Compound interest
Growth earning growth, with the deposits separated from the dividends of patience.
Future value
$196,665
Why time does more than deposits
FV = P(1+r)ⁿ + PMT × ((1+r)ⁿ − 1) ÷ r r = APR ÷ 12 The receipt splits the result into what you handed over and what growth added — because that ratio is the whole story of compounding. Stretch the years input and watch growth's share pass deposits' share; that crossover arrives far earlier than intuition expects.
Assumptions on the counter
- Constant rate across the term; markets do not behave this smoothly.
- Contributions land at month-end; no taxes or fees deducted.
- Negative rates allowed to model real (inflation-adjusted) returns.
Questions people ask
What is the compound interest formula?
For a starting balance P growing at monthly rate r for n months: FV = P(1+r)ⁿ. Monthly contributions add an annuity on top: PMT × ((1+r)ⁿ − 1) ÷ r. This calculator runs both together.
How much will $10,000 be worth in 20 years?
Left untouched at 7% compounded monthly, roughly $40,400. Add $300 a month and the same horizon reaches about $236,000 — of which only $82,000 came from deposits.
Is compounding monthly different from annually?
Yes, slightly in your favor: money earns interest earlier. A 7% APR compounded monthly behaves like about 7.23% over a year.