Calculating a mortgage installment involves understanding the terms of the loan, including the loan amount (principal), the interest rate, and the loan term (duration). A common method for calculating the monthly mortgage payment uses the formula for a fixed-rate mortgage, which is often:

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Where:

• M is the monthly payment.
• P is the principal loan amount.
• r is the monthly interest rate (annual interest rate divided by 12 months).
• n is the number of payments (loan term in years multiplied by 12 months/year).

This formula calculates the monthly payment that will be consistent over the term of the loan, covering both interest and principal repayment, so that by the end of the term, the entire loan is paid off.

Here is a calculator for example Link