A mortgage is a loan secured against a home: the lender provides most of the purchase price up front, and you repay it in monthly installments over 15 to 30 years, with the house itself as collateral. If you stop paying, the lender can foreclose — sell the house to recover the loan — which is why mortgage rates are lower than unsecured borrowing rates: the collateral cuts the lender's risk. The monthly payment has up to four components, remembered by the acronym PITI: Principal (the amount borrowed), Interest (the lender's charge), property Taxes, and homeowners Insurance. This calculator computes the principal-and-interest portion, which is the part determined by the loan amount, the interest rate, and the term. Taxes and insurance vary by location and property, so estimate them separately and add them on — lenders do exactly this when they quote your total monthly housing cost. Most US lenders collect taxes and insurance through an escrow account: each month you pay one-twelfth of the annual bills along with principal and interest, and the lender pays the tax authority and insurer on your behalf. Escrow protects the lender's collateral (unpaid taxes can trigger a tax lien ahead of the mortgage), and it smooths your cash flow — no $5,400 property-tax shock in December. Your monthly payment can still change yearly as tax assessments and insurance premiums move, even on a 'fixed-rate' loan; only the principal-and-interest slice is truly fixed. Most US lenders collect taxes and insurance through an escrow account: each month you pay one-twelfth of the annual bills along with principal and interest, and the lender pays the tax authority and insurer on your behalf. Escrow protects the lender's collateral (unpaid taxes can trigger a tax lien ahead of the mortgage), and it smooths your cash flow — no $5,400 property-tax shock in December. Your monthly payment can still change yearly as tax assessments and insurance premiums move, even on a 'fixed-rate' loan; only the principal-and-interest slice is truly fixed.
The payment is calculated with the standard amortization formula: M = P x r x (1+r)^n / ((1+r)^n - 1), where P is the loan amount after your down payment, r is the monthly interest rate, and n is the number of payments. Because interest is charged on the remaining balance each month, early payments are mostly interest and later payments are mostly principal. On a $360,000 loan at 6.75 percent, the first monthly payment of $2,334.95 contains about $2,025 of interest and only $310 of principal — you are 87 percent interest in month one. By the final year, the proportions have flipped almost entirely. This front-loading is not a trick; it is the mathematical consequence of charging interest on a shrinking balance while keeping the payment constant. It does mean, though, that selling or refinancing in the first few years builds very little equity beyond your down payment and any market appreciation. Watch the equity curve, not just the payment. Equity is the home's value minus the loan balance, and early on it grows almost entirely through the down payment and market appreciation rather than your payments — recall that after 10 years on the $360,000 example you still owe about $309,000 despite paying roughly $280,000. Extra principal payments bend this curve sharply upward because they skip you past the interest-heavy years. Biweekly payment plans (26 half-payments = 13 full payments a year) are just a disguised way of making one extra payment annually; you can replicate the effect free by adding one-twelfth to each monthly payment yourself. Watch the equity curve, not just the payment. Equity is the home's value minus the loan balance, and early on it grows almost entirely through the down payment and market appreciation rather than your payments — recall that after 10 years on the $360,000 example you still owe about $309,000 despite paying roughly $280,000. Extra principal payments bend this curve sharply upward because they skip you past the interest-heavy years. Biweekly payment plans (26 half-payments = 13 full payments a year) are just a disguised way of making one extra payment annually; you can replicate the effect free by adding one-twelfth to each monthly payment yourself.
Down payment size matters twice over. Putting 20 percent down on a $450,000 home means borrowing $360,000 instead of $450,000, which lowers every single monthly payment for 30 years — and in the US it also lets you avoid private mortgage insurance (PMI), a monthly surcharge lenders add when the down payment is below 20 percent. PMI protects the lender, not you, and typically costs 0.5 to 1 percent of the loan amount per year until your equity reaches 20 percent. The trade-off is liquidity: cash sunk into a down payment is not available for emergencies, moving costs, or investments that might earn more than the mortgage rate. First-time buyers often stretch to 20 percent for the PMI savings, but keeping a six-month emergency fund intact matters more than crossing that threshold — a homeowner with no cash buffer is one furnace replacement away from credit-card debt. Discount points let you buy down the rate: one point costs 1 percent of the loan ($3,600 on $360,000) and typically cuts the rate about 0.25 percent. Whether points pay off is a break-even calculation — divide the upfront cost by the monthly saving. If points cost $3,600 and save $60 a month, break-even is 60 months; sell or refinance sooner and you lose money. Points make sense when you are certain to stay put past break-even and have cash to spare; they are a bad deal when that cash would otherwise clear high-interest debt or when a move is likely within a few years. Discount points let you buy down the rate: one point costs 1 percent of the loan ($3,600 on $360,000) and typically cuts the rate about 0.25 percent. Whether points pay off is a break-even calculation — divide the upfront cost by the monthly saving. If points cost $3,600 and save $60 a month, break-even is 60 months; sell or refinance sooner and you lose money. Points make sense when you are certain to stay put past break-even and have cash to spare; they are a bad deal when that cash would otherwise clear high-interest debt or when a move is likely within a few years.
The biggest decision is usually 15-year versus 30-year. A 15-year loan carries a higher monthly payment but typically a lower interest rate and dramatically less total interest; a 30-year loan costs far less per month but you pay interest for twice as long. On a $400,000 loan, a 30-year term at 6.25 percent costs $2,462.87 a month with $486,633 of total interest, while a 15-year term at 5.75 percent costs $3,321.64 a month with only $197,895 of total interest — a savings of about $288,738 for a payment roughly $859 higher. Neither choice is universally right. The 15-year builds equity fast and frees you from payments a decade and a half sooner, but the higher payment reduces your flexibility if income drops. The 30-year keeps payments low and lets you invest the difference — which can win if investments outpace the mortgage rate — but most borrowers spend the difference instead of investing it, so be honest with yourself. A middle path exists: take the 30-year for flexibility and make extra principal payments whenever you can, effectively creating your own shorter term without the contractual obligation. Refinancing follows the same break-even logic: a refinance that saves $200 a month but costs $6,000 in closing costs needs 30 months to pay off (6,000 / 200). Only refinance if you will keep the loan past that point, and beware of 'resetting the clock' — refinancing a 25-years-remaining loan into a fresh 30-year term lowers the payment but can raise total interest despite the lower rate. Adjustable-rate mortgages (ARMs) tempt with lower initial rates, typically fixed for 5-7 years then adjusting; they suit buyers who will definitely sell before adjustment, and endanger everyone else. When in doubt, the boring 30-year fixed remains the default for good reason. Refinancing follows the same break-even logic: a refinance that saves $200 a month but costs $6,000 in closing costs needs 30 months to pay off (6,000 / 200). Only refinance if you will keep the loan past that point, and beware of 'resetting the clock' — refinancing a 25-years-remaining loan into a fresh 30-year term lowers the payment but can raise total interest despite the lower rate. Adjustable-rate mortgages (ARMs) tempt with lower initial rates, typically fixed for 5-7 years then adjusting; they suit buyers who will definitely sell before adjustment, and endanger everyone else. When in doubt, the boring 30-year fixed remains the default for good reason.