Cost of idle money.

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. Frequency and unit price pull the total in different directions. The biggest surprise for most people is how small recurring amounts compound into large annual figures — that's where this calculation earns its keep.

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

Treat the output as one point on a wider map. Run it three times — a pessimistic case, a central case, and a stretch case — and plan against the pessimistic one. That habit alone separates people who stick with an investment plan from those who bail at the first wobble.

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. Treat the number as one scenario, not a forecast.