Sharpe Ratio = (Portfolio Return − Risk-Free Rate) ÷ Standard Deviation of Portfolio Returns
The Short Version
Raw returns don't tell the whole story — a 15% return achieved with wild swings is very different from a 15% return achieved smoothly. The Sharpe ratio measures return earned per unit of risk (volatility) taken, making it possible to compare investments or portfolios on a risk-adjusted basis.
Explain It Like I'm 10
Imagine two roller coasters both end up at the same height as where they started, taking the same amount of time. One is a smooth, gentle ride the whole way. The other has terrifying drops and sudden loops the entire time. Even though they 'end up' the same, most people would say the smooth ride was the better experience for the same result. The Sharpe ratio applies that same idea to investing — it rewards getting a good return WITHOUT a wild, scary ride along the way.
Step-by-Step Example
- 1 Portfolio A returns 12% for the year, with a 'risk-free' savings rate of 4%, and swings around by a standard deviation of 10%.
- 2 Sharpe Ratio = (12% − 4%) ÷ 10% = 0.8.
- 3 Portfolio B also returns 12%, with the same 4% risk-free rate, but swings around wildly with a standard deviation of 25%.
- 4 Sharpe Ratio = (12% − 4%) ÷ 25% = 0.32.
- 5 Even though both portfolios made the exact same 12% return, Portfolio A gets a much better Sharpe ratio because it achieved that return far more smoothly, with much less risk along the way.
Quick Recap
- ✓ Higher Sharpe ratio = more return for each unit of risk taken.
- ✓ The 'risk-free rate' is typically approximated using short-term government Treasury yields.
- ✓ Useful for comparing funds or strategies, not just individual stocks.