Compound Interest Calculator

1. Enter your initial investment amount (the starting principal).
2. Enter a monthly contribution amount (or leave at 0 for lump-sum only).
3. Set the annual interest rate as a percentage.
4. Choose the number of years for the investment period.
5. Select the compounding frequency (monthly, quarterly, or annually).
6. View the final balance, total contributions, interest earned, and year-by-year breakdown.

What Is Compound Interest?

Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest, which only earns returns on your original deposit, compound interest allows your money to grow exponentially because you earn interest on your interest. Albert Einstein reportedly called it the "eighth wonder of the world." Whether you are saving for retirement, building an emergency fund, or growing a college fund, understanding compound interest is essential to making informed financial decisions and setting realistic long-term goals.

How Compound Interest Is Calculated

This calculator uses the standard compound interest formula for the initial principal: A = P(1 + r/n)^(nt), where P is the principal, r is the annual interest rate (as a decimal), n is the number of compounding periods per year, and t is the number of years. For regular monthly contributions, the future value of an annuity formula is applied: FV = PMT x [((1 + r/n)^(nt) - 1) / (r/n)]. Together, these formulas calculate how your initial investment and ongoing deposits grow over time, accounting for interest that compounds at your chosen frequency (monthly, quarterly, or annually).

Common Use Cases

• Projecting the growth of a savings account or certificate of deposit (CD)
• Estimating how much a monthly investment will be worth in 10, 20, or 30 years
• Comparing the impact of different compounding frequencies on returns
• Planning for major financial goals like retirement, college tuition, or a home down payment
• Understanding how the Rule of 72 estimates the time needed to double your money
• Evaluating investment opportunities with different interest rates

Simple Interest vs. Compound Interest

Simple interest is calculated only on the original principal. If you invest $10,000 at 6% simple interest for 20 years, you earn $600 per year, totaling $12,000 in interest and a final balance of $22,000. Compound interest, on the other hand, reinvests the interest earned so that future interest is calculated on a growing balance. The same $10,000 at 6% compounded annually for 20 years grows to approximately $32,071, yielding $22,071 in interest. With monthly compounding, the result climbs even higher to about $33,102.

The difference becomes even more dramatic over longer time periods and with regular contributions. Adding just $200 per month to that same $10,000 investment at 6% compounded monthly over 20 years produces a final balance of roughly $125,500, of which more than $67,000 is pure interest. This is why financial advisors consistently emphasize starting early. Even modest monthly contributions can produce life-changing wealth when given decades to compound. The year-by-year breakdown in this calculator illustrates how interest earned accelerates over time as your balance grows.

Plan your financial goals further with our Savings Calculator, map out long-term wealth with the Retirement Calculator, or evaluate investment performance using the ROI Calculator.