Opportunity cost of a one-off purchase — what that money could have grown to.

A 2,000 purchase foregone and invested at 7% for 20 years compounds to roughly 7,740 — nearly 4× the original. That's not a reason to never spend; it's a reason to spend intentionally. Routine purchases go under the radar; the tool surfaces the real long-run cost.

What the result means

Primary is the future value foregone. Secondary shows the compound growth multiple and the difference between future value and original spend. The bigger the rate and longer the horizon, the more dramatic the gap.

How to use this honestly

Not every spend needs to be questioned — this is behavioural finance, not a rule. Use it for recurring or borderline purchases: the third streaming subscription, the annual upgrade, the 'maybe I don't need this but it's only £X' item. The tool makes the long-run cost visible so the decision is deliberate.

Run it with sensible defaults

Using purchase amount of 2,000, annual return of 7%, years of 20 years, the calculation works out to 7,739.37. Nudge the inputs toward your own situation and the output recalculates instantly. The defaults are meant as a starting point, not a recommendation.

The levers in this calculation

The inputs — Purchase Amount, Annual Return, and Years — do not pull with equal force. Two inputs usually tip the answer one way or the other. Identify which ones matter most by flipping each value past a round threshold and watching whether the winning option changes.

How the math works

Standard compound growth formula. The foregone future value is the purchase amount grown at the return rate for the stated horizon. Does not model inflation — for real-purchasing-power comparison, use a real return rate (nominal minus inflation). The working is transparent — you can verify every step yourself in the formula section below. No black box, no opaque "proprietary model".

How to use this beyond the first run

Re-run the calculation once a year. Life changes — pay rises, new expenses, interest-rate shifts — and the figure that looked right 12 months ago often isn't today. Annual recalibration keeps the plan honest.

What this doesn't capture

The calculation assumes a steady savings rate and a stable interest rate. Real saving journeys include emergencies, windfalls, and rate changes — especially in easy-access products. The figure is a direction of travel, not a guarantee.