Compound interest calculator for lump sums and savings plans - with tax and inflation view
Work out final amount and interest for a lump sum or monthly savings plan - year-by-year chart, time-to-target answer, German tax and inflation view.
A compound interest calculator answers the question behind every savings account: what does your money grow to when the interest itself earns interest? Run a lump sum or a monthly savings plan, follow the path year by year, switch on German capital gains tax or inflation, and see when you reach a target amount.
You deposit one amount once and let the interest compound over the years.
Interest is credited once per year; mid-year deposits earn simple pro-rata interest. This is how most banks calculate.
Show formula
A = P · (1 + r/n)^(n·t) A = final amount, P = principal, r = annual interest rate, n = compounding periods per year (1 or 12), t = duration in years.
When do you reach your goal?
With these settings you reach €100,000 after 59 years.
Uses your current settings: mode, crediting interval, contribution increase and tax all apply.
Show year table
| Year | Paid in | Interest this year | Balance |
|---|---|---|---|
| 1 | €10,000 | €400 | €10,400 |
| 2 | €10,000 | €416 | €10,816 |
| 3 | €10,000 | €433 | €11,249 |
| 4 | €10,000 | €450 | €11,699 |
| 5 | €10,000 | €468 | €12,167 |
| 6 | €10,000 | €487 | €12,653 |
| 7 | €10,000 | €506 | €13,159 |
| 8 | €10,000 | €526 | €13,686 |
| 9 | €10,000 | €547 | €14,233 |
| 10 | €10,000 | €569 | €14,802 |
Legal note: not legal or tax advice
This calculator produces a simplified projection, not investment or tax advice. The assumptions (constant interest rate, yearly taxation of credited interest, no loss offsetting) are a rough model of reality. Interest rates change, and the German allowance and tax rates may change too. Ask a tax advisor for concrete decisions.
How does compound interest add up?
Compound interest means last year's interest earns its own interest this year. Your balance doesn't grow in a straight line; it accelerates, barely noticeable in the first years, clearly visible after 15 or 20. The longer the duration and the higher the rate, the bigger the share the interest contributes by itself.
Three examples, calculated with annual crediting (monthly for the savings plan):
| Setup | Paid in | Final amount | Interest share |
|---|---|---|---|
| €10,000 once, 4 percent, 10 years | €10,000 | €14,802 | €4,802 |
| €10,000 once, 7 percent, 25 years | €10,000 | €54,274 | €44,274 |
| €100 monthly, 5 percent, 30 years | €36,000 | €83,226 | €47,226 |
The second row makes the point: same starting capital, but at 7 percent over 25 years the interest contributes more than four times what you paid in. The chart shows this as stacked year bars, starting capital at the bottom, contributions above it, the growing interest share on top. The target section below it answers the reverse question: when do you reach, say, €100,000 with your settings? The answer comes in years and months.
What is the compound interest formula?
The base formula is A = P · (1 + r/n)^(n·t). A is the final amount, P your principal (the starting capital), r the annual interest rate as a decimal, n the number of times interest is credited per year (1 for annual, 12 for monthly), and t the duration in years.
A = P · (1 + r/n)^(n·t) €10,000 · (1 + 0.04)¹⁰ = €14,802
For a savings plan the formula alone isn't enough, because new money arrives every month. Each contribution is paid in at the end of the month and starts compounding the month after; the calculator sums this up month by month. With annual crediting it follows common bank practice: mid-year deposits earn simple pro-rata interest, the credit lands at year end and compounds from the next year on. An optional annual increase grows your contribution by a fixed percentage each year, for example alongside salary raises.
Does German tax apply to savings interest?
If you save in Germany, yes. Interest above the saver's allowance is subject to the flat-rate capital gains tax: 25 percent plus the solidarity surcharge, 26.375 percent combined. Church tax pushes the rate to just under 28 percent. Your bank withholds the tax directly, in every year interest is credited.
- Saver's allowance: €1,000 of interest per person and year stays tax-free, €2,000 for jointly assessed couples. With an exemption order (Freistellungsauftrag) on file, your bank applies the allowance at source; without one, tax is withheld first and reclaimed through your tax return.
- Example: €50,000 at 4 percent earns €2,000 interest a year. €1,000 stays free; the rest carries €263.75 in tax (without church tax).
- The deduction slows compounding: what the bank withholds can't earn interest the following year.
Source: Finanzamt NRW on the saver's allowance and exemption order.
The tax mode here works exactly that way: tax comes off the balance year by year, not once at the end. In the chart it appears as a yellow cap on top of the bars, so you keep seeing what's missing from the gross interest. The model is deliberately simplified (no loss offsetting, no personal-rate option) and is not tax advice.
If your monthly contribution goes into an ETF rather than a savings account, the ETF calculator continues the math with fund costs (TER) and German fund tax. For quick percentage questions in between, there's the percentage calculator.
Frequently Asked Questions
What does monthly instead of annual compounding change?
With monthly compounding, one twelfth of the rate is credited every month and starts earning interest immediately. €10,000 at 4 percent grows to €14,908 in 10 years instead of €14,802, about €106 more. The gap widens with higher rates and longer durations, but stays modest at typical savings rates.
What is the difference between the nominal and the real final amount?
Nominal is the number on your account at the end. Real is what that number buys in today's money. At 2 percent inflation, €14,802 after 10 years is worth about €12,143 of today's purchasing power. The inflation view shows both figures side by side without changing the nominal math.