What Is Compound Interest?
The Compound Interest Calculator shows how an investment grows over time when interest is earned on both the original amount and the interest already accumulated. Enter an initial amount, an optional monthly contribution, the interest rate (monthly or yearly) and the period, and the tool instantly projects the final balance, the total you invested and the interest earned, plus a year-by-year breakdown. It is perfect for planning savings, retirement or any long-term goal, and it runs entirely in your browser so your figures stay private.
Compound interest is interest earned not only on your original deposit but also on the interest that has already accumulated. Each period, the gains are added to your balance and start earning their own returns, so the money grows faster and faster over time. Albert Einstein is often quoted as calling it the most powerful force in finance — and over decades the compounding effect can dwarf the amount you actually put in.
Calculating Interest
- Enter the initial amount and an optional monthly contribution.
- Set the interest rate (per month or per year) and the period.
- Read the final amount, total invested and interest, plus the yearly breakdown.
Common Scenarios
- Long-term investors projecting how a portfolio or retirement fund could grow over the years.
- Savers comparing accounts to see how regular monthly contributions accelerate growth.
- Students and educators illustrating the power of starting to invest early.
- Anyone setting a financial goal and wanting a realistic estimate of future value.
The Formula Behind Compound Growth
For a lump sum, the future value is A = P × (1 + r/n)^(n·t), where P is the principal, r the annual rate, n the number of times interest compounds per year, and t the number of years. When you add regular contributions, each one compounds for the time remaining until the end, and the calculator sums every deposit plus its growth to give the final balance.
The key insight is the exponent: because growth is exponential, time matters more than the amount. Investing a smaller sum for longer often beats a larger sum invested late, which is why starting early is so valuable. More frequent compounding (monthly rather than yearly) also nudges the result higher.
Advantages
- Plan savings, investments and retirement goals with realistic projections.
- See the impact of regular monthly contributions and of time on compounding.
- Runs entirely in your browser — your financial figures are never uploaded.
Financial Calculations Stay With You
Compound interest is calculated using financial formulas implemented in JavaScript running in your browser. Your principal, rate, and time period are processed entirely locally — your financial data never reaches our servers.