About MortgageMath & our methodology

MortgageMath is a free, independent set of mortgage calculators. We do not sell loans, collect applications or receive referral fees from lenders. Every figure is computed from the formulas below.

Monthly payment

Principal and interest use the fixed-rate amortization formula M = P × r(1 + r)ⁿ ÷ ((1 + r)ⁿ − 1), with r = annual rate ÷ 12 and n = years × 12. Amortization schedules apply each payment to the month's interest first and the remainder to principal.

Taxes, insurance and PMI

Property tax (1.1% of home value per year) and homeowners insurance (0.35% per year) are planning estimates; actual amounts vary widely by state, county and property. PMI is estimated at 0.5% of the loan per year when the down payment is under 20%.

Affordability

Affordability pages follow the 28/36 guideline: housing costs up to 28% of gross monthly income and all debt payments up to 36%. They assume 20% down and a 30-year fixed loan.

Example interest rate

Narrative figures use an example rate of 6.5% so pages stay comparable. It is not a quote or a prediction — every page includes a table across rates and a calculator where you can enter your own.

Disclaimer & contact

Results are educational estimates, not financial advice or a loan offer. Confirm terms with a licensed lender. Spotted an error? Contact us. Last reviewed 2026-10-15.