What is a Sharpe ratio, and when does it matter?

Most investors evaluate their portfolio with one number: total return. If the balance went up 15% this year, that's a good year. The problem with this approach is that it ignores the ride you took to get there: how much volatility you absorbed to earn that return.

Two portfolios can return identical amounts over a year while having completely different risk profiles. The Sharpe ratio is how you tell the difference.

The formula, plainly stated

The Sharpe ratio was developed by economist William Sharpe in 1966:

Sharpe Ratio = (Portfolio Return − Risk-Free Rate) / Standard Deviation of Portfolio Returns

Breaking that down:

• Portfolio Return is your actual return over the period.
• Risk-Free Rate is the return on an essentially risk-free investment, conventionally the 3-month US Treasury bill rate. In early 2026 this is approximately 4.3%.
• Standard Deviation measures how much your portfolio's returns vary month to month. A portfolio that returns exactly 0.6% every month has a standard deviation of zero. A portfolio that swings between +8% and -6% month to month has a high standard deviation.

The result is a measure of excess return per unit of risk. A higher Sharpe ratio means you're getting more return for each unit of volatility you're absorbing.

A concrete comparison

Consider two portfolios over a one-year period where the risk-free rate is 4.3%:

Portfolio A returns 14%. Monthly returns vary widely, some months +6%, some months -4%. Annualized standard deviation: 22%. Sharpe ratio: (14% − 4.3%) / 22% = 0.44

Portfolio B returns 10%. Monthly returns are steadier, rarely moving more than 1.5% in either direction. Annualized standard deviation: 8%. Sharpe ratio: (10% − 4.3%) / 8% = 0.71

Portfolio A returned more. Portfolio B has a better Sharpe ratio. Which is better depends on what you care about, but if you have a shorter time horizon or low risk tolerance, Portfolio B may be the right choice even though it returned less.

When Sharpe ratio doesn't matter much

For a long-horizon investor in their 30s with stable income, short-term volatility is mostly noise. A 20% drawdown is uncomfortable but irrelevant if you're not selling for 30 years. In this context, obsessing over Sharpe ratios is counterproductive. You're optimizing for smoothness when you should be optimizing for long-term expected return.

When it does matter

The ratio matters more as your time horizon shortens. Near retirement, a sharp drawdown early in the distribution phase can permanently impair a portfolio through sequence-of-returns risk. Retirees drawing down assets can't wait for a recovery. Here, lower volatility has real dollar value, and a high Sharpe ratio is genuinely meaningful.

As a rough benchmark: above 1.0 is generally considered good. Above 2.0 is excellent. The S&P 500 has a long-run Sharpe ratio of approximately 0.4-0.5. A well-constructed diversified portfolio typically does better.

How Prismfolio calculates this

The Prismfolio web app calculates your portfolio's Sharpe ratio using trailing 12-month return data from your connected holdings. It uses the current 3-month T-bill rate as the risk-free benchmark and calculates standard deviation from monthly return history.

You don't have to pull the formula up in a spreadsheet. It's there automatically, alongside your allocation breakdown and fee analysis, so you can see not just how your portfolio did, but how efficiently it did it.