The Sharpe ratio is one of the most quoted numbers in trading, and one of the most misunderstood. At its core it answers a simple, important question: how much return did a strategy earn for the amount of risk it took? This guide explains what the Sharpe ratio measures, how to interpret it without over-reading it, and where it quietly falls short.
What the Sharpe ratio measures
The Sharpe ratio compares a strategy's return, above a risk-free baseline, to the volatility of that return. In plain terms, the numerator is "how much extra you earned" and the denominator is "how bumpy the ride was." Divide one by the other and you get return per unit of risk — a single figure that lets you compare strategies that would otherwise be hard to line up side by side.
The intuition is that volatility is a cost. A strategy that grinds out steady gains is worth more, all else equal, than one that reaches the same destination by lurching up and down, because the smooth path is easier to hold and less likely to shake you out. The Sharpe ratio rewards that smoothness explicitly: two strategies with identical returns will score differently if one was calmer than the other.
Because it condenses reward and risk into one number, the Sharpe ratio is a natural starting point for comparison. It's the reason a strategy with lower raw returns can legitimately be judged superior to a higher-returning but wilder alternative.
How to read the number
A higher Sharpe ratio is generally better — it means more return for each unit of volatility endured. Very roughly, values below one are often seen as unremarkable, while higher values suggest a more efficient use of risk. But these rules of thumb are context-dependent, and treating any threshold as a hard pass/fail line is a mistake.
What matters far more than the absolute value is the basis for comparison. A Sharpe ratio is only meaningful against strategies measured over the same period, in the same market conditions, using the same assumptions. A number computed over a gentle bull run will flatter almost anything; the same strategy tested through a crash may look entirely different.
Treat the Sharpe ratio as a relative tool, not an absolute grade. Its job is to help you rank strategies against each other under matched conditions, not to certify that any one strategy is objectively "good."
Where the Sharpe ratio falls short
The Sharpe ratio has real blind spots, and knowing them keeps you from over-trusting it. Because it treats all volatility as bad, it penalizes big upside moves exactly as harshly as big downside moves — even though few investors complain about sudden gains. The Sortino ratio was designed to address this by counting only downside volatility.
