Compound interest calculators answer "what will this grow into". People saving for something specific are asking the reverse: the amount is fixed and the unknown is the monthly payment, or the number of months. This page solves the same formula from the other end.

How to use it

1. 1 Pick which side you know: the deadline, or the monthly amount.
2. 2 Enter the target and anything you have set aside already.
3. 3 Add an interest rate only if the money will actually earn one.

The two questions are the same equation

Target, starting balance, monthly payment, months and rate are five quantities in one relationship; know four and the fifth follows. Switching modes here does not change the maths, only which one is left blank.

A monthly rate is not the annual one divided by twelve

Interest earned in one month earns interest in the next, so the twelve monthly steps have to multiply back to the yearly figure rather than add to it. Dividing by twelve overstates growth slightly every month, and over years the drift becomes visible. This page compounds properly.

Interest matters less than you expect over short horizons

Over two years at a typical savings rate, interest contributes a small fraction of the total; almost all of it is money you put in. Compounding needs time before it does the heavy lifting, so for a goal a year or two away the deposit is the lever that actually moves.

Inflation moves the target while you save

A price quoted today will not be the price in three years, so a long goal is worth setting a little above the current figure. This calculator works in today's money and does not guess at future prices - that judgement is yours, and it belongs in the target you type.