What Is the Sharpe Ratio?
Developed by Nobel laureate William Sharpe, the ratio answers a simple question: how much return did an investment deliver per unit of risk taken? Raw returns are misleading on their own, because a fund that earns 15 percent with wild swings may be a worse investment than one that earns 10 percent smoothly. The Sharpe ratio makes that trade-off explicit.
Because it standardizes performance, the ratio is the default way allocators screen funds. A pension deciding between a bond strategy and an equity strategy cannot compare headline returns directly, but it can compare Sharpe ratios to see which manager is compensating investors better for the risk being run.
How to Calculate It
Sharpe Ratio = (Portfolio Return - Risk-Free Rate) / Standard Deviation of Portfolio Returns. If a fund returns 12 percent while Treasury bills yield 4 percent and the fund's annualized volatility is 10 percent, its Sharpe ratio is (12 - 4) / 10 = 0.8. The numerator is the excess return earned for taking risk; the denominator is the amount of risk taken.
As rough benchmarks, a Sharpe ratio near 1 is considered good for a liquid strategy over a full cycle, while sustained ratios above 2 are rare and usually signal either exceptional skill or hidden risk. The measure has known weaknesses: it penalizes upside volatility the same as downside, and strategies that sell tail risk can post high Sharpe ratios for years before a single blowup.
Why It Matters in Practice
Hedge funds, mutual funds, and trading desks are all judged on Sharpe. Multi-manager platforms such as Citadel and Millennium explicitly manage portfolio managers to Sharpe targets, cutting capital from teams whose risk-adjusted returns slip. Allocators use the ratio alongside drawdown and correlation statistics when deciding where to place capital.
For interviews, be ready to compute the ratio from given inputs and to critique it. Strong candidates note that Sharpe assumes returns are roughly normal, that it can be gamed by smoothing or illiquid marks, and that alternatives like the Sortino ratio, which uses only downside deviation, address some of these flaws.
