Opportunity Cost Calculator
Cost of idle money.
Calculate opportunity cost of money sitting idle versus invested. Enter amount sitting idle to see opportunity cost: foregone returns from not investing money.
What this tool does
Opportunity cost on idle cash is the return foregone by not investing it. This calculator shows what that forgone return amounts to in real terms. Enter the amount sitting idle, the annual return rate available through an alternative investment, and your time horizon in years. The tool then estimates the total return you would not receive by keeping the money uninvested over that period. The result represents the cumulative value of returns missed during your holding period. The alternative return rate and time horizon are the primary drivers—higher rates or longer periods increase the foregone amount significantly. For example, someone comparing cash holdings against a bond or equity return over five years would see how much growth the idle cash position costs in that scenario. The calculation assumes consistent annual returns and does not account for taxes, inflation, or changes in the alternative investment's performance. It serves for educational illustration of how time and return rates interact.
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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.
Opportunity cost calculator quantifies the value of money sitting idle vs being invested. 10,000 cash earning 0% vs invested at 7% over 10 years: foregone returns = 9,672. The 'cost' of not investing nearly equals the original principal. Inflation-adjusted (2-3% loss to inflation), idle cash actively destroys value.
Example: 20,000 in checking account 'just in case' for 20 years. At 7% alternative return: future value would be 77,394. Opportunity cost = 57,394 - nearly 3x the principal lost to inaction. Even safer alternatives (5% bonds) cost 33,066. Cash beyond emergency fund is one of the most expensive 'safe' choices in personal finance.
Opportunity cost applies beyond cash: choosing one investment means foregoing returns on alternatives. Buying property = giving up stock market returns on the down payment. Paying off low-rate mortgage = giving up index fund returns. Always frame major financial decisions through opportunity cost lens. The question isn't 'is X a good return?' but 'is X better than my next best alternative?'
A worked example
Try the defaults: amount sitting idle of 20,000, alternative annual return of 7%, years of 20 years. The tool returns 57,393.69. You can adjust any input and the result updates as you type — no submit button, no reload. That's the real power here: seeing how sensitive the output is to one or two assumptions.
What moves the number most
The result responds to Amount Sitting Idle, Alternative Annual Return %, and Years.
The formula behind this
Future value of amount at alternative return rate over years, minus original amount. Everything the calculator does is shown in the formula box below, so you can check the math against your own spreadsheet if you want.
Using this well
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.
££20,000 idle vs invested at 7% for 20y costs 57,393.69.
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
This calculator models the opportunity cost of holding idle money by computing how much purchasing power or investment growth is foregone. It applies compound interest mathematics: the calculator raises one plus the annual return rate to the power of the number of years, multiplies by the initial amount, then subtracts the original amount. This yields the net gain that could have been realised under the alternative investment scenario. The model assumes a constant annual return rate throughout the period, treats growth as smooth and uninterrupted, and does not account for fees, taxes, inflation adjustment, or volatility. Results represent a simplified projection based on stated assumptions and should not be interpreted as a prediction of actual outcomes.
References
Frequently Asked Questions
How much cash is too much?
Opportunity cost of paying off debt?
Why people ignore opportunity cost?
Inflation impact?
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