Compound Interest Calculator

Compound interest is the most misunderstood concept in personal finance. The definition is simple: you earn interest not just on your original deposit, but also on the interest it has already earned. Over short periods this distinction barely matters. A thousand dollars at 7 percent for one year earns about seventy dollars whether it compounds annually or daily. Over thirty years, however, the same thousand dollars at 7 percent grows to roughly $7,610 with monthly compounding. That extra $6,610 above the original principal is the mechanism behind long-term investing. This calculator gives you the final balance, total interest earned, and effective annual yield (APY) for any combination of principal, rate, term, and compounding frequency.

1. 1Enter your starting principal: the amount you are investing today.
2. 2Enter the annual interest rate as a percentage (a 7% savings account is entered as 7, not 0.07).
3. 3Enter how many years you plan to leave the money invested.
4. 4Choose the compounding frequency. Most savings accounts compound monthly or daily.
5. 5The calculator converts the annual rate to a per-period rate by dividing by the frequency.
6. 6It then applies the compound interest formula to compute the final balance, the total interest, and the effective annual yield.

• Retirement savings projection: Plug in your current investment balance, your expected annual return, and the years until retirement. The result tells you what you would have if you made zero additional contributions. Compare that to your retirement target to see how much you need to add per year.
• Comparing savings account offers: Two banks advertise the same 4.5% rate, but one compounds daily and the other monthly. The daily-compounding APY is 4.604%; the monthly one is 4.594%. On a $50,000 balance that is $5 per year. Small, but a useful sanity check when comparing offers.
• Understanding credit-card debt growth: Credit cards compound daily on the unpaid balance. A $5,000 balance at 20% APR grows to about $6,100 in a year if no payment is made. The same compounding that grows your savings hurts you on debt.
• Education fund planning: A $10,000 deposit made when a child is born, compounding at 6% annually for 18 years, becomes about $28,500 by college age. Use the calculator to back-solve: what lump sum today grows to your target by a specific year?

• Confusing rate with yield: When a bank quotes "5% interest, compounded monthly," your actual annual return is 5.116%, not 5%. Always check whether the advertised number is APR or APY.
• Underestimating long time horizons: The difference between investing for 25 years versus 30 years at the same rate is often 50% or more in final value. The last five years compound on the largest balance. Time matters more than most people expect.
• Assuming nominal returns survive inflation: A 7% nominal return in a 3% inflation environment is a 4% real return. Long-term projections that ignore inflation overstate purchasing power. Mentally subtract expected inflation from the nominal rate to get real growth.

• Start as early as possible, even with small amounts: $200 per month invested from age 25 to 65 at 7% becomes about $525,000. Starting at 35 instead yields about $245,000 with identical contributions. The first decade is the most valuable because it compounds the longest.
• Use the Rule of 72 for fast mental math: Divide 72 by the rate to get the doubling time in years. 72 ÷ 6% = 12 years. 72 ÷ 8% = 9 years. Useful for sanity-checking long-term projections without a calculator.
• Reinvest dividends and interest: Many brokerages and savings accounts let you auto-reinvest dividends and interest. Doing so converts the simple-interest version of the formula into the compound version, which is precisely what powers the long-term growth shown above.