Compound Interest Calculator
Project the future value of an investment with compound interest — pick a compounding frequency (annually to continuous), add optional monthly contributions, and see a year-by-year growth table.
Input
Optional — regular monthly deposits added on top of the principal.
Output
| Metric | Value |
|---|---|
| No data yet | |
| Year | Contributions | Interest | Balance |
|---|---|---|---|
| No data yet | |||
Guides
What is compound interest?
Compound interest is interest earned not just on your original principal, but on the interest that's already accumulated. Over long enough time horizons, this "interest on interest" effect can dwarf the original deposit — the core reason long-term investors are told to start early.
How this tool works
- Enter your principal amount — the money you're starting with.
- Enter the annual interest rate as a percentage.
- Set the time period in years.
- Choose a compounding frequency — annually, quarterly, monthly, weekly, daily, or continuous compounding (the theoretical limit as compounding periods approach infinity).
- Optionally add a monthly contribution to model regular deposits on top of the principal.
The Summary shows the future value, total amount contributed, and total interest earned. The Year-by-Year Growth table breaks down contributions, interest, and running balance for every year of the projection — copyable and downloadable as CSV.
Compounding frequency matters
The same rate compounded more often yields a (slightly) higher return, because interest starts earning its own interest sooner. Monthly compounding at 7% annually outperforms annual compounding at the same nominal rate; continuous compounding is the mathematical ceiling on that effect, expressed as A = Pe^(rt).
The contribution formula
With no contributions, future value is the standard compound-interest formula A = P(1 + r/n)^(nt). With recurring monthly contributions, the tool adds the future value of an ordinary annuity on top, converting your monthly deposit into an equivalent per-compounding-period contribution.
Common uses
- Comparing how different compounding frequencies affect long-term growth
- Modeling a savings or investment account with regular monthly deposits
- Estimating how long it takes a lump sum to double or triple at a given rate
- Teaching the mechanics of compound interest
Privacy
This tool runs entirely in your browser. Your numbers are never uploaded to a server.