The problem the Sharpe ratio solves
Two portfolios can post the same headline return while carrying very different amounts of risk — comparing raw returns alone hides that difference. The Sharpe ratio compares an investment's excess return, its return above a risk-free benchmark such as Treasury bills, to the volatility of its returns, measured by standard deviation. It answers how much reward an investor received for each unit of risk taken, letting portfolios with different risk levels be compared on a common scale.
William F. Sharpe introduced the measure in a 1966 paper in the Journal of Business, originally calling it the reward-to-variability ratio. Sharpe was later awarded the 1990 Nobel Memorial Prize in Economic Sciences for his work on the theory of financial asset pricing.
The formula and a worked example
The Sharpe ratio subtracts the risk-free rate from the portfolio return to isolate the excess return — the compensation for taking risk — then divides that excess by the standard deviation of returns. The result is a dimensionless number: excess return earned per percentage point of volatility.
A portfolio returning 8% when the risk-free rate is 3% has an excess return of 5 percentage points; with a standard deviation of 12%, the Sharpe ratio is (8 − 3) ÷ 12 ≈ 0.417. All three inputs must cover the same time period, typically annualized, for the ratio to be meaningful.
- Sharpe ratio = (Rp − Rf) ÷ σp
- Rp = portfolio return (8%), Rf = risk-free rate (3%), σp = standard deviation of returns (12%)
- Sharpe ratio = (8 − 3) ÷ 12 ≈ 0.417
Reading the number
There is no official regulatory threshold for a good Sharpe ratio, but rule-of-thumb ranges are widely used in investment practice when comparing portfolios measured over the same period.
| Sharpe ratio | Common interpretation |
|---|---|
| ≥ 1.0 | Commonly described as good — at least one unit of excess return per unit of volatility. |
| 0.5 – 1.0 | Adequate — positive risk-adjusted return, but less excess return per unit of risk. |
| < 0.5 | Poor — little or no excess return relative to the volatility taken (negative if the portfolio underperformed the risk-free rate). |
Where the Sharpe ratio breaks down
Because the Sharpe ratio uses standard deviation, it penalizes upside and downside volatility equally and assumes returns are approximately normally distributed — it can overstate the quality of strategies with smooth returns but rare large losses, such as option-selling strategies, where standard deviation understates true downside risk. It also isn't a forecast: it summarizes historical (or assumed) risk and return and doesn't predict future performance. A negative Sharpe ratio, which occurs whenever return falls below the risk-free rate, is also tricky to rank — among underperforming portfolios, higher volatility makes the ratio less negative without indicating better performance.
Related measures modify the risk denominator for other use cases: the Sortino ratio divides excess return by downside deviation only, useful for asymmetric return distributions, while the Treynor ratio uses beta instead of total volatility.
Domande frequenti
What is a good Sharpe ratio?
In common investment practice, a Sharpe ratio of 1.0 or higher is often described as good, between 0.5 and 1.0 as adequate, and below 0.5 as poor. These are practitioner rules of thumb rather than official standards, and sensible benchmarks vary by asset class, strategy, and market environment.
Who invented the Sharpe ratio?
William F. Sharpe introduced it in a 1966 paper, "Mutual Fund Performance," published in the Journal of Business, originally calling it the reward-to-variability ratio. Sharpe later received the 1990 Nobel Memorial Prize in Economic Sciences for his contributions to the theory of financial asset pricing.
Can the Sharpe ratio be negative?
Yes. A negative Sharpe ratio occurs whenever the portfolio's return is below the risk-free rate, meaning the investor took on volatility but earned less than a risk-free alternative. Ranking portfolios by negative Sharpe ratios is problematic, because among underperforming portfolios a higher standard deviation makes the ratio less negative without indicating better performance.
What is the difference between the Sharpe ratio and the Sortino ratio?
The Sharpe ratio divides excess return by total standard deviation, penalizing upside and downside volatility equally, while the Sortino ratio divides excess return by downside deviation only, penalizing only returns below a target. The Sortino ratio is sometimes preferred for strategies with asymmetric return distributions.
Fonti
- Sharpe WF. Mutual Fund Performance. Journal of Business, 1966;39(1):119–138.
- Sharpe WF. The Sharpe Ratio. Journal of Portfolio Management, 1994;21(1):49–58.
- CFA Institute. Portfolio Risk and Return — CFA Program Curriculum. cfainstitute.org.
- U.S. Securities and Exchange Commission, Investor.gov. Assessing risk and investment performance. investor.gov.