Finance

Compound Interest Calculator

Calculate compound interest growth with monthly contributions, different compounding frequencies, and a 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. Albert Einstein is often (perhaps apocryphally) credited with calling it "the eighth wonder of the world" — and with good reason. The compounding effect means that over long periods, even modest interest rates can multiply wealth dramatically.

Simple interest is calculated only on the original principal. Compound interest is calculated on the principal plus all accumulated interest. Over time, this difference becomes enormous.

The Compound Interest Formula

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

Where: A = final amount, P = principal, r = annual interest rate (decimal), n = compounding periods per year, t = time in years.

With monthly contributions: each contribution also compounds for the remaining period, so the full formula becomes a sum of geometric series — which this calculator handles automatically.

Compounding Frequency Makes a Difference

More frequent compounding means slightly higher returns. On a $10,000 investment at 7% for 20 years:

  • Annual compounding: $38,697
  • Monthly compounding: $40,127
  • Daily compounding: $40,163

The difference between monthly and daily compounding is negligible. The real power of compounding comes from time — not compounding frequency.

The Rule of 72 — Quick Mental Math

Divide 72 by the annual interest rate to estimate how many years it takes to double your money. At 7% interest, your money doubles approximately every 72 ÷ 7 = 10.3 years. This works remarkably well for rates between 2% and 20%.

Why Regular Contributions Matter More Than Timing

Adding $200/month to an investment at 7% over 30 years results in approximately $242,000 — from only $72,000 in contributions. The extra $170,000 is pure compound interest. Starting 10 years earlier (40 years total) grows that to over $525,000. Time in the market consistently beats timing the market.

Financial Disclaimer: Results are estimates only and should not be relied upon as financial or investment advice. Consult a licensed financial advisor for guidance specific to your situation.

Frequently Asked Questions

What is the difference between compound and simple interest?
Simple interest is calculated only on the original principal. Compound interest is calculated on both the principal and the accumulated interest. Over time, compound interest produces dramatically higher returns due to the "interest on interest" effect.
How often does compound interest compound?
Compounding frequency varies by product. Savings accounts typically compound daily or monthly. Most investment accounts compound monthly or quarterly. The more frequently interest compounds, the slightly higher the effective annual return.
What is the Rule of 72?
The Rule of 72 is a quick mental math shortcut: divide 72 by the annual interest rate to estimate how many years it takes to double your money. At 6% interest, your money doubles in roughly 12 years (72 ÷ 6 = 12).
What interest rate should I use?
For long-term stock market investments, the historical average return of the S&P 500 is approximately 7-10% per year after inflation. For savings accounts, check your bank's current APY. For loan calculations, use the stated APR.
How does monthly contribution affect compound growth?
Regular monthly contributions dramatically accelerate compound growth. Each contribution immediately starts earning interest and compounds for the remaining investment period. Even small monthly amounts ($50-$200) can add tens of thousands to your final balance over 20-30 years.