Compound interest calculator
How much you will have if you start with a lump sum, add a fixed amount every month and let interest earn interest. Move the sliders and watch for the year when the money starts working harder than you do.
The starting values are an example and are deliberately invented, with round figures so the arithmetic can be redone by hand. They are not data from any real company.
Year by year
Deposits, accumulated interest and the total at the end of each year.
Monthly compounding: each month the deposit is added and interest is applied at the annual rate divided by twelve.
This calculator runs entirely in your browser. No data is sent to any server, no sign-up is needed and nothing is stored.
The formula
Final capital = C₀ × (1 + r/12)^(12·n) + D × [((1 + r/12)^(12·n) − 1) ÷ (r/12)]
How it is calculated
Compound interest means the interest of each period is added to the capital and, from then on, earns interest too. Starting with €10,000, adding €200 every month at 6% a year compounded monthly, after 20 years there is €125,510: €58,000 is what you put in (€10,000 up front plus 240 deposits of €200) and €67,510 is interest. More than half of the result did not come out of your pocket.
The formula has two parts. The first, C₀ × (1 + r/12)^(12·n), is the starting capital growing on its own: the €10,000 becomes €33,102. The second is the sum of the monthly deposits, each growing for the time it has left: the €200 a month becomes €92,408. The calculator does exactly that, month by month, which is why the table matches the formula to the euro.
Why years matter more than the return
With the same €10,000 and €200 a month at 6%: after 10 years there is €50,970; after 20, €125,510; after 30, €261,129; after 40, €507,873. The third decade adds more than the first two combined (€135,619 against €125,510), and the fourth almost as much as the previous three (€246,744 against €261,129). It is not a curve that climbs gently: it does almost nothing at the start and almost everything at the end.
The return matters too, but less than it looks and in the opposite direction to the one people usually watch: over 20 years, dropping from 6% to 3% leaves €83,868 instead of €125,510, and rising to 9% gives €193,669. A point of return that no guaranteed product offers is made up for by years that do depend on you.
The rule of 72 is the shortcut: divide 72 by the annual return to get the years it takes capital to double. At 6% that is 12 years; the exact figure is 11.9. It is for thinking on your feet, not for planning.
Monthly or annual, and what the calculator leaves out
This calculator compounds every month, which is how a savings plan with regular deposits works. Compounding once a year gives slightly less: the €10,000 on its own, at 6% for 20 years, would be €32,071 with annual compounding against €33,102 with monthly. The difference is real, but small compared with the difference years make.
It does not include inflation, taxes on interest or capital gains, or product fees, and it assumes the return is the same every year. In the stock market it is not: a 6% average can be a +20% followed by a −15%, and the order in which they arrive changes the result when there are deposits. The number that comes out is a scenario, not a forecast.
Frequently asked questions
What is compound interest?
It is interest calculated on the starting capital plus the interest already accumulated. With simple interest, interest is always calculated on the original capital; with compound interest, each period starts from the larger capital the previous one left. With €10,000 at 6%: with simple interest, after 20 years there is €22,000; with annual compounding, €32,071.
How is compound interest calculated with monthly deposits?
Month by month: the accumulated capital earns the annual rate divided by twelve, then the deposit is added. As a formula, the starting capital grows as C₀ × (1 + r/12)^(12·n) and the deposits as A × [((1 + r/12)^(12·n) − 1) ÷ (r/12)]. The table on this page runs that calculation and shows it at the end of each year. At a 0% return the closed formula cannot be used (it divides by r/12) and the deposits simply add up; the table does exactly that.
What return should I use?
The one you genuinely expect from the product, before tax and after fees, and it is worth trying several. A deposit or a savings account has a known rate; a fund or a stock portfolio does not: its historical average return does not guarantee the future one. This calculator does not recommend any rate.
How long does it take money to double?
Roughly 72 divided by the annual return in percent: at 6%, about 12 years; at 3%, 24; at 9%, 8. That is the rule of 72, an approximation of the exact formula ln(2) ÷ ln(1 + r), which at 6% gives 11.9 years.
Does it account for inflation and taxes?
No. The result is in nominal euros and before tax. For a rough idea in today’s euros, subtract expected inflation from the return: 6% with 2% inflation behaves roughly like a real 4%. Taxes depend on the product and the country and cannot be put into a general formula.
Other calculators
WACC Calculator · DCF Calculator: intrinsic value per share · P/E Ratio Calculator: reported vs adjusted · Dividend Calculator: annual income and reinvestment · CAGR Calculator: compound annual growth rate · FIRE Calculator: your financial independence number
Do it on a real company
Celsmar computes this from the accounts companies file with their regulator, and shows which line every figure comes from. Free to start, no card.
A calculation tool for informational purposes. It is not financial advice nor a recommendation to buy or sell, and its result depends entirely on the assumptions you enter.