Starting from the answer
A compound interest calculator asks what a sum becomes. This one starts from the number you want and works backwards: how many years at your current pace, or how much a month to arrive on schedule.
Adding years is the cheapest lever
Three things move a goal closer — paying in more, waiting longer, or earning more. The return is the one you cannot simply decide on, and raising it means taking on more risk. Try the numbers both ways and you will usually find that two or three extra years does as much as a large increase in the monthly amount.
The assumption drives the answer
This assumes the return you type in arrives every single year, unchanged. Real markets do not, and neither tax nor inflation is in here. Run it once with a deliberately pessimistic return, and treat the output as a way to compare assumptions rather than a plan.
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Frequently asked questions
Q. Why does a small change in the return move the answer so much?
Because the growth is exponential — the rate sits in the exponent, so a difference of one percentage point compounds year after year. Over long horizons that is precisely why the assumed return deserves more scrutiny than the monthly amount.
Q. What if the target is below what I already have?
Then there is nothing to calculate; you are there. The tool needs a target above the starting amount to have a gap to close.
Q. Is the monthly figure paid at the start or the end of the month?
At the end of each month, the ordinary-annuity convention. Paying at the start of the month earns one extra month of growth on every instalment and so needs slightly less.