Compound Interest Calculator
See how a lump sum and regular deposits grow over time. Enter your starting balance, interest rate, how often it compounds, and any monthly contribution.
Compound interest is interest earned on your interest as well as your original money, which is why savings and investments can snowball over long periods. The two biggest levers are time and rate: the longer money compounds and the higher the rate, the more dramatic the growth. Adding regular contributions accelerates it further. This calculator combines a starting balance, a compounding rate and optional monthly deposits to project a future value.
How compound interest works
Each period, interest is added to the balance, and the next period's interest is calculated on that larger balance. The more often it compounds, the slightly higher the effective return.
Contributions: FV = PMT × [((1 + r/n)^(n·t) − 1) ÷ (r/n)]
where r = rate, n = times compounded per year, t = years
| $10,000 at 5% | Value |
|---|---|
| After 10 years | $16,470 |
| After 20 years | $27,126 |
| After 30 years | $44,677 |
Worked example
$10,000 to start, $100 a month, 5% compounded monthly for 10 years:
| Starting balance grows to | $16,470 |
| Contributions grow to | $15,528 |
| Total future value | ≈ $31,998 |
Why starting early matters
Because growth compounds, money invested earlier does far more work than the same amount invested later. A dollar saved in your twenties can multiply many times over by retirement, while the same dollar saved a decade later has far less time to grow. This is why financial planners stress starting early and contributing consistently, even in small amounts — time in the market is the ingredient you can't get back.