How a mortgage payment is calculated
A fixed-rate mortgage charges the same interest rate for the whole loan term, with a level monthly payment split between principal (paying down what you borrowed) and interest (the lender's charge for lending it). Early in the loan, most of each payment goes toward interest; later on, more goes toward principal — even though the total payment amount doesn't change.
The formula, in plain language
M = P × [r(1 + r)^n] / [(1 + r)^n − 1]
- M — the monthly principal & interest payment
- P — the loan amount (home price minus down payment)
- r — the monthly interest rate (annual rate ÷ 12, as a decimal)
- n — the total number of monthly payments (loan term in years × 12)
Worked example
Suppose you buy a $400,000 home with $80,000 down (20%), a 30-year term, at a 6.5% annual rate:
- Loan amount: $400,000 − $80,000 = $320,000
- Monthly interest rate: 6.5% ÷ 12 = 0.5417%
- Number of payments: 30 × 12 = 360
- Monthly principal & interest: $2,022.62
- Total paid over 30 years: $728,142.36
- Total interest paid: $408,142.36 — more than the loan amount itself
Try these exact numbers in the calculator above to see the full amortization schedule.