The Sharpe ratio summarises average excess return relative to the variability of those returns. It is a historical performance measure, not a test that distinguishes skill from luck on its own.

Use Consistent Periods

For each equally spaced period, calculate the strategy's return and subtract a matching benchmark return. Divide the average of those differences by their standard deviation. William Sharpe's explanation distinguishes historical measurements from expectations and explains why the period and comparison benchmark matter.

For example, suppose a set of hypothetical monthly excess returns has a mean of 0.5% and a standard deviation of 2%. The monthly Sharpe ratio is 0.5 / 2 = 0.25. This is an illustrative calculation, not an investment result.

Do not mix an annual return in the numerator with daily volatility in the denominator. Do not omit the benchmark simply because its rate seems small. Include trading costs consistently and explain how deposits and withdrawals are excluded from the return calculation.

Annualisation Has Assumptions

Multiplying a monthly figure by the square root of 12 gives an annualised estimate only under assumptions about how returns scale over time. Serial correlation can invalidate that shortcut. Overlapping positions, infrequent valuations and smoothed prices can make a strategy look less variable than it is.

Report the observation frequency, sample dates, benchmark and annualisation convention beside the ratio. A daily figure and an annualised figure are not directly comparable.

Why There Is No Universal Passing Score

A high ratio in a short selected period may disappear in another period. A ratio based on stale marks or omitted costs may look strong for the wrong reason. A strategy exposed to rare large losses may have a smooth record until one occurs.

Before comparing two strategies, align the measurement period and cost treatment. Then examine drawdowns, leverage, liquidity and the distribution of returns. Sharpe uses both positive and negative variability; it does not directly measure the largest possible loss.

You can calculate a ratio without normally distributed returns. The limitation is that mean and standard deviation may describe skewed or heavy-tailed outcomes poorly; normality is not a prerequisite for performing the arithmetic.

Applying It to an Evaluation Account

A nominal evaluation balance is not necessarily capital you own or the amount you can lose before a breach. Dividing P&L by that balance can create returns that are difficult to compare with a personal brokerage account.

Document the denominator, whether results are simulated, and the programme's actual loss budget. A Sharpe ratio does not establish compliance with daily loss, trailing loss or payout conditions. Do not turn an attractive ratio into a claimed probability of passing.

Frequently Asked Questions

Should I increase size when Sharpe improves? A ratio alone is insufficient. Check whether the change comes from a short sample, changed exposure or a few unusual observations, and keep position decisions within the existing risk plan.

What if return variability is zero? The division is undefined. This can occur in a very short record or a series with unchanged marks; it does not mean the strategy has risk-free profitability.

What else should I track? Use net expectancy, equity drawdown, exposure and the largest losses. Each answers a different question; none replaces the underlying trade record.

Risk Disclaimer: Trading involves substantial risk of loss. Past performance is not indicative of future results. See our full risk disclaimer.