When you’re about to buy something big, you look at the price: $1,000, $2,000, $5,000. But that number is only half the story. Spending a sum today means giving up everything that sum could have become over time. Economists call this the opportunity cost: the value of the best alternative you say no to. For money, the simplest alternative is to let it grow. This tool puts a number on what you’re giving up.
How it works
The idea is to compare two paths for the same money: spend it now, or invest it and leave it alone for some years. The second path is just compound growth:
future value = amount × (1 + return)^years
The “compound” part is what makes it interesting: each year the return is earned on the earlier years’ returns too, so the figure accelerates as time passes. The gap between that future value and the price you pay today is the opportunity cost — the growth you forgo by spending instead of investing.
A concrete example
You’re about to spend $2,000 on something non-essential. Suppose you could instead invest it at 7% a year and leave it for 20 years. Here’s what happens:
- those $2,000 would grow to about $7,740;
- of which roughly $5,740 is pure growth the money made on its own;
- in other words, every dollar spent today is worth about $3.87 of your future self.
That doesn’t make the purchase wrong — a trip or an object you use and love can be worth the money. It means the true cost, measured over the long run, is several times the sticker. Seeing it in black and white helps you decide whether it’s worth it.
The part that matters
Opportunity cost isn’t an argument for never spending: it’s a way to choose with the math in front of you instead of by feel. Three honest things to keep in mind:
- The return is an assumption, not a promise. You pick it to reason in orders of magnitude. No investment guarantees a fixed rate and markets can fall — risk and return rise together. For a cautious estimate, lower the percentage.
- Time does almost all the work. Over 5 years the gap is modest; over 20 or 30 it becomes large, because compounding has more time to pile up. The farther out the horizon, the heavier the opportunity cost — which is also why putting off investing is itself expensive.
- It’s in today’s dollars, before tax and inflation. The calculation doesn’t subtract inflation or the tax usually owed on investment gains. Read the totals as an optimistic order of magnitude, useful for comparing choices, not a guaranteed check.
A note on the kind of spending: this is built for a large, one-time purchase. If the spending is small but repeats — a daily coffee, a subscription — it accumulates differently, contribution after contribution. For that there’s a dedicated tool: the cost of a habit.
Change the values above to match your real purchase — the amount, a return you think is realistic, the years you want to look across — and watch the opportunity cost grow. Often the surprise isn’t the price, but what you’re giving up without noticing.