Compound Interest Calculator
Put in a starting amount, whatever you add each month, and the return you expect. The calculator shows what the balance becomes — and, more usefully, how much of it is your own money versus interest that arrived on its own.
Compounding is unremarkable for the first few years and then stops being unremarkable. The year-by-year table below is there for exactly that reason: it shows the year your interest starts to outpace your contributions, which is the only milestone in investing that really changes the shape of the curve.
How this is calculated
Interest is applied at the end of each compounding period on the balance at that moment. Contributions land at the end of their period, so a contribution earns nothing in the period it arrives.
The rate is treated as constant, which no real investment is. Nothing here models taxes, platform fees, loan defaults or inflation — this is arithmetic on the numbers you enter, not a forecast or investment advice.
Rounded to whole euros. Figures assume the rate you entered holds for the entire period and that every contribution is made on schedule — a projection, not a promise.
Why the curve bends
The textbook formula is A = P(1 + r/n)^(nt) — starting amount P, annual rate r, n compounding periods per year, t years. The exponent is where everything happens: doubling your rate roughly doubles your interest, but doubling your time horizon does something much larger, because each year's interest becomes principal for every year that follows.
Regular contributions are handled as an annuity on top of that. Each one compounds only for the time it stays invested, so a contribution in year one does far more work than an identical one in year twenty. That is why starting early beats saving harder later, and why the first few years of a plan feel so unrewarding — you are buying exponent, not interest.
Compounding frequency matters less than you think
At 8% a year, switching from annual to monthly compounding lifts the effective yield to about 8.3%. Switching from monthly to daily adds a couple of hundredths of a point. It is worth understanding, but it is not worth choosing a platform over. Your contribution rate and your horizon dominate the result.
What this calculator quietly assumes away
A constant rate, no fees, no taxes, no missed contributions and no inflation. Real crowdlending returns arrive lumpy: loans default, recoveries take months, and interest sits as idle cash until it is reinvested. If you want a number you can plan around, run the projection again at a rate two or three points below the advertised one and treat that as the realistic case.
Keep going
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P2P Lending Calculator & Comparison
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Platform finder
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Taxes by country
What the tax office takes before compounding gets to work on the rest.
Common questions
What is compound interest?
Compound interest is interest earned on interest. Instead of paying your returns out, you leave them invested, so each period the balance that earns interest is larger than the one before. Over long horizons this turns a straight line into a curve.
How is compound interest calculated?
The base formula is A = P(1 + r/n)^(nt), where P is the starting amount, r the annual rate, n the number of compounding periods per year and t the number of years. Regular contributions are added on top as an annuity, each one compounding for the time it stays invested.
Does compounding frequency matter?
Less than most people expect. Moving from annual to monthly compounding at 8% adds roughly a third of a percentage point of effective yield. Your contribution amount and your time horizon move the final number far more than the compounding interval does.
Do P2P lending platforms really compound?
Only if you reinvest. Most crowdlending platforms pay interest into your cash balance, where it earns nothing until it is put back into loans. Auto-invest tools automate that, but idle cash between reinvestments, loan defaults and late payments all mean realised returns come in below the advertised headline rate.