Volatility Impact Calculator
Cost of volatility on long-term returns.
Calculate the drag of volatility on long-term returns using the arithmetic-minus-geometric return gap — why volatile portfolios end lower than averages suggest.
What this tool does
Volatility creates a cash drag through the gap between arithmetic and geometric returns — same average, lower compound. This calculator shows that cost by comparing a smooth, steady return path against one with realistic ups and downs. Enter your starting amount, expected annual return, volatility measured as standard deviation, and time horizon. The tool calculates the cumulative shortfall in your currency between what you'd have with perfectly smooth returns versus what fluctuating returns actually deliver. The result illustrates how volatility alone — independent of whether your average return is higher or lower — reduces compound growth over time. Principal size and time horizon are the primary drivers of the total cash impact. The calculation uses a geometric return approximation and is valid for small-to-moderate volatility ranges. Actual outcomes may vary depending on the sequence and timing of returns.
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Formula Used
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Disclaimer
Results are estimates for educational purposes only. They do not constitute financial advice. Consult a qualified professional before making financial decisions.
Two portfolios with the same average return can end at very different places. A portfolio averaging 10% with 5% volatility lands differently than one averaging 10% with 20% volatility. The geometric return — what actually compounds — drops as volatility rises. 100,000 at 10% average return over 30 years: arithmetic FV is 1,745,000; with 20% volatility, realistic geometric FV is 1,006,000 — a 739,000 difference despite the same headline average. Volatility is a real cost.
Quick example
With principal of 100,000 and average annual return of 10% (plus volatility of 20% and years of 30), the result is 738,674.54. Change any figure and watch the output shift — it's often more useful to see the pattern than to memorise the formula.
Which inputs matter most
You enter Principal, Average Annual Return, Volatility (std dev), and Years. Not every input has equal weight. Adjusting one input at a time toward extreme values shows which ones move the result most.
What's happening under the hood
Geometric return approximation: arithmetic mean minus half the variance. Valid for small-to-moderate volatility; actual geometric return simulated from return distributions can differ slightly. The formula is listed in full below. If the number looks off, you can retrace the calculation by hand — that's the point of showing the working.
Where this fits in planning
This is a "what-if" tool, not a forecast. Use it to test ideas before committing: what happens if the rate is 2% lower than hoped, what happens if you add five more years. The value is in the scenarios you run, not the single answer you get from the defaults.
What this doesn't capture
Steady-rate math ignores real-world volatility. Actual returns are lumpy; sequence-of-returns risk matters most in drawdown; fees and taxes drag on compound growth; and behaviour changes in drawdowns can reduce outcomes below the projection. The number represents one scenario rather than a forecast.
With £100,000 invested at 10 annual return over 30 years, volatility of 20 reduces your expected outcome to 738,674.54.
Inputs
This example uses typical values for illustration. Adjust the inputs above to match a specific situation and see how the result changes.
Sources & Methodology
Methodology
Geometric return approximation: arithmetic mean minus half the variance. Valid for small-to-moderate volatility; actual geometric return simulated from return distributions can differ slightly.
References
Frequently Asked Questions
Why does volatility reduce compound returns?
Typical equity volatility?
Can I avoid volatility drag?
Is this the same as sequence risk?
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