Compound Interest
Calculate compound interest with optional monthly contributions and a year-by-year breakdown.
About Compound Interest
Compound Interest projects how a balance grows over time, with optional recurring monthly contributions and a year-by-year breakdown of principal versus earned interest. Use it to model a savings goal, compare what a deposit becomes at different rates, or see the real effect of adding a little each month. Everything is computed in your browser, so your figures stay private.
- Category
- finance
- Input
- No file input — params only.
- Output
- Outputs: application/json.
- Cost
- Free, runs in your browser
- Memory
- low
Common uses
- Project what a $10,000 deposit becomes after 10 years at 5% with monthly compounding
- See how adding $200 a month changes the final balance of a retirement account
- Compare two savings accounts side by side by running the same starting amount at each APR
- Build a year-by-year table showing how much of your growth comes from contributions versus interest
- Estimate how long it takes to reach a target balance by adjusting the rate and contribution
- Sanity-check a bank's quoted return before opening a fixed-rate savings product
Frequently asked questions
Does it support monthly contributions or only a single deposit?
Both. You can model a one-time principal, ongoing monthly contributions, or a combination of the two.
What compounding frequency does it use?
It compounds on the schedule you set; the year-by-year breakdown shows the running balance so you can verify the curve at each step.
Is APR the same as the rate I enter?
You enter the annual rate. Because the result compounds, the effective annual yield can be slightly higher than the nominal APR you typed, especially at higher compounding frequencies.
Does anything I enter get sent to a server?
No. The projection is calculated locally in your browser. No amounts, rates, or goals leave your device.
What does it output?
JSON containing the final balance, total contributions, total interest earned, and a per-year breakdown you can copy into a spreadsheet.
Keywords
- compound
- interest
- investment
- savings
- finance
- calculator
- APR