How to use the compound interest calculator
Enter what you start with, what you will add each month, the yearly interest or growth rate, and how many years you will leave it. The chart splits each year's balance into the money you put in and the interest it has earned, so you can see the interest share grow over time.
Deposits are counted at the end of each month. The rate is an assumption: a savings account pays a known rate, while investments rise and fall, so treat long-term results as an illustration.
The 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. Each monthly deposit then grows in the same way for the months that remain after it is paid in.
Worked example
1,000 left for 10 years at 5%, with interest added monthly, becomes 1,000 × (1 + 0.05 ÷ 12)120 = 1,647.01. With simple interest it would reach only 1,500, so compounding earned an extra 147.01.
Results are estimates for planning. They are not financial advice, and a lender’s own figures may differ because of fees and rounding.
Common questions
What is the rule of 72?
A quick estimate of how long money takes to double: divide 72 by the interest rate. At 6% it takes about 12 years.
Does compounding daily make a big difference?
Less than you might expect. At 6%, daily compounding earns only slightly more than monthly. The rate and the time matter far more.
Does this allow for inflation or tax?
No. To see growth in today's money, subtract the expected inflation rate from the interest rate you enter.