What Is Compound Interest?
Compound interest is interest calculated on both your initial principal and the interest you've already earned. Unlike simple interest — which only earns on the original amount — compound interest causes your balance to grow exponentially over time. This is what Albert Einstein is often (perhaps apocryphally) credited with calling "the eighth wonder of the world."
The longer your money compounds, the more dramatic the effect. A $10,000 deposit at 5% simple interest earns $500 every single year. The same deposit at 5% compound interest earns $500 in year one, but $525 in year two, $551 in year three — and so on, accelerating every year.
The Compound Interest Formula
When you add regular monthly contributions (as our calculator supports), the formula extends to account for the future value of an annuity — each contribution earns compound interest from the moment it's deposited.
How Compounding Frequency Affects Growth
The more frequently interest compounds, the more you earn. Here's $10,000 at 5% over 10 years under different frequencies:
| Frequency | Times/Year | Balance After 10 Years | Interest Earned |
|---|
| Annually | 1 | $16,288.95 | $6,288.95 |
| Quarterly | 4 | $16,436.19 | $6,436.19 |
| Monthly | 12 | $16,470.09 | $6,470.09 |
| Daily | 365 | $16,486.65 | $6,486.65 |
Daily compounding earns about $16 more than annual compounding over 10 years on a $10,000 balance — the difference grows significantly with larger balances and longer time horizons.
The Power of Starting Early
Time is the most powerful variable in the compound interest formula. An investor who starts at age 25 and contributes $200/month at 7% until age 65 will end up with significantly more than someone who starts at 35 and contributes the same amount — even though the late starter contributes for the same number of total years. The early years of compounding are irreplaceable.
Frequently Asked Questions
What's the difference between APR and APY?
APR (Annual Percentage Rate) is the simple annual rate before compounding. APY (Annual Percentage Yield) accounts for compounding and reflects what you actually earn in a year. For savings accounts, banks advertise APY because it's the higher number. Our calculator uses APY — enter the rate as shown on your savings account or investment.
How often does a typical savings account compound?
Most high-yield savings accounts and money market accounts compound daily and credit interest monthly. Traditional bank savings accounts typically compound monthly. CDs (certificates of deposit) vary — some compound daily, others monthly or quarterly. Always check the terms of your specific account.
Does compound interest work the same way on debt?
Yes — and it works against you. Credit card debt typically compounds
daily on a high APR (often 20–30%), which is why balances grow so quickly when only minimum payments are made. The same mathematical force that builds wealth through savings also amplifies debt. Use our
Credit Card Payoff Calculator to see how quickly you can eliminate debt.
What interest rate should I use for long-term investment projections?
For S&P 500 index fund projections, financial planners commonly use 7% as a conservative real return (after inflation) or 10% as a nominal historical average. For high-yield savings accounts, use the current APY (which changes with Fed rate decisions). For CDs, use the locked rate on the specific term you're considering. Always use conservative estimates for retirement planning.
Are the results on this calculator accurate?
Our calculator uses the standard compound interest formula and is mathematically accurate. However, real-world results will vary due to factors like taxes on interest income, variable interest rates, inflation eroding purchasing power, and the timing of contributions. The results here are projections intended for educational planning — not guaranteed outcomes. Always consult a financial advisor for major financial decisions.
What is the Rule of 72?
The Rule of 72 is a quick mental math shortcut: divide 72 by your interest rate to estimate how many years it takes for money to double. At 6%, money doubles in roughly 12 years (72 ÷ 6). At 8%, it doubles in about 9 years. It's a handy approximation — our calculator gives you the exact figure.