How a mortgage payment actually works
Your monthly payment has two parts that behave very differently. Interest is the annual rate divided by twelve, times whatever you still owe. Principal is the part that shrinks your balance. Early in a 30-year loan, interest dominates: on a $340,000 loan at 6.75%, the first payment sends about $1,913 to interest and only $293 to principal. Each month the balance drops a little, the interest slice shrinks, and the principal slice grows. That shifting split is called amortization, and the schedule above shows it playing out year by year.
What moves your monthly payment
Four things matter, roughly in this order: the loan amount, the interest rate, the term, and the add-ons. Rate is the quiet giant. A single percentage point on a $340,000 loan changes the payment by about $230 a month, roughly $80,000 over 30 years. Term is the classic trade: a 15-year loan at the same rate raises the monthly payment by about a third but cuts total interest by more than half. Then come the add-ons that surprise first-time buyers. Property tax and home insurance are usually escrowed, so your lender collects a twelfth of each bill monthly. PMI appears when your down payment is under 20%, and HOA dues sit outside the loan but are very real in the budget.
Why extra payments punch above their weight
Extra principal payments attack the balance directly, which shrinks every future interest charge. An extra $200 a month does not just save a little interest each month. It shortens the loan, deleting the most expensive payments at the end, so the interest saved is always a multiple of the extra cash you put in. Biweekly payments work through a back door: 26 half-payments a year equals 13 full monthly payments, one extra payment annually without thinking about it. Try the chart above. The gap between the two lines is money you keep.
When refinancing makes sense
Refinancing only pays if you stay long enough for the monthly savings to cover the closing costs. Divide closing costs by monthly savings for the breakeven point in months. If that number is 24 and you move in 18 months, refinancing loses money. Two traps to watch. First, resetting the term restarts the amortization clock, so a lower rate on a fresh 30-year loan can cost more total interest than your current loan even with a lower payment. Compare total interest, not just the payment. Second, closing costs are often rolled into the new loan, raising the balance you pay interest on. The breakeven tool above keeps both visible.
Frequently asked questions
How much house can I afford?
Lenders commonly use the 28/36 rule: housing costs under 28% of gross income, total debt under 36%. It is a ceiling, not a target. Run this calculator with your real tax and insurance numbers before trusting a pre-approval letter.
Is a 15-year mortgage always better than a 30-year?
It builds equity far faster and slashes total interest, but the higher payment reduces flexibility. Many buyers take the 30-year and make extra principal payments, approximating the 15-year payoff while keeping the lower required payment as a safety net.
When does PMI go away?
On a conventional loan you can usually request cancellation at 20% equity, and the lender must drop it at 22% of the original value. Extra payments get you there sooner.
Do biweekly payments really save that much?
Yes, because they smuggle in a 13th monthly payment each year. On a typical 30-year loan that cuts roughly six years off the term and tens of thousands in interest.