What Is the Sharpe Ratio? Measuring Risk-Adjusted Returns

What is the Sharpe ratio?

Developed by Nobel laureate William Sharpe in 1966, the Sharpe ratio measures return earned per unit of risk taken:

Sharpe Ratio = (Return − Risk-Free Rate) ÷ Standard Deviation of Returns

The risk-free rate (typically the 3-month T-bill yield) represents the return available with zero risk. Subtracting it gives you the "excess return" — what you earned above what you could have earned for free. Dividing by standard deviation tells you how much volatility (risk) you took to achieve that excess return.

Why it matters: two funds, same return

Fund A returned 15% last year with 10% standard deviation → Sharpe = (15% − 5%) ÷ 10% = 1.0
Fund B returned 15% last year with 25% standard deviation → Sharpe = (15% − 5%) ÷ 25% = 0.4

Same headline return. Fund A delivered it with dramatically less risk. Fund B required investors to endure 2.5× the volatility for the same reward. A rational investor prefers Fund A.

Sharpe ratio benchmarks

Below 0.5: Poor risk-adjusted return — you're being underpaid for the volatility you're taking.
0.5–1.0: Acceptable. S&P 500 long-run average sits here.
1.0–2.0: Good. Achieved by skilled managers and well-constructed portfolios.
Above 2.0: Very good. Rare to sustain over long periods.
Above 3.0: Excellent — or something wrong with the measurement (strategy exploiting tail risk).

The Sortino ratio: an improvement

The standard deviation in the Sharpe denominator penalises both upside and downside volatility equally. But investors don't mind upside surprises — they only dislike losses. The Sortino ratio uses only downside deviation, making it a fairer comparison for strategies with positively skewed returns.

Key takeaways

Sharpe ratio = excess return ÷ standard deviation. Higher = better risk-adjusted performance.
Above 1.0 is good; the S&P 500 averages 0.5–0.6 long-run.
Two portfolios with the same return can have very different risk profiles — Sharpe captures this.
Limitation: assumes normal return distribution; strategies hiding tail risk can look deceptively high-Sharpe.

Frequently Asked Questions

What is the Sharpe ratio formula?

Sharpe Ratio = (Portfolio Return − Risk-Free Rate) ÷ Portfolio Standard Deviation. The risk-free rate is typically the current yield on 3-month US Treasury bills. Standard deviation measures the volatility of returns over the measurement period.

What is a good Sharpe ratio?

Generally: above 1.0 is considered good, above 2.0 is very good, and above 3.0 is excellent. The S&P 500 has historically averaged a Sharpe ratio of around 0.5–0.6. Most actively managed funds struggle to sustain Sharpe ratios above 1.0 over long periods.

What are the limitations of the Sharpe ratio?

Sharpe ratio uses standard deviation as its risk measure, which assumes returns are normally distributed. In reality, markets have fat tails — extreme events happen more often than normal distribution predicts. Strategies that appear high-Sharpe by collecting steady small gains (and hiding rare catastrophic losses) can be dangerously misleading.

What is the difference between Sharpe and Sortino ratio?

The Sortino ratio modifies Sharpe by only penalising downside volatility (negative deviations), not total volatility. Since investors don't mind upside volatility, Sortino gives a fairer picture for strategies with positively skewed returns.