How Mortgage Calculations Work
A mortgage payment is rarely just "loan amount divided by months." Lenders build it from several moving parts, and understanding each one helps you see exactly where your money goes every month.
The Core Formula: Principal and Interest
The foundation of every mortgage payment is the amortization formula, which spreads your loan balance and interest evenly across every monthly payment for the life of the loan:
Here, P is your loan amount (home price minus down payment), r is your monthly interest rate, and n is your total number of payments. Early payments are mostly interest; later payments are mostly principal — even though the total payment stays the same.
Property Tax and Insurance
Most lenders also collect property tax and homeowners insurance as part of your monthly payment, holding the funds in an escrow account and paying the bills on your behalf when they're due. This means your true monthly cost is usually higher than the principal-and-interest figure alone.
Why Extra Payments Save So Much
Because interest is calculated on your remaining balance, extra payments toward principal early in the loan reduce the interest charged for every remaining month — which is why even small extra payments can shave years off a 30-year mortgage.
Try It Yourself
Use our Mortgage Calculator to see a full breakdown of principal, interest, taxes, and insurance for your own numbers.