How to use the compound interest calculator
- Enter your starting amount, the annual interest rate and the number of years.
- Choose how often interest is compounded. Savings accounts often compound daily or monthly; many bonds compound half-yearly.
- Add a regular monthly or yearly contribution if you will keep saving, and choose whether you pay in at the start or end of each period.
- Read the final balance, how much of it you paid in, and how much is interest. The chart and table show the growth year by year.
The compound interest formula
Without contributions, the future value is A = P × (1 + r/n)n×t, where P is the starting amount, r the annual rate as a decimal, n the number of compounding periods per year and t the number of years. 10,000 at 5% compounded monthly for 10 years grows to 10,000 × (1 + 0.05/12)120 = 16,470.09. Compounded daily it reaches 16,486.65, and continuously (A = Pert) 16,487.21: more frequent compounding helps, but only slightly.
With regular contributions, each deposit grows for the time remaining. The calculator works month by month, applying the equivalent monthly growth for your compounding frequency, so any combination of contribution and compounding schedules is handled. 100 a month at 5% compounded monthly for 10 years adds 15,528.23 to the example above.
The effective annual rate (APY) shows the true yearly growth after compounding. Results ignore taxes, fees and inflation, which reduce real returns. Investment returns are not guaranteed; use a realistic average rate. Everything runs in your browser.
What is the rule of 72?
Divide 72 by the interest rate to estimate how many years it takes to double your money. At 6% that is about 12 years.
What is the difference between APR and APY?
APR is the nominal yearly rate; APY (effective annual rate) includes compounding. 5% APR compounded monthly is 5.116% APY.
Does contributing at the start of the month make a difference?
Yes, a little: each deposit earns one extra period of interest. Over decades the difference becomes noticeable.