How a mortgage works, in plain terms

A mortgage looks like one thing from the outside (a monthly payment) and is actually two things underneath: a loan, which is a promise to pay money back, and a lien, which is a claim the lender has on the property if the loan isn't repaid.1 The loan part is what people usually mean when they say "mortgage." The lien part is what makes the deal work for the lender, because it means the lender can foreclose on the house if the loan goes unpaid.

The borrower receives a lump sum from the lender (the principal) and agrees to pay it back over a set term, typically 15 or 30 years, with interest charged each month on the outstanding balance.

Where each monthly payment actually goes

A standard fixed-rate mortgage has a constant monthly principal-and-interest payment. The composition of that payment changes over time, even though the dollar amount doesn't. In the early years, most of each payment is interest, because the outstanding balance is still large. In the later years, most of each payment is principal, because there's less balance to charge interest against. The line where they cross is called the crossover point, and on a 30-year loan at typical rates it lands roughly halfway through the term. The amortization visualizer at /tools/amortization shows this curve for any loan size and rate.

A typical mortgage payment also includes property tax (held in escrow by the lender and paid to the local taxing authority twice a year), homeowners insurance (also escrowed and paid annually), and, when the borrower puts down less than 20%, private mortgage insurance (PMI). Together these are sometimes called PITI: principal, interest, taxes, insurance.

The Loan Estimate is the document that matters

When a buyer applies for a mortgage, federal law requires the lender to send a Loan Estimate within three business days.2 This is a standardized three-page form that shows the loan amount, interest rate, monthly payment, estimated closing costs, and a comparison of total costs over the first five years across competing offers. The Loan Estimate is the apples-to-apples document for comparing lenders. Quotes that don't show up there (rate without points, payment without taxes, "out-the-door" cost without escrow setup) are usually not the same number.

Three days before closing, the lender sends a Closing Disclosure with the actual final numbers. Comparing the Loan Estimate to the Closing Disclosure is how a buyer catches changes that should have been disclosed earlier.

Rate is one number among several

A 7.0% mortgage rate sounds higher than 6.5%. It is. But the rate is only one of the costs the lender is charging. A loan with a slightly higher rate and zero points can cost less over time than a loan with a lower rate and two points paid up front, depending on how long the borrower keeps the loan. A point is a fee equal to 1% of the loan amount, paid at closing in exchange for a lower rate.

The APR (annual percentage rate) on the Loan Estimate folds in the rate plus the points and most lender fees, expressed as a single yearly rate. APR is closer to the true cost of the loan than the headline rate, though it has its own quirks (it assumes the borrower keeps the loan for the full term, which most borrowers don't).

What a lender is actually checking

Mortgage underwriting looks at four things: credit (history of paying debts back on time), capacity (income relative to total debts), collateral (the property itself, verified by appraisal), and cash (down payment plus reserves).3 The numerical thresholds vary by program (conforming conventional loans typically require a 620+ credit score, FHA goes lower, VA has its own standards) but the four buckets are what every underwriter is checking against.

The most consequential of the four for most borrowers is the debt-to-income ratio (DTI). Lenders typically want total monthly debt payments, including the new mortgage payment, to land below about 43% of gross monthly income for a conforming loan, with some flexibility above and below depending on compensating factors. A buyer with a 30% DTI has more room than one at 42%, even if the credit score and down payment are identical.

Rates aren't constant and they aren't predictable

The 30-year fixed rate published every Thursday by Freddie Mac is an average, not a quote. Individual lenders price differently based on their own funding costs and risk appetite. The published average has moved meaningfully even within single quarters in recent years.4 A rate quoted today is usually only good for a specific number of days unless the borrower locks it, which is itself a financial decision (a lock has a cost, expressed either as a slightly higher rate or as a fee).

The honest position on rate forecasts is that nobody (not lenders, not economists, not the Fed itself) has been reliably right about where 30-year mortgage rates would be 12 months from any given date. Long-term planning around rates is mostly an exercise in modeling sensitivity rather than predicting a number.