Compound Interest Calculator — See How Your Money Actually Grows

Why Compound Interest Is Called the Eighth Wonder

Compound interest earns interest on your interest. A $10,000 investment at 8% earns $800 the first year. The second year, you earn 8% on $10,800 — that's $864. Each year, the growth accelerates. After 30 years, that $10,000 becomes $100,627. You earned $90,627 without contributing another penny.

The Power of Monthly Contributions

Add $200/month to that $10,000 starting investment at 8% for 20 years:

• You contribute: $10,000 + ($200 x 240 months) = $58,000
• Your balance: $168,187
• Interest earned: $110,187

Interest earned nearly doubles what you put in. That's the magic. Try different numbers with our compound interest calculator.

Starting Early vs Starting Late

$200/month at 8%:

• Start at 25, retire at 65: $702,856
• Start at 35, retire at 65: $298,072
• Start at 45, retire at 65: $118,589

Starting 10 years earlier more than doubles the result. Time beats amount — $200/month for 40 years beats $400/month for 20 years.

Run Your Own Numbers

Open the compound interest calculator and play with different scenarios. Change the starting amount, monthly contribution, interest rate, and time period to see how each factor affects your final balance.

Frequently Asked Questions

How much is $200 per month for 20 years?

At 8% annual return: $118,589. You contribute $48,000 total, interest adds $70,589. At 10%: $151,874 ($103,874 in interest). Time is the biggest factor.

What is the Rule of 72?

Divide 72 by your interest rate to estimate doubling time. At 8%, money doubles in ~9 years. At 10%, ~7.2 years. At 6%, ~12 years.

How often should interest compound?

More frequent = slightly more growth. The difference between monthly and annual compounding on $10K at 8% over 20 years is about $1,200. Not huge, but free money.