Calculate final capital, initial capital, interest rate, or duration with the interest calculator. With compound interest, sub-annual compounding, tax calculation, and step-by-step solution.
Available
Inputs
Enter values
Live
Calculate final capitalHow much capital is available after the term?
€
The amount invested today.
€
The desired final amount.
%
The nominal annual interest rate.
Investment period in full years.
Compounding and tax
Additional months for the investment period (0-11).
%
Capital gains tax incl. surcharge (standard: 26.375%).
For illustration only – your own calculation is what counts.
Explanation
How Interest Calculator works
The interest calculator computes final capital, initial capital, interest rate, or duration for lump-sum investments. It supports simple interest and compound interest with various compounding frequencies, as well as optional capital gains tax calculation.
Simple interestK_end = K × (1 + r × t)
K is the initial capital, r the annual interest rate as a decimal, and t the duration in years. With simple interest, only the initial capital earns interest.
Compound interestK_end = K × (1 + r/n)^(n × t)
n is the number of compounding periods per year (e.g., 12 for monthly compounding). Interest is added to the capital after each period.
Effective annual rater_eff = (1 + r/n)^n - 1
This calculator's effective annual rate accounts for compounding frequency; fees and ancillary costs are not included.
The tax calculation deducts the saver's allowance from the gross yield and applies the capital gains tax rate to the remaining amount. The net result shows the yield after taxes.
Knowledge
Understanding interest with confidence
Simple interest, compound interest, and the influence of compounding frequency on yield.
With simple interest, capital grows linearly: the same amount of interest is added each year. With compound interest, it grows exponentially because the interest is added to the capital and earns interest itself in the next period.
Simple interest: K_end = K x (1 + r x t) — linear growth.
Compound interest: K_end = K x (1 + r/n)^(n x t) — exponential growth.
More frequent compounding (monthly instead of annually) slightly increases the yield.
Rule of 72: Divide 72 by the interest rate to get the approximate doubling time in years.
At 6% interest, capital doubles in approx. 12 years (72 / 6 = 12).
Knowledge
Taxes on capital income
Capital gains tax, solidarity surcharge, and saver's allowance in Germany.
In Germany, capital income is subject to a flat capital gains tax of 25% plus 5.5% solidarity surcharge on the tax. This results in a total tax rate of 26.375% (without church tax).
Saver's allowance: EUR 1,000 per person, EUR 2,000 for married couples (since 2023).
Income below the allowance remains tax-free.
The tax is automatically withheld by the bank (withholding tax).
An exemption order ensures that the allowance is considered.
Practice
Practical examples
For illustration only – your own calculation is what counts.
Fixed deposit investment
EUR 10,000 is invested for 5 years at an interest rate of 3.5% with annual compounding. What is the final capital?
Result: After 5 years, the final capital is approx. EUR 11,876.86 with annual compound interest.
Annual compounding€11,876.86
Quarterly compounding€11,903.40
Monthly compounding€11,909.43
Savings account over 2 years
EUR 25,000 is held for 2 years in a savings account at 2% interest with monthly compounding.
Result: After 2 years with monthly compounding, the final capital is approx. EUR 26,019.
2% interest, 10 years€12,189.94
4% interest, 10 years€14,802.44
6% interest, 10 years€17,908.48
Savings goal: How much to invest?
EUR 50,000 should be available in 10 years. The interest rate is 4% with annual compounding. How much must be invested today?
Result: To reach EUR 50,000 in 10 years, approx. EUR 33,780 must be invested today.
Determine interest rate
An investment grew from EUR 10,000 to EUR 12,500 in 5 years. What annual interest rate was achieved?
Result: The achieved annual interest rate is approx. 4.56% with annual compound interest.
Doubling time
EUR 10,000 should double at 5% compound interest. How long does it take?
Result: At 5% compound interest, the capital doubles in approx. 14 years and 2 months.
Short-term investment with tax
EUR 50,000 is invested for 1 year at 3%. Capital gains tax of 26.375% applies, with an allowance of EUR 1,000.
Result: After deducting capital gains tax (minus EUR 1,000 allowance), the net yield is approx. EUR 1,368.13.
Notes
Common mistakes
Confusing nominal rate with effective rate
The nominal rate is the stated interest rate. This calculator's effective rate accounts for compounding frequency, but not fees or ancillary costs. With monthly compounding, it is higher than the nominal rate.
Always use the effective rate for comparisons and consider the compounding frequency.
Underestimating the compound interest effect over long terms
Many savers calculate linearly (simple interest) instead of exponentially (compound interest). At 5% over 20 years, the difference exceeds 65% — the simple model massively underestimates the yield.
Always calculate with compound interest for multi-year investments, as interest is added to the capital and earns interest itself.
Assuming gross interest as net return
Capital income exceeding the saver's allowance (EUR 1,000/person) is subject to 26.375% capital gains tax. Those who assume the gross rate as their actual return overestimate their yield.
Always calculate the net yield after deducting capital gains tax (26.375%) and considering the saver's allowance.
Not considering inflation
A nominal return of 3% with inflation of 2.5% corresponds to an exact real return of about 0.49%. The difference of 0.5 percentage points is only an approximation for small rates.
Calculate the real return for the same period exactly as (1 + nominal return) / (1 + inflation rate) - 1.
Ignoring months in duration calculation
When the term is specified in months, the interest period deviates from full years. Those who simply convert months to years (e.g. 18 months = 1.5 years) without considering the compounding frequency may get incorrect results.
Always enter the exact term in months or convert correctly to years, considering the sub-annual compounding frequency.
FAQ
Frequently asked questions
What is the difference between simple interest and compound interest?
With simple interest, only the initial capital earns interest. With compound interest, the interest is added to the capital and earns interest itself in the next period, leading to exponential growth.
What does sub-annual compounding mean?
With sub-annual compounding, interest is credited more than once per year, e.g., monthly or quarterly. This results in a slightly higher total yield than annual compounding.
How does the compounding frequency affect the yield?
The more frequently interest is compounded, the higher the yield due to the compound interest effect. The difference is most pronounced at high interest rates and long terms.
What is the effective annual rate?
This calculator's effective annual rate accounts for compounding frequency. Fees and ancillary costs are not included. It is higher than the nominal rate when compounding occurs more than once per year.
How is capital gains tax calculated?
Capital gains tax is 25% plus 5.5% solidarity surcharge on the tax, totalling 26.375%. It is levied on capital income exceeding the saver's allowance.
What is the saver's allowance?
The saver's allowance is EUR 1,000 per person (EUR 2,000 for married couples). Capital income up to this amount remains tax-free.
When is compound interest particularly beneficial?
The compound interest effect is stronger the longer the term and the higher the interest rate. Even small interest rate differences lead to large result differences over decades.
Can I also calculate the required interest rate?
Yes. In the 'Calculate interest rate' mode, enter the initial capital, target capital, and duration. The calculator determines the required annual interest rate.
How accurate is the calculation?
The calculation uses exact mathematical interest formulas. It serves as guidance and does not replace individual financial advice or bank conditions.
Limits
Limitations
No account fees considered: The calculator does not account for custody fees, account management fees, or transaction costs that reduce the actual yield.
Constant interest rate over the entire term: The calculation assumes an unchanged interest rate. With variable investments (savings accounts), the rate can change at any time.
No individual financial advice: The results serve as mathematical guidance and do not replace personal advice from a financial or tax advisor.
Simplified tax calculation: Church tax, loss offsetting, and the more favourable assessment rule are not considered. Actual tax liability may differ.
Sources
Sources and references
§ 246 BGB — Gesetzlicher ZinssatzBundesministerium der Justiz
Statutory interest rate of 4% per year for a debt bearing interest, unless a different rate is specified.
View sourceRetrieved: 07/23/2026 · Verified on: 07/23/2026 · Primary source
§ 248 BGB — ZinseszinsBundesministerium der Justiz
Regulation on the prohibition of compound interest for certain contract types and its exceptions in banking.
View sourceRetrieved: 07/23/2026 · Verified on: 07/23/2026 · Primary source
§ 20 EStG — Einkünfte aus KapitalvermögenBundesministerium der Justiz
Tax law basis for the taxation of interest income as capital income.
View sourceRetrieved: 07/23/2026 · Verified on: 07/23/2026 · Primary source
§ 32d EStG — AbgeltungssteuerBundesministerium der Justiz
Regulation of the flat tax rate of 25% on capital income.
View sourceRetrieved: 07/23/2026 · Verified on: 07/23/2026 · Primary source
Deutsche Bundesbank — BasiszinssatzDeutsche Bundesbank
Current and historical base interest rates from the Deutsche Bundesbank per section 247 BGB.
View sourceRetrieved: 07/23/2026 · Verified on: 07/23/2026 · Primary source
Compare interest savings, net returns and liquidity using a transparent break-even point, without equating certain and risky income.
Choose your course
You set the course.
Rechnerpilot counts views and actions anonymously – without cookies and without recognition – and improves through that regardless of your choice. In addition, you can allow Google Analytics, which is loaded only after your consent. You can change your choice at any time.
Your calculation inputs and results are transmitted on neither path.
