What Is the Sharpe Ratio? A Plain-English Guide

The Sharpe ratio is a strategy's return above the risk-free rate, divided by how much its returns fluctuate (their volatility). Higher means more reward for each unit of risk taken.

Sharpe ratio = (average return - risk-free rate) / standard deviation of returns.

The top measures the excess return over a safe baseline (like short-term government bonds). The bottom measures volatility - how much the returns bounce around. For comparability, it's usually annualised.

Strategy A returns 20% a year with a volatility of 10%. Strategy B also returns 20%, but with a volatility of 40%. Assume a risk-free rate of 4%.

Strategy A's Sharpe = (20% - 4%) / 10% = 1.6. Strategy B's Sharpe = (20% - 4%) / 40% = 0.4.

Same headline return, but A's is four times higher quality by this measure - it delivered the gain far more smoothly. If you had to live through B's swings (and stay invested), A is the easier and arguably safer route to the same place.

A backtest's total return tells you nothing about how bumpy the road was. The Sharpe ratio captures that, so you can prefer a strategy that earns its money calmly over one that earns the same amount through gut-wrenching volatility.

It's also a rough guard against cherry-picked, fragile results. A strategy with a great return but a tiny Sharpe is often relying on a few explosive moments rather than a consistent edge.

Where the Sharpe ratio can mislead:

• Treating volatility as the only risk. Sharpe punishes upside swings as much as downside ones; it doesn't single out the losses that actually hurt.
• Comparing Sharpe ratios computed over different periods or frequencies. Daily, monthly, and annual figures aren't directly comparable unless consistently annualised.
• Reading it in isolation. A high Sharpe over a short, calm stretch can vanish in a crisis. Pair it with maximum drawdown for the full picture.