Watch money grow on itself: enter a starting balance, a monthly contribution, an annual return, and the years, and get the final balance, the total you put in, and the part that came from interest alone — plus a year-by-year growth table. Updated live as you type.
Show growth by year
| Year | Invested | Interest | Balance |
|---|
How it works
Interest is credited monthly, and each month’s contribution lands right after it — the ordinary-annuity convention most savings accounts and retirement calculators follow. The engine runs a month-by-month simulation rather than one big formula, so what you see in the year table is literally the account’s history: the same dollars compounding on top of each other, every month, for the whole term.
Everyday examples
The famous one: $100 a month at a 10% average return for 40 years becomes about $632,000 — of which you contributed $48,000. That’s compound interest’s whole pitch: the last decade earns more than the first three combined. Slower view: $100,000 parked at 7% for 30 years grows to about $811,650 without you touching it. Debt is this in reverse: the loan payoff calculator runs the identical math with interest working against you.
A note on rates: use a long-term average, not a best year — US stock market history averages near 10% nominal, bonds less, savings accounts less still. Built in-house in plain JavaScript — no widgets, no tracking, nothing you type leaves your device.