How This Mortgage Calculator Works
This calculator answers the question every homebuyer asks first: what will I actually pay each month? The number a lender advertises — the principal-and-interest payment — is only part of the check you write. In the U.S., most homeowners also pay property tax and homeowner's insurance through an escrow account, plus private mortgage insurance (PMI) if the down payment is under 20%, and HOA dues in many condos and planned communities. This tool adds all of those together so the headline number matches real life.
The core of the math is the standard fixed-rate amortization formula lenders everywhere use:
M = P × r(1+r)n / ((1+r)n − 1)
where
M = monthly principal & interest, P = loan amount (price − down payment), r = monthly interest rate (APR ÷ 12), n = number of payments (years × 12).
Worked example: a $450,000 home with a $90,000 (20%) down payment leaves a $360,000 loan. At 6.5% APR over 30 years (360 payments), the monthly rate is 0.5417% and the formula gives a principal-and-interest payment of about $2,275. Add 1.1% annual property tax ($412/mo), $2,400/year insurance ($200/mo), and no PMI (20% down avoids it), and the true monthly cost is roughly $2,887 — about 27% more than the P&I figure alone. That gap is exactly why this calculator exists.
Each month, your P&I payment is split: part covers that month's interest (balance × monthly rate), and the rest reduces the principal. Early on, most of the payment is interest; over time the split flips. The year-by-year amortization table shows this shift, plus the point where PMI drops off — modeled at 78% loan-to-value, the automatic cancellation point under the federal Homeowners Protection Act.
Practical tips
One extra monthly payment per year on a 30-year loan can cut roughly 4–5 years off the term and save tens of thousands in interest, because early extra dollars skip decades of interest charges.
A 15-year loan at a lower rate saves a fortune in interest, but the higher payment reduces your flexibility. If job security is uncertain, a 30-year loan with voluntary extra payments gives you the optionality.
Taxes, insurance, PMI, and HOA routinely add 25–40% on top of P&I. Budget on the total payment, not the advertised one.
Avoiding PMI saves hundreds monthly, but a bigger down payment also means a smaller loan, less total interest, and a cushion if home values dip.
Multiple mortgage inquiries in a short shopping window count as a single hard inquiry for credit scoring, so get 3–5 Loan Estimates and compare APR — not just the note rate.