Sharpe ratio

What the Sharpe ratio means

The Sharpe ratio is a way of judging a return by the amount of ups and downs it took to get there. Rather than asking only how much an investment earned, it asks how much it earned for each unit of volatility. It is named after the economist William Sharpe, who proposed it in the 1960s, and it is common in fund fact sheets, academic papers and portfolio software.

Two portfolios can both return 8 percent in a year. One may have climbed steadily, while the other swung wildly and ended at the same place. Most people would prefer the steadier path. The Sharpe ratio turns that preference into a number: higher means more return per unit of volatility, and lower means the return came with a bumpier ride.

It is used by fund analysts comparing managers, by traders reviewing a strategy's history and by investors comparing portfolios. It is a backward-looking statistic that depends heavily on the time period chosen.

How it works

The formula in words is:

Sharpe ratio = (average return of the portfolio - risk-free rate) ÷ standard deviation of the portfolio's returns

The steps are:

1. Choose a period and a frequency, such as monthly returns over three years.
2. Calculate the average return per period.
3. Subtract the risk-free rate for the same period. This is the return available on something nearly riskless, commonly a short-term government bill. The result is called the excess return.
4. Calculate the standard deviation of the periodic returns, which measures how widely they scatter around the average.
5. Divide the excess return by the standard deviation.
6. If you want an annual figure from monthly data, multiply by the square root of 12. The same scaling applies to other frequencies, such as the square root of 252 for daily data.

Rough conventions are often quoted: below 1 is considered modest, between 1 and 2 is considered good, and above 2 is exceptional, though these thresholds are rules of thumb and depend on the asset class and the period. A negative number means the portfolio earned less than the risk-free rate.

A worked example

These numbers are illustrative only. Compare two funds over a year.

Fund A returned 9 percent with a standard deviation of 12 percent. Fund B returned 7 percent with a standard deviation of 5 percent. The risk-free rate over the year was 2 percent.

1. Fund A excess return = 9 - 2 = 7 percent. Sharpe = 7 ÷ 12 = 0.58.
2. Fund B excess return = 7 - 2 = 5 percent. Sharpe = 5 ÷ 5 = 1.00.

Fund A made more money in absolute terms but took far more volatility for it. Fund B delivered less return but a much smoother ride, so its Sharpe ratio is higher.

Now suppose an investor can use leverage. If Fund B is levered by a factor of 2, both its excess return and its volatility roughly double, ignoring financing costs: excess return 10 percent, standard deviation 10 percent, and the Sharpe ratio is still 1.00. That illustrates a key point: the ratio measures efficiency, not the size of return.

Why it matters

The ratio helps compare strategies on equal terms. A high-return strategy that takes huge risks looks less attractive once volatility is considered. It also stops you from judging a manager by one lucky year.

For retail traders, it can be used on a trading journal. Take your monthly returns, compute the Sharpe ratio, and see whether the profit came from a stable process or from a few outsized swings. Costs matter here: spreads, commissions and financing reduce returns and can quickly lower the ratio for frequently trading strategies. Brokers with different pricing therefore produce different Sharpe ratios for the same strategy.

It also supports decisions on diversification. Adding an asset that does not move in step with the rest of a portfolio can raise the Sharpe ratio even if its own return is modest, because it reduces overall volatility.

How to use it in practice

1. Compute it on net returns, after all fees, spreads and taxes where possible.
2. Use enough data. A few months of results say very little; several years across different market conditions are better.
3. Match the risk-free rate to the currency and period of your returns.
4. Compare like with like: similar assets, same frequency, same period.
5. Look beyond the single number at drawdown, the longest losing streak and the distribution of returns.
6. Treat unusually high ratios from short histories with caution, especially when they come with promotional material.
7. Recalculate regularly to see whether results change as new data arrives.

Common mistakes and misconceptions

• Treating a high ratio as a guarantee. It describes the past and can fall sharply when conditions change.
• Comparing ratios across different time spans or frequencies without annualising consistently.
• Ignoring the shape of returns. Strategies that sell insurance-like risk can show smooth results and a high Sharpe ratio until a rare large loss occurs.
• Using gross returns. Costs can turn an attractive ratio into a mediocre one.
• Assuming that volatility equals risk. Standard deviation treats upside swings and downside swings the same, although investors mostly dislike the downside.
• Mixing up the Sharpe ratio with the risk-reward ratio, which is a trade planning concept.

Risks and limitations

The Sharpe ratio assumes returns are roughly normally distributed, which real markets frequently violate. Fat tails, sudden gaps and illiquidity can make the standard deviation understate true risk. Smoothed or infrequently priced assets can look artificially stable. The ratio can be gamed by selling options or taking hidden tail risk. It also says nothing about the size of potential loss in a single event. Alternatives such as the Sortino ratio, maximum drawdown and value at risk capture different aspects of risk and are often used alongside it.

How Brokeli looks at it

Brokeli does not rate brokers by trading performance, and we do not publish performance claims for strategies. The Sharpe ratio is useful to us as a way of explaining why costs matter: lower spreads and commissions lift net returns and can improve the ratio for a given strategy. We therefore record fee schedules in the published research snapshot so you can model your own net results.