Working backward from a number
Most growth calculators start with a deposit and show where it ends up. This one starts with the goal. Enter a target, a timeline and an assumed rate, and it solves for the monthly deposit that gets there. For $1,000,000 in 40 years at a hypothetical 7% compounded monthly, starting from zero, that's about $380.98 a month.
The formula
First, subtract what your starting amount will grow to on its own: need = target − start × (1 + i)^n, where i is the monthly rate and n the number of months. Then solve the monthly-deposit formula for the payment: payment = need × i ÷ ((1 + i)^n − 1). If the starting amount alone reaches the target, the page shows $0.
The rate is the assumption
Everything depends on the rate. At 12% compounded monthly over 40 years, reaching about $1.18 million takes just $100 a month. At 7%, it takes almost four times as much for less money. Neither rate is a promise; real returns vary and can be negative for years at a time. Try a few rates to see how sensitive the answer is.
What a future million is worth
A million dollars 40 years from now won't stretch as far as a million does today. At 3% inflation, it's worth about $307,000 in today's dollars. The page shows that figure as an illustration, so the target can be read in real terms. The inflation page explains the math.
Checking a monthly amount
Already have a monthly amount in mind? Enter it in the "check a monthly amount" field and the page shows how many months it would take to reach the target at the same rate.
Not a plan
This is arithmetic under steady assumptions, not a financial plan or advice. Fees, taxes and changing returns are left out.
Sources
- SEC Investor.gov: Compound Interest Calculator
- U.S. Bureau of Labor Statistics: CPI Inflation Calculator
Last reviewed: Sep 28, 2026