Most tools that show you a Sharpe ratio use a hardcoded risk-free rate. Corvo does not. Here is exactly how the calculation works and why it matters.
The Formula
The Sharpe ratio is: (Portfolio Return - Risk-Free Rate) / Portfolio Volatility.
Each piece of this matters. The portfolio return is your annualized return over the selected time period, calculated from the daily price history of each holding weighted by your allocation. The denominator is your annualized volatility, measured as the standard deviation of daily returns scaled to an annual figure using the square root of 252 trading days. The risk-free rate is where most tools cut corners.
Why the Risk-Free Rate Matters
The risk-free rate is the return you could earn without taking any risk. In practice, this is approximated by short-term U.S. Treasury yields, specifically the 3-month T-bill.
In a near-zero interest rate environment, the risk-free rate approaches 0%, which makes almost any positive return look good on a risk-adjusted basis. With short-term rates above 4%, the calculation changes substantially. A portfolio returning 8% per year with a risk-free rate of 4.5% has an excess return of only 3.5%. The same portfolio in a 0% rate environment had an excess return of 8%. The Sharpe ratios are not comparable without accounting for this.
Using a hardcoded rate of 2% or 3% produces a number that looks authoritative but is systematically wrong whenever rates have moved. Most retail tools use a fixed rate because it is simpler to implement. The result is Sharpe ratios that are inflated during high-rate environments and deflated during low-rate environments.
How Corvo Pulls Live Rate Data
Corvo pulls the current 3-month T-bill yield using the ^IRX ticker via yfinance. This gives a real-time approximation of the risk-free rate that reflects actual market conditions at the time of your analysis. The ^IRX ticker tracks the annualized yield of 3-month U.S. Treasury bills, which is the standard proxy for the risk-free rate in academic and institutional finance.
The rate is fetched as part of each analysis run. If the live rate is unavailable due to a data outage, Corvo falls back to a conservative estimate based on recent historical values. In normal conditions, your Sharpe ratio reflects the actual current cost of holding risk assets instead of parking cash in risk-free instruments.
Good vs Bad Sharpe Ratios
Broadly accepted thresholds for a correctly calculated Sharpe ratio:
- Below 0.5 - Poor. You are taking on meaningful volatility without adequate compensation in returns.
- 0.5 to 1.0 - Adequate. Returns are somewhat justified by the risk taken.
- 1.0 to 2.0 - Good. Solid risk-adjusted performance, better than most diversified indexes.
- Above 2.0 - Excellent. Rare in practice for diversified portfolios over multi-year periods.
These thresholds assume a correctly calculated risk-free rate. A Sharpe ratio calculated with a hardcoded 0% risk-free rate will always appear better than one calculated against the current 4%+ T-bill yield. If you are comparing Corvo's Sharpe number against another tool's, verify which rate they are using before drawing conclusions.
Comparing Two Portfolios
The Sharpe ratio is most useful when comparing two portfolios with different return and volatility profiles. Portfolio A returning 22% with a Sharpe ratio of 0.8 is not necessarily better than Portfolio B returning 14% with a Sharpe ratio of 1.4. The second portfolio generated better returns per unit of risk, which means it is more efficient at converting volatility into returns.
This is what Corvo's benchmark comparison is designed to show: not just whether you beat the index, but whether you did so at a reasonable cost in volatility. Beating the S&P 500 by 3% while taking on twice the volatility is not a win. Matching the S&P 500 with half the volatility is.