See how money grows over time. Enter a starting balance, a monthly contribution, an interest rate, and the number of years — and watch compounding do the work.
Estimate only — assumes a steady rate compounded monthly and on-time contributions. Real returns vary and aren't guaranteed; this is general information, not investment advice.
Compound interest means you earn returns on your returns, not just your original money. The longer the runway, the more the later years do the heavy lifting — which is why starting early beats saving more later.
Future value = Start × (1 + r)ⁿ + Contribution × ((1 + r)ⁿ − 1) ÷ r
…where r is the monthly rate and n the number of months. This calculator does it for you.
Compound interest is the reason a modest amount saved consistently can grow into a large sum: you earn returns not only on the money you put in, but on the returns that money has already generated. Each period, your interest is added to the balance, and the next period's interest is calculated on that bigger balance. Over years, this snowball effect does far more work than the contributions themselves.
For a single lump sum, future value is A = P(1 + r/n)nt, where P is the starting principal, r is the annual rate (as a decimal), n is how many times a year interest compounds, and t is the number of years. When you also add money every month, each contribution compounds for the time remaining — the calculator sums the growth of your starting balance and of every monthly deposit for you.
Start with $1,000, add $100 a month, and assume a 7% annual return for 10 years. Your starting $1,000 grows to about $1,970. Your $100-a-month deposits — $12,000 contributed in total — grow to roughly $17,300. Altogether that is about $19,300, of which you contributed $13,000 and roughly $6,300 is growth. Leave it another 10 years and the growth portion grows much faster than the contributions, because compounding accelerates over time.
Divide 72 by your annual return to estimate how many years it takes money to double. At 7%, that is about 10 years; at 9%, about 8 years. It is an approximation, but a useful one for sanity-checking a plan without a calculator.
Use this to set a monthly savings target for a goal, to compare "start now" against "start in five years" (the earlier start almost always wins), or to see how sensitive your plan is to the return rate. Change one input at a time to build intuition for which lever matters most — usually it is time.
Your balance grows by the periodic rate each period, and each period's growth is added to the base for the next one. With regular contributions, each deposit also compounds from the moment it's added. This calculator combines both.
Use a realistic expected return for where the money sits — a savings account rate for cash, or a long-run market average (often quoted near 7% real) for diversified investing. Lower is safer to plan around.
Related: monthly investing growth benchmark (what $X/month becomes) · savings goal calculator · debt payoff calculator · home & money tools.
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Yes. The Compound Interest Calculator is completely free, runs in your browser, and never asks for a signup, email, or download. Enter your numbers and the result updates instantly.
It uses standard formulas and recalculates the moment you change a value, so the math is exact for the inputs you enter. Treat the result as a reliable planning estimate; for decisions with legal or tax consequences, confirm the figures with a qualified professional.
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Last reviewed: September 2026. Free to use, no signup — Compound Interest Calculator by ToolWise Digital.
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