Compound Interest Calculator
Calculate compound interest and investment growth with monthly contributions. Free, accurate, and it runs entirely in your browser.
- ✓ Runs in your browser — nothing uploaded
- ✓ No sign-up
- ✓ Free, no limits
This compound interest calculator shows how a lump sum plus optional monthly contributions grows over time, so you can see the real effect of compounding on savings and investments. Enter your starting amount, an annual interest rate, how long you'll invest, and any monthly deposit — the future value, total contributions and total interest earned update instantly.
Compounding is interest earning interest. This tool compounds monthly using the standard formula: the future value of the principal is P × (1 + i)^N, and regular deposits add PMT × [((1 + i)^N − 1) ÷ i], where i is the monthly rate and N is the number of months. Worked example: $1,000 at 12% for one year with no deposits compounds monthly to $1,126.83 — noticeably more than simple interest's $1,120, because each month's interest itself earns interest.
Use it to compare savings scenarios, plan a retirement contribution, or sanity-check a bank's projection. Everything is computed locally in your browser, so your figures never leave your device and you get the same deterministic result every time. This is an estimate for planning, not financial advice — real accounts vary in how they compound and tax returns.
Frequently asked questions
How is compound interest calculated?
The principal grows by P × (1 + i)^N, where i is the periodic interest rate and N is the number of periods. This calculator compounds monthly and adds any recurring monthly contribution on top.
What's the difference from simple interest?
Simple interest is paid only on your original principal. Compound interest is paid on the principal plus previously earned interest, so it grows faster over time.
Are my numbers sent to a server?
No. The calculation runs entirely in your browser — nothing you enter is uploaded or stored anywhere.
Is this financial advice?
No. It's an estimate for planning. Real accounts differ in compounding frequency, fees and tax treatment, so treat the result as a guide, not a guarantee.