How the payment is calculated
The payment comes from the standard annuity formula: the loan times the periodic rate, divided by one minus (1 + rate) to the power of minus the number of payments. The periodic rate is the yearly rate divided by the number of payments per year, which is how banks quote a nominal rate. At 0% the loan is simply split into equal parts.
Each payment first covers the interest on the open balance, rounded to the cent, and the rest repays the loan. Since the payment is rounded too, the last payment is a little higher or lower so the balance lands on exactly zero. The effective rate shown next to the result is the nominal rate compounded over a year. It leaves out fees, so the rate in your loan contract can be higher.
Initial repayment rate
Mortgages in Germany and Austria are often agreed as an interest rate plus an initial repayment rate, for example 4% interest and 2% repayment. The yearly payment is then 6% of the loan, split over the payments of the year. Pick Repayment rate under Term to enter a loan this way, and the tool works out how long it runs.
Fixed-rate periods and extra payments
Many mortgages fix the rate for 5, 10 or 15 years only. Enter that period and the result shows what you still owe when it ends, which is the amount you need to refinance. The schedule marks the payment where it happens.
Extra payments and special repayments go straight to the principal and never past what is still owed. A special repayment is paid together with the first regular payment on or after its date. The savings compare the same loan with and without them, and the chart draws the balance without them as a dashed line.