Subscription to Investment Calculator

15 a month cancelled and invested at 7% for 30 years grows to roughly 18,300. That single 15 decision compounded across a career is meaningful money. Most households have two or three unused subscriptions — the combined FV tends to be surprising once calculated.

How to use it

Enter the monthly subscription cost, an expected annual return, and your horizon. The tool projects the future value assuming the monthly amount is invested each month instead.

Use it for auditing

Run the tool for each questionable subscription. Anything over 10/month that you don't actively use compounds into a surprising figure. The decision isn't about the 10 — it's about the FV that 10 quietly represents.

A worked example

Try the defaults: monthly subscription of 15, annual return of 7%, years of 30 years. The tool returns 18,299.56. 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 Monthly Subscription, Annual Return, and Years. The rate and the time horizon usually dominate — compounding means a small change in either reshapes the final figure more than a similar shift in contribution size. Test this by doubling one input at a time.

The formula behind this

Ordinary annuity future value with monthly compounding. Assumes the monthly amount is invested at the end of each month — standard approximation for regular investing. Everything the calculator does is shown in the formula box below, so you can check the math against your own spreadsheet if you want.

Why the behavioural angle matters

Most personal finance mistakes are behavioural, not mathematical. You know the math; the hard part is acting on it consistently. Calculators like this one are useful because they externalise a private feeling into a public number — and public numbers are easier to argue with than vague feelings.

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. The output is a prompt for thinking rather than a precise prediction.