Compound Interest Calculator 2026
Enter your starting balance, what you can save each month, and the interest rate you expect. The result updates automatically, no account needed.
Estimated future value
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Total contributed
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Total interest
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How compound interest works
Compound interest means the interest earned in one period becomes part of the capital for the next period, and starts earning interest of its own. Over enough years, that difference against simple interest (where only the original balance earns anything) becomes large, even at the same rate.
This calculator assumes monthly compounding: the annual rate you enter is divided by 12 and applied to the running balance every month. If you choose "end of the month" (the common case for savings accounts and monthly ETF contributions), that month's interest is calculated before your contribution lands, so the contribution only starts earning interest the following month. "Start of the month" means the contribution counts toward that same month's interest.
Why starting early matters more than the amount
In a compound-interest projection, time matters more than the monthly amount. Doubling your monthly contribution can at most double the portion that comes from your own deposits; doubling the number of years can multiply the final result by far more than that, because every euro invested earlier passes through more compounding cycles. That's why "start now, with less" usually beats "wait and start with more" later on.
The rule of 72
A quick way to estimate how long a lump sum takes to double (with no further contributions) is to divide 72 by the annual interest rate. At a 4% rate, for example, 72 ÷ 4 = 18 years. It's an approximation, but close enough for quick decisions, and a handy shortcut for comparing different interest rates against each other.
What this calculator doesn't do
It doesn't account for tax on the interest earned (rules and rates vary a lot by country and by financial product), doesn't adjust for inflation, and assumes a constant interest rate for the whole period, which real, volatile markets rarely deliver. It's meant to give you a clear picture of the mechanics of compound interest, not to predict the exact value of a real investment.
Frequently asked questions
Does this calculator account for taxes on my interest?
No. Tax treatment of interest and investment gains varies a lot by country and by financial product, so this calculator shows the gross growth before any tax. Your actual take-home return will be lower once tax is applied.
What's the difference between contributing at the start or end of the month?
It changes whether that month's contribution earns interest in the same month it's added. With "end of the month" (the more common default for savings accounts and monthly investment contributions), that month's interest is calculated first, and your contribution only starts earning interest the following month. With "start of the month", the contribution is added before interest is calculated, so it earns interest immediately.
What is the rule of 72?
A quick way to estimate how many years it takes a lump sum to double, with no further contributions: divide 72 by your annual interest rate. At 4%, that's 72 ÷ 4 = 18 years. It's an approximation, but useful for comparing different rates quickly.
Why does starting earlier matter more than contributing more?
Because every euro invested earlier passes through more compounding cycles. Doubling your monthly contribution can at most double the portion of your final balance that comes from your own deposits, but doubling the number of years can multiply the final result by far more than that.
Does this assume a constant interest rate every year?
Yes. It applies the single annual rate you enter, divided by 12, every month for the whole period. Real markets are volatile and rarely deliver a constant return, so treat the result as a model of how compounding works, not a prediction of an actual investment's future value.
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