Compound Interest Calculator
See how your savings or investment grows with compound interest and regular monthly deposits
Last updated: October 22, 2026
How to Use the Compound Interest Calculator
How to use the compound interest calculator
- Enter your initial deposit. This is the amount you start with. It can be zero if you only plan to make monthly deposits.
- Add a monthly deposit (optional). The amount you plan to add at the end of every month.
- Enter the interest rate. Use the nominal annual rate quoted by your bank or investment.
- Set the time. Enter the years, and extra months if needed.
- Choose how often interest compounds. Savings accounts often compound daily or monthly. Pick the one that matches your account.
- Read the results. You will see your future value, how much you put in, how much was interest, and a year-by-year table.
Results are estimates: real returns on investments vary, and this calculator assumes a fixed rate with no fees or taxes.
Formula & Methodology
Compound interest formula
Without regular deposits:
A = P × (1 + r ÷ n)^(n × t)
- A = future value
- P = initial deposit (principal)
- r = annual interest rate as a decimal (5% = 0.05)
- n = times interest is compounded per year
- t = number of years
Worked example
You deposit $10,000 at 5%, compounded monthly, for 10 years, with no extra deposits.
- Rate per period: 0.05 ÷ 12 = 0.004167
- Number of periods: 12 × 10 = 120
- Growth factor: (1.004167)^120 = 1.6470
- Future value: 10,000 × 1.6470 = $16,470.09
- Interest earned: $16,470.09 − $10,000 = $6,470.09
With monthly deposits
Each deposit is added at the end of the month and then earns interest for the months that remain. The calculator works month by month, so every deposit compounds correctly whichever compounding frequency you choose.
Effective annual yield (APY)
APY = (1 + r ÷ n)^n − 1
APY shows the true yearly return once compounding is included. A 5% rate compounded monthly has an APY of 5.116%.
The Rule of 72
Divide 72 by the annual rate to estimate how many years it takes to double your money. At 6%, money doubles in about 72 ÷ 6 = 12 years.
About Compound Interest Calculator
Our compound interest calculator is a free, easy-to-use tool designed to help you see how your savings or investment grows with compound interest and regular monthly deposits. For professionals or those who need quick calculations, our calculator provides accurate results instantly.
Frequently Asked Questions
What is compound interest?
Compound interest is interest earned on both your original deposit and on the interest already added to it. Because the base grows each period, your balance grows faster over time than with simple interest, which only ever pays on the original amount.
How do you calculate compound interest?
Use A = P(1 + r/n)^(nt), where P is the deposit, r is the annual rate as a decimal, n is the number of compounding periods per year and t is the number of years. $10,000 at 5% compounded monthly for 10 years grows to $16,470.09.
How does compounding frequency affect growth?
The more often interest is compounded, the slightly more you earn. At 5% a year on $10,000 for 10 years, annual compounding gives $16,288.95, monthly gives $16,470.09 and daily gives $16,486.65. The difference between frequencies is small compared to the effect of time and rate.
What is the difference between APY and the interest rate?
The interest rate is the nominal yearly rate. APY (annual percentage yield) includes the effect of compounding, so it is always equal to or higher than the rate. Banks use APY to let you compare accounts fairly.
What is the Rule of 72?
It is a quick way to estimate how long money takes to double: divide 72 by the annual interest rate. At 8%, money doubles in about 9 years; at 4%, in about 18 years.
Do monthly deposits make a big difference?
Yes. Regular deposits are often the largest part of long-term growth. Adding $200 a month to $10,000 at 5% for 10 years turns it into about $47,500, far more than the $16,470 you would have without deposits.
Does this calculator include taxes or fees?
No. It assumes a fixed rate and no taxes, fees or withdrawals. Investment returns are not guaranteed and can be lower in some years, so use the result as an estimate rather than a promise.
Your result will appear here
Loading…