Compound Interest Calculator
Calculate how your investments grow over time with the power of compounding. Model retirement savings, education funds, and any long-term financial goals.
Compound Interest Calculator
Calculate investment growth with compound interest and regular contributions
Investment Parameters
Starting amount to invest
Expected annual return
Investment duration
Expected annual inflation
Regular Contributions
Regular contribution amount
How to Use the Compound Interest Calculator
1
Enter Your Principal
Input your initial investment or current savings balance. This is the starting amount on which interest will first be calculated.
2
Set the Interest Rate
Enter the annual interest rate (APY). For savings accounts, use the APY shown by your bank. For investments, use an expected annual return (the S&P 500 has historically averaged ~10% annually).
3
Choose Time Period
Select the investment horizon in years. Even small changes in time dramatically affect the result — this demonstrates why starting early is so critical.
4
Add Regular Contributions
Enter monthly or annual contributions if you plan to add money regularly. Consistent contributions combined with compounding create powerful wealth-building results.
Compound Interest Formula Explained
A
Final amount (principal + interest)
P
Principal (initial investment)
r
Annual interest rate (as decimal, e.g., 5% = 0.05)
n
Compounding frequency (12 = monthly, 365 = daily)
t
Time in years
Example: $5,000 invested at 7% annual return, compounded monthly for 30 years:
A = 5,000 × (1 + 0.07/12)^(12×30) = $38,061 — growing from $5,000 to over $38,000 without adding a single dollar!
Why Compound Interest Matters for Your Wealth
The Rule of 72
Divide 72 by your annual return to estimate when your money doubles. At 8% return, your money doubles every 9 years. At 12%, every 6 years.
Start Early, Win Big
A 25-year-old investing $5,000/year at 7% will have $1.06M at 65. A 35-year-old doing the same will only have $505K — half as much, despite investing for 10 fewer years.
Tax-Advantaged Accounts
In a 401(k) or Roth IRA, your gains compound without annual taxes. This "triple compounding" (principal + interest + tax savings) makes tax-advantaged accounts immensely powerful.
Frequently Asked Questions
What is compound interest?
Compound interest is interest calculated on both the initial principal and the accumulated interest from previous periods. Unlike simple interest (calculated only on principal), compound interest causes your money to grow exponentially — often called "interest on interest".
What is the compound interest formula?
The formula is A = P(1 + r/n)^(nt), where A is the final amount, P is the principal, r is the annual interest rate (decimal), n is the number of compounding periods per year, and t is the number of years. For monthly compounding: A = P(1 + r/12)^(12t).
How often should interest compound for maximum growth?
More frequent compounding produces higher returns. Daily compounding yields slightly more than monthly, which yields more than quarterly, which yields more than annual compounding. For a $10,000 investment at 5% over 10 years: annual compounding gives $16,289 vs. daily compounding giving $16,487.
What is the Rule of 72?
The Rule of 72 is a quick mental shortcut to estimate how long it takes for an investment to double. Divide 72 by the annual interest rate. For example, at 6% annual return, your money doubles in approximately 12 years (72 ÷ 6 = 12).
How does compound interest affect loans?
Compound interest works against borrowers. On loans like credit cards, interest compounds on the unpaid balance, meaning unpaid interest gets added to your balance and starts accruing its own interest. This is why carrying credit card debt is so costly.
What is the difference between APR and APY?
APR (Annual Percentage Rate) is the simple annual interest rate without compounding. APY (Annual Percentage Yield) reflects the actual return after compounding. APY is always higher than APR when compounding occurs more than once per year.
How do I maximize the power of compound interest?
Start early (time is the most powerful factor), invest regularly (dollar-cost averaging), reinvest all dividends and interest, minimize fees (even 1% fees significantly impact long-term growth), and choose tax-advantaged accounts like 401(k) or IRA to let gains compound without annual tax drag.