Compound Interest Calculator

Free Compound Interest Calculator

This compound interest calculator projects how much an investment grows over time, including optional monthly contributions. Enter your starting amount, interest rate, time horizon, and how often interest compounds — and the tool shows the final balance, total interest earned, and a year-by-year breakdown. It is ideal for retirement planning, savings goals, and comparing investment options.

The Compound Interest Formula

Compound interest is interest earned not only on your principal but also on the interest already added to your account. The classic formula is:

A = P × (1 + r/n)^(n × t)

Where A is the future amount, P is the principal, r is the annual interest rate (decimal), n is how many times interest compounds per year, and t is the time in years. This calculator extends the formula to include monthly contributions, since most real-world investing involves regular deposits.

How to Use the Calculator

Enter your principal (starting amount), rate (annual interest as a percentage), and the number of years. Pick how often interest compounds — daily, monthly, quarterly, semi-annually, or annually. Optionally add a monthly contribution to model regular saving. The result updates live, and the year-by-year table shows exactly how the balance, contributions, and interest evolve.

Why Compounding Frequency Matters

The more often interest compounds, the more you earn — though the difference between monthly and daily is small in practice. For example, $10,000 at 7% for 10 years grows to roughly $20,097 with annual compounding, $20,140 with monthly, and $20,168 with daily. The bigger lever is time: doubling the years roughly quadruples the gain, because interest compounds on interest.

Where People Use Compound Interest Calculations

• Retirement planning — projecting 401(k), IRA, or pension growth
• Savings goals — house down payment, emergency fund, college fund
• Investment comparison — choosing between savings accounts and CDs
• Debt understanding — credit cards and loans compound the other direction
• Education — teaching kids how saving builds wealth over time

Frequently Asked Questions

What is the difference between simple and compound interest?

Simple interest is calculated only on the original principal, so the interest earned is the same each year. Compound interest is calculated on the principal plus any interest already added, so your earnings grow faster every year. Long-term, compound interest dramatically outperforms simple interest.

How do I find my investment's real return?

Subtract inflation (typically 2-3% per year) from your nominal return. A 7% annual return is closer to a 4-5% real return after inflation. For long horizons, run the calculator twice — once with your nominal rate, once with the inflation-adjusted rate — to see both numbers.

Is this calculator accurate?

Yes. It uses the standard compound interest formula and accumulates each compounding period with full floating-point precision. Real-world returns will vary because actual investment rates fluctuate, but for projection purposes the math is exact.

Why include monthly contributions?

Because that is how most people actually save. Including regular deposits is critical to showing realistic retirement and savings projections — without it, you would only see the growth of a one-time deposit, which is rarely the full picture.