Portfolio Volatility Calculator

How to estimate portfolio volatility from your asset allocation

Knowing your portfolio's expected return is only half the picture. The other half is risk - specifically, how much your portfolio's value is likely to fluctuate from year to year. This portfolio volatility calculator estimates the annualised standard deviation of a three-asset-class portfolio composed of stocks, bonds, and cash, using your entered allocations and each asset's individual volatility. The result gives you a practical sense of the range of outcomes your portfolio might experience in a given year.

The method used here is a simplified version of the Markowitz portfolio variance formula. For a fully accurate calculation, you would need the correlation matrix between each pair of assets. Since correlations change over time and vary by market environment, this calculator uses a simplifying assumption of zero correlation between asset classes - meaning it calculates the root of the sum of squared weighted volatilities. This approach is conservative in some market conditions and may understate volatility in periods when asset classes move together, such as during broad market stress events. For a first-order estimate of portfolio risk, it is a useful and practical starting point.

The formula is: portfolio volatility = square root of ((wS x volS)^2 + (wB x volB)^2 + (wC x volC)^2), where w represents the weight as a decimal fraction and vol represents the volatility percentage. Each term represents the squared contribution of each asset class to portfolio variance, and the square root converts variance back into the same units as standard deviation (percentage points of return). The result is interpreted as: in approximately two-thirds of years, the portfolio return should fall within the expected return plus or minus this volatility figure.

The 1 standard deviation range on a $100,000 portfolio is then computed by applying that volatility percentage as a gain and a loss to the portfolio base value. If portfolio volatility is 10%, the range would be $90,000 to $110,000 - representing the band within which the portfolio would be expected to fall roughly 68% of the time under a normal distribution. Actual distributions of returns are not perfectly normal and can have fat tails, meaning extreme events happen more often than a standard normal distribution would predict.

What volatility figures to use for each asset class

Volatility inputs matter significantly and should reflect realistic long-run estimates rather than any single short period. Broad global equity indexes have historically shown annualised volatility in the range of 15% to 20%. During calm bull market years, equity volatility can drop below 10%. During financial crises, it can spike above 30%. A planning assumption in the 16% to 18% range is commonly used for a diversified equity allocation in a developed-market portfolio.

Government bond volatility depends heavily on the duration of the bonds held. Short-duration bonds (1 to 3 years) may show volatility below 3%. Intermediate-duration bonds (5 to 10 years) typically show volatility in the 5% to 8% range. Long-duration bonds (20+ years) can show volatility of 10% or more. For a diversified bond fund, a volatility assumption of 5% to 8% is a reasonable middle ground. Cash and cash equivalents - money market funds, short-term deposits, or treasury bills - have volatility very close to zero, typically below 1%.

When you have a real portfolio, you can look up the annualised standard deviation of the specific fund or ETF you hold. Most fund fact sheets and financial data platforms publish this figure as part of the risk profile, often alongside a 1, 3, and 5 year volatility figure. Using actual fund volatility data rather than asset class averages will produce more accurate results for your specific holdings.

How allocation changes affect portfolio volatility

The most direct lever for changing portfolio volatility is the equity allocation. Because stocks typically have the highest volatility and carry the most weight in many portfolios, shifting even 10 percentage points between stocks and bonds can noticeably reduce the overall portfolio volatility figure. This is the basic mechanism behind "de-risking" a portfolio as you approach a financial goal or retirement.

The diminishing returns of diversification are visible in this calculator's output. Adding bonds to a 100% equity portfolio reduces volatility significantly because the low-volatility bond component pulls the weighted total down substantially. Moving from 80% to 60% stocks produces a more modest further reduction. Adding cash as the third asset reduces volatility further still, but the marginal risk reduction from cash is small because cash volatility is near zero.

It is worth noting that in periods of market stress, the zero-correlation assumption tends to break down. Stocks and bonds often fall together during crises, meaning the actual volatility experienced in a bad year can exceed the figure this calculator produces. This is why many financial advisers recommend stress-testing portfolios against historical crisis scenarios - not just statistical estimates - to ensure you can tolerate the worst realistic outcome, not just the average expected range.

This calculator is for educational and illustration purposes only. It uses simplified assumptions and does not account for correlation between asset classes, taxes, fees, or inflation. Actual portfolio risk will differ from estimates. This is not financial advice. Consult a qualified financial adviser for a personalised risk assessment.