Compound Interest Calculator
See how compound interest grows your money over time.
All calculations happen locally in your browser — nothing is uploaded, stored or tracked.
Enter the initial deposit
What you start with.
Add monthly contributions
Regular amount you add.
Set rate and years
See the balance and interest earned year by year.
What Is Compound Interest?
Compound interest is interest earned on interest: each period's interest is added to the principal, so the next period earns on a larger base. Over long horizons this growth is exponential — the famous "eighth wonder" effect that makes time the most powerful factor in investing.
The calculator models an initial deposit, regular monthly contributions and annual compounding, and produces a year-by-year table showing the balance, contributions and interest earned.
The Formula
P is the initial deposit, r the annual rate, n the compounding frequency, t the years, PMT the regular contribution. The first term grows the lump sum; the second accumulates the contributions.
Your data never leaves your device.
Every calculation on this site runs in your browser using vanilla JavaScript. Nothing is uploaded, stored or tracked.
- 100% local, client-side calculations
- No data stored on any server
- No tracking, no analytics, no ads scripts
- Works offline once loaded
Frequently Asked Questions
How does compound interest work?
Interest is added to your balance each period, and the next period's interest is calculated on the larger balance — growth accelerates over time.
What is the compound interest formula?
A = P(1 + r/n)^(nt) for a lump sum, plus the contribution annuity term when you add money regularly.
Does compounding frequency matter?
Yes, modestly — daily or monthly compounding earns slightly more than annual at the same nominal rate. This calculator compounds annually.
How long does it take to double my money?
The rule of 72: divide 72 by the annual rate. At 7%, money doubles in about 10.3 years.
Is this financial advice?
No — it is a math tool. Returns are not guaranteed; the rate you enter is an assumption.