What happens to the default $250,000 loan?
Leave the inputs where they start and the calculator describes a $250,000 loan at 6.5% for 30 years. The payment works out to $1,580 a month. That figure holds for all 360 payments, but what it buys changes the whole time. The first payment sends $1,354 to interest and $226 to principal, so after a month of paying you still owe $249,774. Interest is charged on the entire outstanding balance, and at the start that balance is nearly everything you borrowed. The split improves slowly from there. Principal does not claim the larger share of a payment until payment 233, in the loan's twentieth year. By the time the balance reaches zero you will have paid about $318,900 in interest on top of the $250,000 you borrowed, a total cost near $568,900. The monthly payment tells you whether you can afford the loan. The total cost tells you what it takes from you over three decades, and that second number is the one most borrowers never look at.
How much difference does the rate make?
More than any other input, relative to its size. The loan amount scales the payment in a straight line: borrow twice as much, pay twice as much. The rate compounds instead. On $250,000 over 30 years, the payment at 5.5% is $1,419 a month and at 7.5% it is $1,748. That two-point spread costs $329 a month and roughly $118,000 in extra interest across the term. Even a quarter point moves the payment about $40 a month at this balance, which is worth chasing when it repeats 360 times. Market rates shift week to week with the bond market, and Freddie Mac publishes a weekly survey that shows where average mortgage rates currently sit. What you personally get quoted depends mostly on your credit score and down payment, plus how your existing debts compare to your income. Two borrowers can walk into the same bank on the same afternoon and leave with different offers. Collect quotes from at least three lenders before you commit. It is tedious, and it routinely saves thousands.
Where do extra payments actually go?
Straight to principal, and that is the entire mechanism. Next month's interest is charged on whatever balance remains, so every extra dollar you pay now shrinks every interest charge between here and payoff. On the default loan, $200 extra a month cuts about $97,600 of interest and ends the loan at month 265 instead of 360, almost eight years early. Timing matters as much as the amount. A dollar of principal removed in year one avoids interest for 29 more years, while the same dollar removed in year 25 avoids it for five. A bonus or tax refund applied early in the term does far more work than the same money applied late. One caution: confirm that your lender applies extra amounts to principal rather than treating them as an early copy of next month's bill. Most handle it correctly when the payment is marked, but it is worth checking your statement the following month.
What does this calculator leave out?
Everything except principal and interest, and for a mortgage that is a lot. Property taxes commonly run 0.5% to 2.5% of the home's value per year. Homeowner's insurance adds $100 to $300 a month in most states, and private mortgage insurance applies when you put down less than 20%, typically 0.3% to 1.5% of the loan balance per year. The CFPB's consumer answers cover how PMI and escrow work in detail. Together these can push a real housing bill $400 to $1,000 above the figure shown here. The model also assumes the rate is fixed for the full term, so an adjustable-rate loan matches it only during the initial fixed period. Closing costs and origination fees are outside the model too, which matters when you are choosing between two offers rather than sizing a single loan. For that decision, the Loan Comparison Calculator puts two offers side by side.