What is the Sharpe ratio?

The Sharpe ratio, developed by Nobel laureate William Sharpe in 1966, measures the excess return of an investment per unit of risk. It is calculated as (portfolio return minus risk-free rate) divided by the standard deviation of portfolio returns. A Sharpe ratio of 1.0 means the investment earned 1% of excess return for each 1% of volatility, which is generally considered good. A ratio above 2.0 is excellent, and sustained ratios above 3.0 are extremely rare and often warrant skepticism.

The risk-free rate in the Sharpe calculation is typically the yield on short-term Treasury bills. By subtracting this rate, the Sharpe ratio isolates the compensation for bearing risk rather than simply holding safe assets. During the zero-rate era of 2009-2021, even modest portfolio returns produced attractive Sharpe ratios because the risk-free rate was near zero. As rates rose in 2022-2023, the bar for justifying equity risk increased because Treasuries offered 5%+ returns with no volatility.

The Sharpe ratio has become the universal language for comparing investment strategies across different asset classes, time periods, and risk profiles. A hedge fund returning 15% with 30% volatility (Sharpe of roughly 0.33 after rates) is arguably worse than a bond portfolio returning 6% with 3% volatility (Sharpe of roughly 0.33). The ratio normalizes for the leverage and risk embedded in different approaches, allowing apples-to-apples comparison.

Practitioners should understand the Sharpe ratio's limitations. It assumes returns are normally distributed, which understates tail risk for strategies with skewed or fat-tailed return profiles. It penalizes upside volatility just as much as downside volatility, which may not reflect investor preferences. And it can be gamed through leverage, illiquidity premiums, or selling tail risk, all of which can inflate the measured Sharpe ratio while hiding latent risks. The Sortino ratio, which only penalizes downside deviation, addresses some of these concerns and is increasingly used alongside the Sharpe ratio.