Compound interest calculator
Work out what a starting amount and a monthly deposit grow to at a fixed interest rate, with interest reinvested or paid out. Built for P2P lending, useful for any fixed-rate saving.
Balance after 10 years
€ 20.514
You paid in
€ 13.000
Interest earned
€ 7.514
Tip: use the rate you actually expect to keep, not the advertised one — the net return calculator works it out. Open net return calculator
These calculators are illustrations, not forecasts or investment advice. P2P loans can lose money, returns are not guaranteed, and these investments are not bank deposits.
How to use it
- Enter your starting amount — what you already have, or 0 if you start from nothing.
- Enter a monthly deposit, the amount you add every month.
- Enter the interest rate you expect, per year.
- Choose the number of years.
- Pick reinvest interest (interest earns interest) or pay out interest (you spend it).
The chart shows each year's balance split into what you paid in and what the interest added. “Show as table” gives the same numbers year by year.
What compound interest actually is
Compound interest is interest earned on interest already received. Simple interest always pays on your original amount; compound interest pays on the amount as it grows, which is why the curve bends upwards the longer you leave it.
This calculator compounds monthly, which matches how most P2P platforms pay: interest arrives every month and, if you reinvest, starts earning from the next month.
The maths behind it:
Final balance = P × (1 + i)ⁿ + M × (((1 + i)ⁿ − 1) ÷ i)
- P — starting amount
- M — monthly deposit
- i — yearly rate ÷ 12
- n — months (years × 12)
A worked example. Start with €1,000, add €100 a month, at 8% a year for 10 years. You pay in €13,000 (€1,000 + 120 × €100) and finish at about €20,514 — roughly €7,500 of it interest. At 4% you would finish near €16,200; at 12% near €26,300. Time and rate both matter, but time compounds.
Reinvesting versus taking the interest out
Switch between the two options and the difference is stark. On the example above, reinvesting produces around €7,510 of interest over ten years; taking the interest out as it arrives produces around €5,560, because each euro of interest stops working the moment it leaves.
On most P2P platforms reinvesting is the default: auto-invest puts repayments and interest straight back into new loans. If you need the income, paying out is a legitimate choice — just be aware that you are trading the compounding effect for cash today.
One P2P-specific catch: money only compounds while it is invested. Repayments that sit in your account waiting for a matching loan earn nothing. That gap is called cash drag, and it is one reason real portfolios lag the calculator.
Why a P2P portfolio rarely follows the curve
This calculator assumes a fixed rate, every payment on time, and nothing taken off. Real P2P lending differs in four ways:
- Defaults. Some borrowers or lending companies do not repay. Losses reduce the rate you actually earn.
- Cash drag. Uninvested money earns nothing, and after a repayment it can take days to be reinvested.
- Fees. Some platforms charge an annual fee or a secondary-market fee.
- Tax. Interest is normally taxable where you live. In Germany that is usually 26.375% including the solidarity surcharge.
Put your advertised rate through our net return calculator first, then bring the result back here — the projection will be far closer to reality. And before committing money to any platform, read our risk page and the platform comparison.
FAQ
- What is compound interest?
- Interest that is added to your balance and then earns interest itself. Over long periods it is the main driver of growth: in the ten-year example on this page, more than a third of the final balance is interest rather than deposits.
- Does this calculator include monthly deposits?
- Yes. Enter any monthly amount and it is added at the end of each month, then earns interest from the following month. Set it to 0 to see a single lump sum grow on its own.
- Monthly or yearly compounding — does it matter?
- A little. At 10% a year, monthly compounding gives an effective rate of about 10.47%, because each month's interest starts earning immediately. This calculator uses monthly compounding, which is how most P2P platforms pay.
- Does interest on P2P platforms compound automatically?
- Only if you reinvest. Most platforms offer auto-invest, which reinvests repayments and interest for you. Bondora's Go & Grow adds interest to the balance daily. If you withdraw interest as it arrives, nothing compounds.
- What interest rate should I use?
- Not the advertised one. Use the rate you expect to keep after defaults, fees, uninvested cash and tax — our net return calculator works that out. Advertised P2P rates of 10–14% often leave considerably less.
- Is my data saved?
- No. The calculation runs in your browser. Nothing you type is sent to us or stored.