Compound Interest Calculator

What Is Compound Interest?

Compound interest is interest earned on both your original investment and the interest it has already generated — essentially, interest earning interest. Albert Einstein allegedly called it the 'eighth wonder of the world,' and while the attribution is debatable, the math isn't. Over time, compound interest creates a snowball effect where your money grows exponentially rather than linearly.

The difference from simple interest is dramatic. With simple interest, a $10,000 investment at 7% earns $700 per year forever — $7,000 after 10 years. With compound interest, that same investment grows to $19,672 after 10 years — nearly double — because each year's interest earns interest the following year. Over 30 years, the gap becomes enormous: $31,000 (simple) vs. $76,123 (compound).

The Rule of 72: A Quick Mental Math Trick

Want to know how long it takes your money to double? Divide 72 by your annual interest rate. At 6% return, your money doubles in about 12 years. At 10%, it doubles in roughly 7.2 years. At 4%, it takes 18 years. This simple formula gives you a surprisingly accurate estimate without needing a calculator.

The Rule of 72 also shows why even small rate differences matter over time. The difference between a 6% and 8% return might seem small, but it means your money doubles every 9 years instead of every 12 — that's one extra doubling over a 36-year career. On a $100,000 portfolio, that's the difference between $800,000 and $1,600,000.

Why Starting Early Matters More Than Starting Big

Consider two people: Alex starts investing $200/month at age 25 and stops at 65 (40 years, $96,000 total contributions). Jordan starts investing $400/month at age 35 and stops at 65 (30 years, $144,000 total contributions). At 7% annual return, Alex ends up with roughly $525,000 while Jordan reaches about $365,000. Alex contributed $48,000 less but ends up with $160,000 more.

The secret is those extra 10 years of compounding. In the early years, growth feels slow — but each year the interest snowball gets bigger, and by year 30-40, the growth is staggering. The best time to start investing was 10 years ago. The second-best time is today.

Understanding Interest Rates as a Borrower

Compound Interest Works Against You Too

The same math that grows your investments also grows your debts. A $5,000 credit card balance at 22% APR, paying only the minimum, takes over 20 years to pay off and costs more than $8,000 in interest. That same $5,000 invested at 7% would grow to nearly $20,000 in 20 years. The net difference is staggering.

This is why financial advisors recommend paying off high-interest debt before investing. If you're paying 22% on credit cards while earning 7% on investments, you're losing 15% net. A personal loan at 8-12% to consolidate credit card debt can save thousands and free up money to invest where compound interest works in your favor.

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