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Compound Interest Calculator

See how your money grows over time with compound interest. Factor in initial investment, monthly contributions, interest rate, and compounding frequency.

How to Use This Calculator

1. Enter your initial investment (principal) amount. 2. Set your planned monthly contribution amount. 3. Enter the expected annual interest/return rate. 4. Choose the investment timeframe in years. 5. Select compounding frequency (monthly is most common for investments). 6. Click Calculate to see your projected growth and yearly breakdown.

Key Formulas

Compound Interest

A = P(1 + r/n)^(nt)

A = final amount, P = principal, r = annual rate, n = compounds per year, t = years.

Future Value with Contributions

FV = P(1+r/n)^(nt) + PMT × [((1+r/n)^(nt) – 1) / (r/n)]

Adds regular contributions (PMT) to the compound interest formula.

Rule of 72

72 ÷ Annual Rate = Years to Double

Quick estimate of how long it takes to double your money at a given rate.

The Power of Compound Interest: How Your Money Grows

Compound interest is the single most powerful force in personal finance. Albert Einstein reportedly called it the eighth wonder of the world. The concept is simple: you earn interest not just on your original investment, but on all the accumulated interest as well.

The difference between simple and compound interest is dramatic over time. A $10,000 investment at 7% simple interest grows to $24,000 after 20 years. The same investment with compound interest grows to $38,697 — over 60% more. Over 40 years, the gap widens to $38,000 vs $149,745.

The Rule of 72 provides a quick mental shortcut: divide 72 by your interest rate to find how many years it takes to double your money. At 7%, your money doubles every ~10.3 years. At 10%, every 7.2 years.

Monthly contributions amplify compound growth enormously. Investing $500/month at 7% annual return grows to $260,000+ in 20 years and $1.2 million+ in 35 years — even though you only contributed $210,000 in total over 35 years. The remaining $1 million+ is compound growth.

Compounding frequency matters but less than you might think. $10,000 at 7% compounded annually yields $19,672 after 10 years. Compounded monthly: $20,097. Compounded daily: $20,138. The real power is in the time horizon and contribution consistency.

Frequently Asked Questions

What is the Rule of 72?

The Rule of 72 is a mental shortcut for estimating doubling time. Divide 72 by the annual interest rate to get approximate years to double. At 6%: 12 years. At 8%: 9 years. At 12%: 6 years. It works well for rates between 2% and 20%.

How much will $10,000 grow in 20 years?

At 7% compounded monthly: $40,387. At 10%: $67,275. At 12%: $96,463. Add $500/month in contributions and at 7% you reach $270,692. The monthly contributions often matter more than the initial investment over long time horizons.

Does compounding frequency matter?

Somewhat, but less than most people think. The difference between annual and monthly compounding on $10,000 at 7% over 10 years is about $425. The difference between monthly and daily is only $41. Contribution consistency and time matter far more.

What is a realistic rate of return?

The S&P 500 has averaged about 10% nominal (7% after inflation) annually since 1926. Bonds average 4–5%. A balanced 60/40 portfolio averages about 8% nominal. Use 7% as a conservative estimate for long-term stock market investments after inflation.