- Combine kurtosis with negative Skewness of returns and you have a strategy whose rare losses are both large and frequent enough to matter.
Reading it
Excess kurtosis above about 3 means tails well beyond normal. Look at the worst 1% of days directly rather than trusting averages.
See also
- Skewness of returns What skewness means for investment returns, how positive and negative skew differ, and why negatively skewed strategies hide their risk.
- Tail ratio The tail ratio compares a strategy's best days with its worst days. How it is calculated from percentiles and what values above or below 1 tell you.
- Standard deviation and volatility What volatility means in investing, how standard deviation of returns is calculated and annualised, and typical volatility for stocks, bonds and leveraged funds.
- Maximum drawdown What maximum drawdown is, how to calculate it, why losses need bigger gains to recover, and historical drawdowns for stocks and leveraged funds.
Pages that link here: Sharpe ratio
Last updated September 30, 2026. Education only, not investment advice.