Interest Calculator

Calculate interest for any time period. Enter payments and interest rates and choose your preferred calculation method.

Start date

End date

Method

Interest settlement

Nr.	Amount (€)	Description	Date
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Nr.	Amount (€)	Description	from	until
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Nr.	Amount (€)	Description	from	until
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Nr.	Rate (%)	effective from	ECB+
1.	%
2.	%
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4.	%
5.	%
6.	%
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10.	%

ECB+: Amount is added to the current ECB base rate (e.g. 5 = ECB+5% for default interest).

Date	Transaction	Payment (€)	Interest (€)	Capital (€)

Overview

The interest calculator computes interest and capital development for any time period. You can enter lump-sum payments, annual payments, monthly payments and individual interest rates (including the ECB base rate). The result shows a complete table of all transactions with the running capital balance.

Step by Step

Enter the start and end date in the format DD.MM.YYYY (e.g. 01.01.2024 to 31.12.2024).
Choose the calculation method (see below).
Choose the interest settlement (see below).
In the Interest Rate tab, enter at least one rate with a date.
Optionally enter payments in the Lump-sum, Annual or Monthly tabs.
Click “Calculate interest”.

The Four Tabs

Tab 1 – Lump-sum Payments

Each row represents a one-time payment on a specific date. Negative amounts are withdrawals (e.g. -500). The date must fall between the start and end date.

Field	Example	Meaning
Amount (€)	1,000.00	Deposited or withdrawn amount (negative = withdrawal)
Label	Initial deposit	Free text shown in the result table
Date	15.03.2024	Date of the transaction

Tab 2 – Annual Payments

For regular yearly payments (e.g. annual premium, savings contribution). The payment is booked from the “from” date every year on the same day, as long as the “to” date (or end date) is not exceeded. If the date falls on 29 Feb, the 28th is used in non-leap years.

Field	Example	Meaning
Amount (€)	500.00	Amount of the annual payment
from	01.01.2024	First payment date
to	31.12.2028	Last possible payment date

Tab 3 – Monthly Payments

For recurring monthly amounts (e.g. savings rate, rent, repayment). The payment is booked monthly on the same day. For end-of-month dates (e.g. the 31st), it is automatically adjusted to the last day of the respective month.

Field	Example	Meaning
Amount (€)	100.00	Monthly amount
from	01.01.2024	First payment date
to	31.12.2024	Last possible payment date

Tab 4 – Interest Rate

Here you define which interest rate applies from which date. Up to 10 different rates can be stored for different periods. The last valid rate before the calculation date is used.

Field	Example	Meaning
Interest rate (%)	3.50	Annual rate in percent
valid from	01.01.2024	This rate applies from this date
ECB+	☑	The entered value is added to the current ECB base rate (§ 247 BGB)

ECB example: ECB base rate is 2.90 %, you enter 5 and enable ECB+ → effective rate = 7.90 % (statutory default interest under § 288 BGB for consumers).

Calculation Methods

Method	Day count	Year basis	Typical use
German (30/360)	Each month = 30 days, year = 360 days	360	German bonds, loans, savings contracts
English (act./365)	Actual calendar days	365	Anglo-American practice, money market

Formula: Interest = Capital × Rate% ÷ Basis × Days

Interest Settlement

Option	Meaning
Annual compound (compounding)	Interest is calculated annually on 31 Dec and added to the capital. The new capital earns interest in the following year.
Annual interest (simple)	Interest is shown on 31 Dec but not added to the capital. No compounding effect; useful for damages and default interest calculations.
Monthly compound (compounding)	Interest is added to the capital monthly (at end of month). Highest compounding effect, typical for building savings contracts.

Result Table – Colour Legend

Green – Payment (deposit or withdrawal)
Blue – Interest posting (compound / monthly)
Orange – Annual / final settlement
Purple – Interest rate change

PDF Export

After the calculation, the button “Create PDF” appears in the result area. The generated PDF contains all settings, a summary with final capital and total interest, as well as the complete transaction table. It can be viewed directly in the browser or downloaded.

Practical Examples

Calculate default interest (§ 288 BGB)

Start date = due date of the claim
End date = today’s date
Tab Lump-sum payments: enter the claim amount
Tab Interest rate: enter 5, enable ECB+, date = start date
Method: German (30/360) · Settlement: Annual interest (simple)

Simulate a savings account

Start date = account opening, end date = desired target date
Tab Lump-sum payments: enter opening balance on start date
Tab Monthly payments: monthly savings rate, from/to = entire period
Tab Interest rate: enter current savings rate
Settlement: Annual compound (compounding)

Negative ECB base rate

The ECB base rate can also be negative (as it was from 2013–2022). In this case, ECB+ reduces the effective interest rate. If the total rate becomes negative, no interest is calculated (the calculator sets it to 0 internally).