A compound interest calculator applies a rate to a balance, then applies the next rate to the new balance, which is the whole idea of compounding. This page does that at the end of each period you choose: once a year, four times a year, twelve times a year, or 365 times a year. It also accepts money added every month, which is the piece a plain "principal times rate" box leaves out. The starting amount can be zero if you only want to see contributions. The monthly amount can be zero if you only want a lump sum to grow. The annual rate is a percent, so 5 means five percent, not 0.05. The term is in years and can be a fraction, up to 50. A rate of zero still adds the contributions, so the future value equals what you put in.
The step is the same every period. Multiply the balance by one plus the annual rate divided by 100 and by the number of periods in a year. Then add the monthly contribution times 12, divided by that same number of periods. Yearly compounding therefore adds a full year of contributions at each step. Monthly compounding adds one month at each step. Daily compounding uses 365 periods in a year, including in a leap year, and adds a small slice of the monthly amount each day. The page rounds money to cents at the end, and it rounds the period count from years times frequency. A quarter of a year compounded monthly is three periods. The worked check used in the tests is a start of 1,000, 12 percent, a quarter of a year, 100 each month, compounded monthly: the balance is 1,333.31, of which 1,300 was contributed and 33.31 is interest.
The defaults are a savings illustration, not a product. Start with 10,000, add 200 a month, earn 5 percent, compound monthly, and wait 10 years. The future value is 47,526.55. You put in 34,000, which is the original 10,000 plus 200 times 120 months. Interest is the difference, 13,526.55. The table lists each year, what was added that year, and the balance at the end of the year. The chart draws the same balances. If you change the frequency to yearly, the path changes because the whole year's contribution arrives before fewer, larger rate steps. Compare frequencies on the same rate before you treat the gap as a reason to pick an account. A bank that compounds daily and charges a fee can leave you behind a bank that compounds monthly and does not.
Use the form from the top. The rate field stops at 40 percent so a missing decimal, typed as 500, cannot draw a chart that looks like a fortune. The starting amount stops at one billion and the monthly amount at ten million, which is enough for a classroom example and for a personal saving plan, and it keeps the page from painting infinity. Years must be greater than zero. After you calculate, read three figures before the chart: the future value, the amount you put in, and the interest. Interest is not a yield you can spend today. It is the gap between the ending balance and the cash you contributed, on these assumptions.
This is not a forecast and not advice. Inflation is not subtracted. Tax on interest is not subtracted. A missed month is not modelled. The rate does not step down after a promotional period. If you are comparing a loan, the loan calculator is the page that builds a payment and an amortization table. A salary estimate, including Malaysian EPF and PCB lines, is the salary calculator, and it does not compound. Those tools answer different questions. Mixing a savings balance into a loan payment is how people talk past each other about "interest."
Nothing you type is uploaded. Refresh the page and the fields return to the defaults. There is no portfolio and no ticker. The directory links the other calculators. If you only needed to know what the formula did with the defaults, you can stop at 47,526.55 and the ten rows. If you change one input, change one input, so you can see which assumption moved the balance.