The Complete Guide to Mortgage Payments and What Determines Them
A mortgage payment is calculated from four inputs — loan amount, interest rate, and loan term determine your principal & interest, using a standard amortization formula. Property tax, homeowners insurance, and (if your down payment is under 20%) PMI are added on top to get your full monthly payment.
Most people find out what their mortgage actually costs the hard way: an online estimate quotes one number, and the number on the loan estimate from an actual lender is a few hundred dollars higher. The gap is almost never a mistake — it's usually property tax, insurance, and PMI, three costs that don't show up in a bare principal-and-interest quote.
This guide breaks down exactly what goes into a mortgage payment, in the order it actually gets added, so the number you calculate matches the number your lender eventually quotes.
The four things that set your principal & interest
Principal & interest — often written "P&I" — is the core of every mortgage payment, and it's set by exactly four numbers: the home price, your down payment (the two together give you the loan amount), the interest rate, and the loan term.
The math behind it is a standard amortization formula: the lender calculates a fixed monthly payment that, if paid every month for the full term at the given rate, pays off the loan to exactly zero on the last payment. Early payments are mostly interest; later payments are mostly principal — the balance owed shrinks slowly at first and quickly near the end.
As a real example: a $320,000 loan (say, a $400,000 home with a $80,000 down payment) at 6.5% over 30 years works out to $2,022.62 a month in principal and interest, and $408,142.36 in total interest paid over the full 30 years — more than the loan amount itself. That figure isn't an approximation; it's the exact output of the standard mortgage formula used industry-wide, and you can reproduce it with the Mortgage Calculator.
What gets added on top: taxes, insurance, and PMI
Property tax is billed by your local government, typically as a percentage of your home's assessed value, and most lenders collect a monthly share of it in advance rather than letting you pay one large annual bill. Rates vary enormously by location — often anywhere from under 0.5% to over 2% of home value per year.
Homeowners insurance works the same way: your lender usually requires it and collects a monthly portion. On that same $400,000 home, a 1.1% property tax rate and $1,400 a year in insurance add roughly $483 a month on top of principal & interest — bringing the full payment to about $2,506.
PMI — private mortgage insurance — only applies if your down payment is below 20% of the home price. It protects the lender, not you, and typically costs 0.5% to 1.5% of the loan amount per year until you've built up enough equity to have it removed. It's the one cost on this list that a larger down payment eliminates entirely.
Why the term you choose changes more than the monthly number
A 15-year loan and a 30-year loan on the same $320,000 balance at the same rate produce very different pictures. The 30-year loan's lower monthly payment comes at the cost of paying far more in total interest over the life of the loan, simply because interest has three times as long to accrue.
There's no universally correct choice — a 30-year term keeps monthly cash flow flexible, while a 15-year term builds equity faster and costs less overall. The right comparison is to run both terms through a calculator with your actual numbers rather than relying on a rule of thumb.