With monthly contributions and compared to simple interest. Free, nothing to install, no sign-up.
Compound interest is interest worked out on what you have built up, not on what you put in at the start: the interest becomes part of the capital and from then on earns interest of its own. This calculator gives you the final figure, takes monthly contributions, and shows alongside it what you would have got with simple interest — which is where the difference shows. It runs inside your browser and downloads as a PDF.
Final amount = starting amount × (1 + rate) to the power of the years, with the rate as a decimal. What matters is not the formula but where the exponent sits: the time is UP THERE, not multiplying. That is why compound interest does not grow in a straight line the way simple interest does — it steepens — and why the last years contribute far more than the first.
A thousand at 6% a year for ten years: 1000 × 1.06 to the power of 10 = 1,790.85. The interest is 790.85. With simple interest the same thousand at 6% over ten years would have given exactly 600. The difference, 190.85, is what the interest itself earned. And that gap grows fast: over twenty years compound gives 2,207 of interest and simple gives 1,200.
If on top of the starting amount you put in something every month, each contribution starts earning from the day it lands, so the first one works for many more years than the last. That is why, over a long period, putting in a little early tends to weigh more than putting in a lot late. The calculator handles each contribution separately, not as if they all arrived at once.
There is a shortcut worth knowing: divide 72 by the percentage and you get roughly the number of years it takes the money to double. At 6%, 72 ÷ 6 = 12 years. It is an approximation and not the exact formula, but if the calculator tells you something far off that, check what you entered.
It assumes a fixed rate every year, and reality almost never is. It takes no account of tax, charges or inflation, and inflation matters: if your money grows 6% and prices rise 3%, what you have actually gained in buying power is far less. This is an arithmetic calculator, not a forecast and not advice about what to do with your money.
With simple interest the interest is always worked out on the original amount and every year earns the same. With compound interest it is added to the capital and from then on earns interest too, so each year earns a little more than the last. Over short periods the difference is small; over long ones it is enormous.
Yes, the box asks for the annual rate, which is how it is normally quoted. The calculator splits it across periods internally so that monthly contributions are handled properly.
No, none of the three. It works out gross growth at a fixed rate. Tax on the returns, the product’s charges and the loss of buying power to inflation all have to be taken off separately, and they can change the real result a great deal.
No. Everything is worked out inside your browser: the amounts you type never leave your device and are not stored anywhere. No account, no email address.
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