The Anatomy of a Mortgage Payment (PITI)
Most homebuyers focus on the "principal and interest" figure a lender quotes, but your actual monthly payment is usually made up of four parts, commonly abbreviated PITI: Principal (the portion that reduces your loan balance), Interest (the lender's charge for borrowing), Taxes (property taxes, typically collected monthly and held in an escrow account), and Insurance (homeowner's insurance, and if your down payment is below 20%, Private Mortgage Insurance or PMI).
Property taxes and insurance can add hundreds of dollars to your monthly payment beyond the loan itself, and they vary enormously by location. Two identical homes with identical loans can have payments that differ by $300–$500/month purely based on local tax rates and insurance costs. When comparing markets or neighborhoods, always ask for an estimate of taxes and insurance — don't rely on the principal-and-interest number alone.
PMI is worth understanding specifically: it protects the lender, not you, in case you default. It typically costs 0.5%–1.5% of the loan amount per year, split into monthly payments, and is usually removable once you reach 20% equity in the home — either through paydown or appreciation. Putting 20% down avoids it entirely.
Fixed vs. Adjustable Rate Mortgages
A fixed-rate mortgage locks in your interest rate for the entire loan term — 15, 20, or 30 years are most common. Your principal-and-interest payment never changes, which makes budgeting simple and protects you if market rates rise. The tradeoff is that fixed rates are usually slightly higher than the initial rate on an adjustable loan.
An adjustable-rate mortgage (ARM) offers a lower introductory rate for a fixed period — commonly 5, 7, or 10 years — after which the rate adjusts periodically based on a market index, usually within caps that limit how much it can move per adjustment and over the life of the loan. ARMs can make sense if you're confident you'll sell or refinance before the adjustment period begins, or if the introductory rate is significantly lower and you want the extra cash flow now.
The risk with ARMs is that "significantly lower now" can become "significantly higher later" if rates rise and you're unable to refinance — for example, if your income drops or home values fall. For most buyers planning to stay in a home long-term, the predictability of a fixed rate outweighs the initial savings of an ARM.
15-Year vs. 30-Year: The Real Cost Difference
A 30-year mortgage spreads your principal over 360 payments, producing the lowest possible monthly payment for a given loan amount and rate. A 15-year mortgage spreads the same principal over 180 payments — roughly doubling the monthly payment, but cutting the loan term in half and typically coming with a meaningfully lower interest rate (often 0.5%–0.75% lower than a 30-year).
The combination of fewer payments and a lower rate makes the total interest difference dramatic. On a $400,000 loan at typical rates, a 30-year mortgage might cost over $300,000 in total interest, while a 15-year mortgage on the same amount might cost under $130,000 — less than half. The catch is the monthly payment: the 15-year version could be roughly 50–60% higher per month.
This is why the decision isn't really "which is better" in the abstract — it's "can your budget absorb the higher 15-year payment without leaving you cash-poor for emergencies, retirement contributions, or other goals?" The Mortgage Calculator's term comparison table shows 10 through 30-year terms side by side using your actual numbers, so you can see exactly where the payment becomes uncomfortable versus where the long-term savings become compelling.
How Much House Can You Actually Afford?
Lenders typically use two ratios to determine how much they'll lend you. The front-end ratio compares your projected housing payment (PITI) to your gross monthly income — lenders generally want this at or below 28%. The back-end ratio (also called debt-to-income, or DTI) compares all your monthly debt payments, including the new mortgage, to your gross income — typically capped around 36–43% depending on the loan program.
Just because a lender will approve you for a certain amount doesn't mean you should borrow that much. Lenders calculate what you can afford based on your debt — they don't know about your other goals: retirement contributions, childcare, travel, or simply having breathing room in your budget. Many financial planners suggest keeping your total housing payment closer to 25% of take-home (after-tax) pay, which is often noticeably lower than what a lender will approve.
The Home Affordability Calculator works backward from your income and existing debts to estimate a comfortable price range — giving you a number to anchor your search around before you fall in love with a home at the top (or beyond) of your budget.
Refinancing: When It Makes Sense
Refinancing replaces your existing mortgage with a new one — usually to get a lower rate, change the loan term, or tap into home equity. The classic rule of thumb is that refinancing makes sense if the new rate is at least 0.5%–1% lower than your current rate, but the real answer depends on closing costs (typically 2%–5% of the loan amount) versus your monthly savings.
Divide your closing costs by your monthly savings to find your "break-even" point in months. If you plan to stay in the home longer than that break-even period, refinancing likely makes financial sense. If you might move or sell before then, the closing costs may exceed what you'd save.
Be cautious about resetting your amortization clock. If you're 10 years into a 30-year mortgage and refinance into a new 30-year loan, you're extending your payoff timeline by 10 years even if your rate drops — which can mean paying more total interest despite the lower rate. Refinancing into a shorter term (or making the same extra payments you would have made anyway) helps avoid this trap.