What this compound interest calculator does
Enter a starting amount, a regular deposit, a rate and a term: the calculator shows the final balance, the total paid in and the interest earned, plus a year-by-year table and a bar chart. Deposits can be monthly or yearly, paid at the start or the end of each period, and interest compounds yearly or monthly. Optionally it converts the result into today's purchasing power. Everything runs locally in your browser.
How is compound interest calculated?
Interest is credited to the balance and earns interest itself from then on. Without deposits:
K = K₀ × (1 + i)ⁿ
where i is the rate per period and n the number of periods. €10,000 at 5% over ten years becomes €16,288.95. Of the €6,288.95 in interest, €5,000 comes from simply paying 5% on the starting amount — the remaining €1,288.95 is interest on interest. That share is what grows disproportionately with time.
What does paying at the start or the end of the period mean?
A deposit at the start of the period sits in the account for the whole period and earns accordingly. One at the end earns nothing in that period. With yearly compounding and twelve monthly deposits this gives the classic equivalent annual deposit:
| Timing | Equivalent annual deposit |
|---|---|
| start of period | R = r × (12 + 6.5 · i) |
| end of period | R = r × (12 + 5.5 · i) |
With r = €100 and i = 6% that is €1,239 versus €1,233 in the first year — from an identical €1,200 paid in. Over twenty years the gap widens accordingly.
Yearly or monthly compounding?
It depends on the product. Savings accounts usually credit interest once a year; many loans and some funds compound monthly. At the same nominal rate, monthly compounding gives a slightly higher final balance because the interest starts working sooner: €10,000 at 5% over ten years becomes €16,288.95 with yearly compounding and €16,470.10 with monthly. The tool uses the nominal rate divided by twelve for that.
What does the inflation adjustment do?
It divides the final balance by (1 + inflation)ⁿ, showing what the amount is worth in today's purchasing power. For scale: Germany's inflation rate averaged 2.2% in 2025, exactly the same as in 2024 (source: Federal Statistical Office, press release of January 2026). At 2% over 20 years, €100 nominal is worth about €67 in real terms.
If inflation and the interest rate are equal, you end up in real terms with exactly the starting amount — the money lost nothing, but gained nothing either.
Honest limits
- The calculation is gross: no withholding tax, no allowance, no account, custody or fund fees, no product charges.
- A constant rate over the whole term is a modelling assumption. Instant-access savings rates move all the time, and shares and funds have no interest rate at all, only fluctuating returns — an average says nothing about any particular path.
- The tool does not round to the cent per booking but computes through with exact decimal arithmetic, so a bank statement can differ by a few cents.
- Terms are rounded to whole years.
Privacy
Amounts, rate and term stay on your device. The calculation runs entirely in the browser and your entries live in local storage only — no server learns anything about your finances.
A model calculation for orientation, not investment advice. Which product suits you depends on your situation, your time horizon and how much risk you can carry.