How Mortgage Payments Are Calculated: A Full Breakdown

A mortgage payment isn't the loan amount divided evenly by the number of months — if it were, the last payment would clear the same amount of principal as the first, and lenders would earn far less interest. Instead, mortgages use an amortization formula that front-loads interest and back-loads principal.

The formula

M = P × [r(1+r)ⁿ] / [(1+r)ⁿ − 1]

• P is the loan principal
• r is the monthly interest rate (annual rate ÷ 12)
• n is the total number of monthly payments (years × 12)

Worked example

A $300,000 loan at 6.5% annual interest over 30 years: r = 0.065/12 ≈ 0.0054167, and n = 360. Plugging in, the monthly payment comes out to $1,896.20. Over the full 360 payments, total payments equal $682,633.47 — meaning $382,633.47 of that, more than the original loan amount itself, is interest.

Why the first payment is mostly interest

In month one, interest is charged on the full remaining balance ($300,000), which at 6.5% annual (0.5417% monthly) works out to $1,625 of interest in that single payment — leaving only $271.20 of the $1,896.20 payment to reduce principal. By contrast, in the loan's final month, almost the entire payment reduces principal, since so little balance remains to charge interest against. This is why paying extra toward principal early in a mortgage saves disproportionately more interest than doing so later.

A shorter-term comparison

Halve the term to 15 years (n=180) at the same rate and principal, and the monthly payment rises to roughly $2,614 — noticeably higher — but total interest paid drops to around $170,500, less than half of the 30-year total. Shorter terms trade higher monthly payments for dramatically less lifetime interest, which is the fundamental trade-off every refinance-vs-keep decision comes down to.

Common mistakes to avoid

• Forgetting that this formula covers principal and interest only — property tax, homeowners insurance, and PMI are separate and often bundled into the actual payment you send each month
• Using the annual rate directly instead of dividing by 12 first, which produces a wildly wrong monthly figure
• Assuming a bi-weekly payment plan simply halves the monthly payment — it actually results in 26 half-payments per year (13 full payments instead of 12), which is what accelerates payoff, not the smaller payment size itself

Try the numbers on your own loan with the mortgage calculator, or model whether refinancing makes sense with the refinance calculator.