Compound interest calculator 2026
Compound interest is interest earned on both the original deposit and on interest already added to the balance. This calculator projects the future value of an initial amount plus monthly contributions at a chosen annual rate, split into money invested and interest earned.
- Future value€109,333.14
- Total invested€58,000.00
- Interest earned€51,333.14
Over time
Illustration only, not financial advice. Assumes a constant rate and regular intervals; real products vary. Verify with a professional.
How the balance grows month by month
The calculator compounds monthly. It converts the annual rate you enter into a monthly rate by dividing by twelve, then applies that rate to the running balance once for every month in the horizon. Your monthly contribution is added at the end of each month, after that month's interest has already been applied, so a fresh deposit does not earn interest for the month it lands in - the standard ordinary annuity convention. Written out, the balance after n months is the initial amount times (1 plus the monthly rate) to the power n, plus the contribution stream compounded the same way, where n is years times twelve. The running total is kept as an exact number internally and only rounded to the nearest cent when it is shown or drawn on the chart, so no rounding error builds up along the way.
A worked example: EUR 10,000 plus EUR 200 a month
Try the calculator's own starting numbers: an initial amount of EUR 10,000, a monthly contribution of EUR 200, an annual rate of 5% and a horizon of 20 years. Compounding that monthly for 240 months gives a future value of EUR 109,333.14. Of that, EUR 58,000 is money you actually put in - the EUR 10,000 initial amount plus 240 payments of EUR 200 - and the remaining EUR 51,333.14 is interest the balance generated on its own. Enter those same four numbers into the fields above and you should land on exactly those three figures, cent for cent, which is a useful way to confirm you are reading the inputs the same way the tool does before you trust it with your own numbers.
What the calculation assumes
The result rests on a short list of simplifications. The annual rate you enter is treated as fixed for the entire horizon - it does not rise, fall or reset partway through. Compounding always happens monthly, whatever the real compounding period of an actual account might be, and every contribution is assumed to arrive on time and in full, in the same amount, every month, with none skipped or increased. No fees, account charges or taxes are subtracted anywhere in the calculation, and the rate itself is nominal - it is not adjusted for inflation before or after the fact. The horizon is also handled as a whole number of years; the tool rounds a fractional entry to the nearest whole year before it starts compounding.
Where the estimate stops matching reality
Real savings and investment products rarely match those assumptions for long. Variable-rate accounts change their rate with market conditions, promotional rates expire, and investment returns - unlike a savings account rate - do not arrive as a smooth constant each year; they swing up and down, and a poor year early in the horizon can cost more than the same poor year near the end, an effect this straight-line projection cannot show. Inflation is not subtracted, so the future value is a nominal figure, not what that money will actually buy. Taxes on interest or gains, and any fees a real product charges, will reduce what you actually keep, and the exact impact depends on rules that differ by country and product, which this country-agnostic tool has no way to know.
Reading the result without over-trusting it
Used well, this figure is a comparison tool, not a forecast. It is most reliable when you hold everything but one variable constant and see how the outcome shifts - raising the monthly contribution, extending the horizon, or comparing two rates side by side under the same assumptions. It is also a reasonable sanity check on whether a number quoted to you by a bank or fund is at least mathematically plausible for the rate and time period stated. What it cannot do is predict what a real account or investment will actually return, guarantee that the rate you entered will hold, or account for what taxes and fees will take out of the final balance. Treat it as a way to see the mechanics of compounding, not a promise about your money.
FAQ
What happens if I leave the monthly contribution at zero?
The calculator still works - it simply compounds the initial amount on its own, with no additions along the way. The formula reduces to the initial amount multiplied by (1 plus the monthly rate) raised to the number of months, the standard lump-sum compound interest calculation. Total invested then equals the initial amount, and the entire remainder of the future value is interest.
Why does a small change in the rate move the result by so much over a long horizon?
Because the rate is applied every single month, and each month's interest itself starts earning interest the following month. Over a handful of years the difference is modest, but over decades this compounding-on-compounding effect widens the gap between, say, a 4% and a 6% assumption far more than the two-percentage-point difference on paper suggests. It is a mechanical property of exponential growth, not a quirk of the calculator.
Does the future value already account for inflation or tax?
No. The figure is a nominal projection: it applies exactly the rate you entered, with nothing subtracted for inflation and nothing subtracted for tax on the interest earned. Both can reduce what the balance is actually worth or what you actually keep, and how much depends on where you live and the type of account, which this tool does not know and does not attempt to guess.
Can I use this to compare two savings or investment offers?
Yes, as long as you run both under the same assumptions - same horizon, same contribution pattern - so the comparison is apples to apples rather than an artifact of different inputs. It is a useful way to see which of two quoted rates leaves you better off mathematically. It cannot tell you which offer is actually available to you, cheaper in fees, or safer, since none of that is part of the calculation.