Calculate loan costs
Enter a loan amount, effective rate and term to model your monthly repayment, interest cost and total amount – right in your browser, with no data transfer.
On this page
The calculator uses a simplified monthly annuity formula. A lender’s individual fees, payout dates and rounding may differ.
The formula behind it
The calculator uses the monthly annuity formula. From the effective annual rate p, a monthly rate is first derived, exponentially: i = (1 + p/100)^(1/12) − 1 This gives the constant repayment over n months: Repayment = K · i / (1 − (1 + i)^(−n)) The total amount is repayment × n; the interest cost is the total amount minus the loan amount. Exactly this formula also underlies the interest card on the home page — there is only one model on this site.
Why the effective rate goes in, not the borrowing rate
The exponential conversion is not a detail: it matches the method by which the effective annual rate is defined under the Price Indication Ordinance (Preisangabenverordnung). If you enter the effective annual rate here, the model fits. If you accidentally enter the borrowing rate (Sollzins), the result comes out too low — because the effective rate also includes the mandatory credit-related costs. The figure you want to compare appears in the offer as the “effective annual rate” (effektiver Jahreszins).
Test two terms
Calculate the same sum once over a short and once over a long term. The monthly repayment falls, the total amount rises — and the ratio between those two movements is the real decision. The short term is only the better one if you can carry the higher repayment in a bad month too. So check it against the repayment headroom from the budget calculator, not against wishful thinking.
What the model does not cover
The model calculates with a constant repayment, a constant interest rate and a payout at the start. It does not account for processing fees outside the effective rate, payment protection insurance, different payout or due dates, or rounding to whole cents per instalment. A lender’s offer may therefore differ slightly. For comparing two offers this is irrelevant — for the last decimal place of your future repayment, the loan agreement is what counts.
- Constant repayment and fixed interest rate
- Payout at the start of the term
- No payment protection insurance included
- No rounding per individual instalment
Frequently asked questions
Why does the interest cost rise with a longer loan term?
With otherwise identical conditions, the outstanding balance is repaid more slowly. Interest is therefore charged for longer, even though the monthly repayment falls. So in the calculator, compare the monthly repayment and total interest cost together.
How is the monthly repayment of an instalment loan calculated?
It is based on the monthly annuity formula: the loan amount, interest rate and term produce a constant repayment in which the interest portion falls and the capital portion rises. The repayment multiplied by the number of instalments gives the total amount. Individual lenders may show slightly different figures because of fees, payout dates and rounding.
Are my inputs stored or transmitted?
No. The calculator runs entirely in your browser; amounts, interest rates and terms are not sent to Thinkstars GmbH or to third parties and are not stored permanently. When you reload the page, the fields are reset to their starting values.
Is the result a binding loan offer?
No. The result is a model calculation based on the values you enter and does not replace an offer, a lending decision or individual advice. Only the pre-contractual information and the loan agreement from your lender are binding.
Selected sources
Your next steps
Maximum repayment
Combine income, spending, current repayments and a safety reserve into a monthly amount that is free on paper – locally in your browser.
Read moreEffective rate
Telling the borrowing rate, the annual percentage rate and the total amount payable apart – and knowing which costs the Price Indication Ordinance includes in the calculation.
Read moreRepresentative example
The yardstick is not the “from” rate but the representative example: the advertiser must be able to expect at least two thirds of contracts to achieve or beat its APR.
Read moreLoan comparison
Set the loan amount and term and compare here using the embedded Verivox comparison. Effective rate, repayment and total amount at a glance.
Read more