Compound Interest Calculator
Enter a starting amount, an annual interest rate, the number of years and how often the interest compounds: annually, semiannually, quarterly, monthly or daily. Add a monthly contribution if you keep saving. The calculator shows the final balance, the interest earned and your total contributions. Compound interest is interest earned on both your original principal and the interest already earned.
Added at the end of each full month.
How to use the Compound Interest Calculator
- Enter your numbers in the Compound Interest Calculator above.
- Press the Calculate button to see the result.
- To try other numbers, change any value and press the button again.
The compound interest formula
A = P (1 + r/n)^(n·t)
P is the starting principal, r is the annual interest rate (as a decimal), n is the number of compounding periods per year, and t is the number of years. A is what the starting principal grows to. A monthly contribution goes in at the end of each full month and compounds from then on at the monthly rate that matches your compounding frequency. The final balance is A plus the grown contributions, rounded to the cent.
Worked example
$10,000 invested at 7% annual interest compounded monthly for 20 years: A = 10,000 × (1 + 0.07/12)^(12 × 20), and the calculator shows a final balance of $40,387.39. The same amount at simple interest would reach only $24,000, so compounding adds over $16,000.
Frequently asked questions
How is compound interest different from simple interest?
Simple interest is calculated only on the principal. Compound interest is calculated on the principal plus any interest already accumulated, which makes the balance grow faster over time.
Does compounding frequency matter?
Yes, though each step up adds less. $10,000 at 7% for 20 years grows to $38,696.84 compounded annually, $40,387.39 compounded monthly and $40,546.56 compounded daily. Going from annual to monthly adds $1,690.55. Going from monthly to daily adds $159.17.
What is the Rule of 72?
The Rule of 72 is a shortcut to estimate doubling time: divide 72 by the annual interest rate to get the approximate number of years for an investment to double. At 8%, money doubles in about 9 years. The calculator shows $10,000 at 8% compounded annually reaching $19,990.05 after 9 years.
Should I include regular contributions?
Yes, if you plan to keep saving. This calculator adds a monthly contribution at the end of each month, and each deposit compounds from then on. With a starting amount of 0, $200 a month for 20 years at 7% compounded monthly is $48,000 of deposits that grow to $104,185.33.