Compound interest calculator

Compound interest in three directions

The same core answers three questions: what becomes of a savings instalment? How long does capital last with monthly withdrawals? And which instalment reaches a target amount? The third question is inverted numerically — on the verified forward calculation, not through a second formula that might deviate from it.

Flat-rate tax (Abgeltungsteuer) and the church tax formula

Investment income attracts a 25 % flat-rate tax (Abgeltungsteuer) plus the solidarity surcharge. Where church tax applies, a special formula takes effect: the church tax reduces the base of the Abgeltungsteuer, so the two influence each other. The calculator resolves that using the statutory formula.

Taxed every year — or once at the end

Interest on instant-access and fixed-term deposits or a bank savings plan is taxed by the bank every year: the saver's allowance is renewed each year, and only the net interest goes on earning interest. That is the calculator's default. If the whole gain is paid out in one sum at the end, only ONE allowance is set against it — the tax is then markedly higher, although the full gross amount earns interest until then. You can choose either case; “final capital” and the chart always show the course before tax, the row “final capital after tax” the case you chose.

The Vorabpauschale concerns funds, not interest

With accumulating funds a minimum yield is taxed annually even without a distribution — calculated from the Bundesbank base rate and capped at the actual increase in value. Equity funds additionally enjoy a partial exemption.

Example: reaching 100.000 € in 15 years at a 5 % return

Instalment required per month	374,13 €
Capital reached with it	100.000,81 €
Total paid in	67.343,40 €

Roughly a third of the closing capital comes from interest. The instalment sits just above the mathematical minimum, because the arithmetic works in whole cents and what is sought is the figure that genuinely holds the target.

What this calculator does not cover

Pretending to be complete is the costliest mistake a financial calculator can make. These points are deliberately absent — either because they need details this page does not ask for, or because they cannot be substantiated in general terms:

• Product costs — entry charges, management fees, custody costs. They erode the return considerably and are not included here.
• Fluctuation. The calculation uses a constant rate. Real markets fluctuate, and with withdrawals the order of returns makes a large difference.
• The return itself is an input, not a promise. No calculator knows the future.

Related calculators

• Daily luxury — what small amounts would have come to over decades
• Loan calculator — the same interest arithmetic with the sign reversed
• For the self-employed — anyone who must provide for themselves needs this calculation first