Opportunity Cost Calculator (what spending now gives up in investment growth)

Understanding the opportunity cost calculator — concept and principle

Opportunity cost is the value of the best alternative you give up. When you spend money instead of investing it, the cost is not just the price but also the growth that money could have earned. This calculator puts a number on that for a one-off purchase, a recurring monthly expense or both.

Investing the $48,000 instead would have produced $30,096 of growth, worth $23,511 in today’s money.

How it is calculated

A one-off amount A could have grown to A × (1 + r)^t.
A monthly amount M could have grown to M × [(1 + i)^n − 1] ÷ i, with i = (1 + r)^(1/12) − 1 and n months.
Value it could have grown to = the two parts added.
Opportunity cost = that value − total spent.
In today's money = opportunity cost ÷ (1 + inflation)^years.

The principle

Every choice to spend is also a choice not to invest. Economists measure the true cost of a decision by what you forgo. With compounding, the gap between spending and investing widens over time, because returns earn further returns. Showing the result in today's money avoids overstating it: part of the future value is simply inflation.

Worked example

Default inputs: one-off 30,000 USD, 150 USD a month, alternative return 6% a year, 10 years, inflation 2.5%.

One-off: 30,000 × 1.06^10 ≈ 53,725 USD.
Monthly: i = 1.06^(1/12) − 1; 150 × [(1 + i)^120 − 1] ÷ i ≈ 24,371 USD.
Total value it could have grown to = 78,096 USD.
Total spent = 30,000 + 150 × 120 = 48,000 USD; opportunity cost = 30,096 USD.
In today's money: 30,096 ÷ 1.025^10 ≈ 23,511 USD.

Each point is the calculator re-run with only “Years” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Years	Value the money could have grown to
2.5	$39,537
5	$50,570
7.5	$63,332
10	$78,096
12.5	$95,176
15	$114,934
20	$164,230

How to read the result

The number is not a verdict. A car, course or holiday may be worth more to you than the growth you give up. Use the figure to make the trade-off visible, especially for recurring costs like subscriptions, which add up over years. Try a cautious return rate to avoid exaggerating the cost.

Common mistakes

Assuming the alternative return is guaranteed.
Ignoring the benefit of what you buy, which can include savings (a reliable car may cut repair costs).
Forgetting taxes on investment returns.
Comparing a nominal future value with today's price without adjusting for inflation.

Keep in mind: Assumes a steady return and ignores taxes; real investments fluctuate. This is a way to think about choices, not financial advice.

Frequently asked questions

What is opportunity cost in simple terms?

What you give up by choosing one option over the next best one.

Is opportunity cost a real cost?

It is not a payment, but it is an economic cost that matters for decisions.

What return should I use?

A realistic return you could actually earn, such as a savings rate or a cautious long-run investment return.

Sources: Mankiw, N. G., Principles of Economics (opportunity cost) · US SEC Investor.gov, Compound Interest Calculator

Checked 9 Nov 2026. Opportunity cost as the value of the best forgone alternative; future value of a lump sum and of monthly payments (ordinary annuity): standard economics and time-value-of-money relations.

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