Compound Interest Calculator
Free to use. Runs in your browser: your files and text are not uploaded.
This tool gives general information only. It is not financial advice.
Enter a starting amount, a monthly addition, an interest rate and a number of years to see the final balance and how much of it is interest.
How to use it
- Type the starting amount.
- Type how much you add each month. Use 0 for none.
- Type the yearly interest rate and the number of years.
- Choose how often interest is compounded.
The formula
For a single amount with no additions:
A = P × (1 + r ÷ n)^(n × t)
Example: 10,000 at 7% a year, compounded yearly, for 10 years. A = 10,000 × 1.07¹⁰ = 19,671.51. The interest earned is 9,671.51.
With simple interest, where interest is never added to the balance, the same money would earn 700 a year, or 7,000 in total. Compounding adds the other 2,671.51.
Adding money every month
With the tool’s starting values, 10,000 plus 100 a month at 7% compounded monthly for 10 years:
- You put in 22,000 in total
- Interest earned is 15,405.09
- Final balance is 37,405.09
Each monthly addition is counted at the end of the month, so it starts earning interest the month after.
How often interest is compounded
More frequent compounding gives slightly more. For 10,000 at 7% over 10 years:
| Compounded | Final balance |
|---|---|
| Yearly | 19,671.51 |
| Monthly | 20,096.61 |
| Daily | 20,136.18 |
Time matters most
Left for 30 years at 7% compounded yearly, the same 10,000 grows to 76,122.55. Tripling the time multiplies the interest earned by nearly seven.
Limits
- The interest rate is assumed to stay the same every year. Returns on shares and funds go up and down, and can be negative.
- Taxes, fees and inflation are not included. A 7% return with 3% inflation buys about 4% more each year, not 7%.
- This tool shows arithmetic. It does not recommend any investment.
Frequently asked questions
What is compound interest?
Interest that is added to your balance, so that later interest is earned on the earlier interest as well as on the money you put in.
What is the compound interest formula?
A = P × (1 + r ÷ n)^(n × t), where P is the starting amount, r is the yearly rate as a decimal, n is how many times a year interest is added, and t is the number of years.
How long does money take to double?
Divide 72 by the yearly interest rate for a quick estimate. At 7% a year, money doubles in roughly 72 ÷ 7 = 10.3 years.