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How to calculate mortage payment

how to calculate mortage payment


Can I pay other than monthly?

Yes, go into Settings. and touch Payment Frequency. You can choose from weekly / bi-weekly up to annual payments

What about Interest-only?

Go into Settings. and touch Term. You can choose to enter the term in months.

Can you translate the calculator into another language?

Yes! If you can help supply the translation, I can add support for more languages.

Reverse calculations

The checkboxes on the left of the input boxes determine whether principal, interest, years or repayment is calculated. By default, the repayment checkbox is ticked, meaning that you can enter data in the principal, interest and years boxes and the repayment will be calculated. Clicking on another checkbox, e.g. principal checkbox, allows you to modify years, interest and repayment to see the principal value calculated as these other values change.


The expenses figure shows the proportionate sum of tax, insurance, HOA and other expenses payable for the period being calculated.

Extra payments

Extra payments can be made on a monthly, annual or one-time basis. They take effect when a valid corresponding month number is given. Enter 1 for the first month, 2 for the second month etc. Entering 0 prevents the extra payment from taking effect.

Interest rates

Enter the nominal interest rate not an APR.

Add five extra interest rates giving the start and end month numbers to indicate when the interest rate is active. Months outside these ranges will use the main interest rate.

For an interest-only period of a loan, enter a start and end month in the interest-only payments section. This will specify a period where no principal is paid,

Loan Settings

Payment frequency - most mortgages are paid monthly. If you change the payment frequency, you should probably change the compound period to match the payment frequency (exceptions are Canadian and some UK mortgages).

Term in years or term in months.

Compound period - monthly is the standard compound period for amortization. This matches monthly payments. In Canada, payment frequency can be monthly but semi-annual compounding is used. In the UK, payments can be made monthly, but some Building Societies still use an annual rest system whereby interest is calculated once per year and the monthly payment is one twelfth of the annual payment due.

Select what impact extra payments have: the default is to reduce term when extra payments are made, but the alternative is to keep term unchanged and reduce the monthly payment instead.

Rounding. no rounding means that full values are stored internally and rounding is only used when displaying values. Rounding means that every value stored internally is rounded to two decimal places using the selected rounding method. These internal values will match the displayed value. In this case, the final payment may be increased to compensate for rounding errors over the life of the loan.

Initial Payment Date - Beginning of period uses the first day of the next month. End of period uses the first day of the month after the next month. End of period is the default for most mortgages.


Karl's Mortgage Calculator uses a standard amortization formula commonly used around the world. The figures generated by the calculator should therefore approximate what your lender will charge you, but you should always contact your lender for exact details on how and when they charge interest on outstanding principal balances.

Category: Bank

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