Compound Interest Calculator
See how an investment or savings balance could grow with compound interest and regular deposits, and how much of the final balance is interest.
Result
Enter your values and select Calculate to see the result.
How to use this calculator
- Enter the starting amount, the annual interest rate and how many years to grow it.
- Choose how often interest compounds.
- Optionally add a regular contribution, how often it is made and whether it is paid at the start or end of each period.
- Open the year-by-year breakdown to see contributions, interest and balance for each year.
Formula
Without contributions: A = P × (1 + r/n)^(n×t) where P is the starting amount, r the annual rate as a decimal, n the compounding periods per year and t the years.
With contributions, the balance is grown one contribution period at a time using the periodic rate i = (1 + r/n)^(n/f) − 1, where f is the number of contributions per year. Continuous compounding uses i = e^(r/f) − 1.
- End of period: interest is added, then the contribution.
- Start of period: the contribution is added first, so it earns interest for that period.
Worked example
10,000 at 5% for 10 years, compounded monthly
10,000 × (1 + 0.05/12)^120 = 16,470.09, so 6,470.09 is interest.
Adding 200 a month
With an extra 200 at the end of each month, the balance after 10 years is 47,526.55: 10,000 starting amount + 24,000 contributions + 13,526.55 interest.
Paying at the start of each month instead gives 47,655.95.
Assumptions and limitations
- The interest rate is assumed constant for the whole period.
- Taxes, fees, inflation and investment risk are not included; real returns on investments vary and can be negative.
- Results are estimates and not financial advice.
Frequently asked questions
How often should interest compound?
Use the frequency stated by your bank or product. More frequent compounding gives slightly more, but the effect is small: 5% compounded monthly is an effective annual rate of about 5.12%.
What is the effective annual rate?
It is the actual yearly growth after compounding, (1 + r/n)^n − 1. It lets you compare products with different compounding frequencies.
Does it matter if I contribute at the start or end of the month?
Yes. Contributions at the start of each period earn one extra period of interest, so the final balance is a little higher.