What a past financial decision has grown into today.

Financial regret has a way of lingering. The car bought on impulse 10 years ago. The stock not bought. The house not purchased when prices were lower. The money lent to a friend and never returned. Looking back, it's easy to quantify the initial cost — but the true cost includes everything that money could have earned in the intervening years.

A 5,000 "mistake" from 15 years ago isn't just 5,000. At 7% compound growth, that's roughly 13,800 today. At 10% (a more aggressive return), it's closer to 20,900. The regret is retroactive — the decision was already made, the money is already spent — but the calculation makes the lesson concrete enough to influence the next decision.

Behavioural research on regret shows it's most useful when channelled into better future decisions rather than ruminated. The tool's purpose isn't to make anyone feel worse; it's to translate abstract "I wish I'd done differently" feelings into specific numbers that inform forward-looking choices.

How to use it

Input the amount of the past decision, how many years ago, and a realistic investment return assumption (7% is common for long-term equity). The tool calculates what that money would be worth today if invested, plus the opportunity cost (the difference from the original amount).

What the result means

The number shows the compounded effect of not having that money working for you. It's illustrative — the calculation assumes the money would have been invested and grown consistently, which is uncertain. But it makes future decisions more vivid: money spent today has a similar multiplier effect 15-20 years from now.

Educational reframing tool. Not a substitute for personalised financial planning.

Quick example

With past amount of 5,000 and years ago of 15 (plus assumed annual return of 7%), the result is 13,795.16. Change any figure and watch the output shift — it's often more useful to see the pattern than to memorise the formula.

Which inputs matter most

You enter Past Amount, Years Ago, and Assumed Annual Return. Not every input has equal weight. Flip one at a time toward extreme values to feel which ones move the needle most for your situation.

What's happening under the hood

Standard compound future value formula. Shows the value today if the past amount had been invested at the given annual rate for the specified number of years. The formula is listed in full below. If the number looks off, you can retrace the calculation by hand — that's the point of showing the working.

Reading the result without judgement

The figure isn't a scorecard. It's a prompt — something to sit with for a few days before deciding whether any habit needs changing. Reflexive reactions ("I need to cut everything") usually don't last; considered ones do.

What this doesn't capture

Behaviour-adjacent math is always an approximation. Human habits are lumpy and context-dependent; the figure here assumes steady behaviour which is a simplification. Treat the output as a prompt for thinking rather than a precise prediction.