Mortgage Calculator

How the Mortgage Calculator Works — and What Actually Goes Into a Monthly Payment

A mortgage is usually the largest financial commitment a person makes in their lifetime — often two or three times their annual salary, paid over 15 to 30 years. Even a small difference in the interest rate or term translates into tens of thousands of dollars. Understanding how your monthly payment is calculated, and which parts of it actually reduce your debt versus which are just the cost of borrowing, is one of the most valuable pieces of financial literacy you can develop. This calculator breaks the payment down into its components and shows you the long-term cost of the loan.

The formula behind it

The base monthly payment — for principal and interest only — comes from the same formula used for any amortizing loan: M = P × [ r(1 + r)^n ] ÷ [ (1 + r)^n − 1 ]. Here, P is the loan amount (the home price minus your down payment), r is the monthly interest rate (annual rate divided by 12), and n is the total number of monthly payments (years × 12). Borrow $400,000 at 6.5% for 30 years, and the formula produces a monthly principal-and-interest payment of about $2,528. The same loan over 15 years has a much higher monthly payment (about $3,484) but saves more than $260,000 in total interest — that's the power of a shorter term.

What “PITI” means

Lenders and real estate agents often use the acronym PITI to describe the full monthly cost of owning a home: Principal, Interest, Taxes, and Insurance. Principal and interest are the loan payment itself. Property taxes are collected monthly into an escrow account and paid to your local government annually — they typically run 0.5% to 2.5% of the home value per year depending on where you live. Homeowners insurance protects against fire, theft, and liability, and usually costs $1,000 to $2,000 per year. Many buyers also pay HOA fees (for condos, townhomes, or planned communities) and PMI (private mortgage insurance, required if your down payment is less than 20%). Together, these add-ons can increase your monthly payment by 30–50% over the base principal-and-interest amount.

Why amortization favors the lender early on

Mortgage payments are structured so that most of your early payments go toward interest, not principal. On a 30-year $400,000 loan at 6.5%, the first payment allocates roughly $2,167 to interest and only $361 to principal. By year 15, the split is closer to 50/50. By the final year, almost all of the payment reduces principal. This is why extra payments early in the loan have an outsized effect — every additional dollar you put toward principal saves you the interest that dollar would have accrued over all remaining years. A single extra payment per year (13 payments instead of 12) can shave 4–5 years off a 30-year mortgage and save tens of thousands in interest. Our calculator shows the yearly amortization schedule so you can see exactly how this plays out.

Limitations to keep in mind

This calculator assumes a fixed-rate mortgage, which is the most common type but not the only one. Adjustable-rate mortgages (ARMs) start with a lower fixed rate for a set period — often 5, 7, or 10 years — then adjust annually based on market conditions, making future payments uncertain. The calculator also doesn't include closing costs (typically 2–5% of the purchase price), property tax changes (assessments often rise over time), maintenance and repairs (budget 1% of the home's value per year), or mortgage tax deductions (interest on up to $750,000 of mortgage debt is tax-deductible for many filers). For a full picture of homeownership cost, add these factors separately. Use this calculator to compare loan offers and understand the true cost of borrowing — then talk to a mortgage professional before signing anything.