The First-Time Buyer's Guide to Mortgage Math (2026)

Buying a first home involves the largest numbers most people will ever sign for, negotiated in a state of excitement, against professionals who do this daily. The antidote is arithmetic. This guide walks the full calculation for a concrete example — a $350,000 home — so that every number in your own deal has a benchmark.

The Core Payment

With 20% down ($70,000), the loan is $280,000. At 6.5% over 30 years, the principal-and-interest payment is $1,769.79 a month — and over the full term, total interest comes to roughly $357,000, more than the amount borrowed. That's not a scam; it's what 6.5% compounded across three decades costs, and it's the first number that should be real to you before anything else.

How Much House Can You Actually Afford?

Lenders will approve you based on gross income and existing debts; your life runs on net income and actual expenses, so do your own math. The long-standing guideline caps housing costs at 28% of gross income — and "housing costs" means PITI: principal, interest, taxes and insurance. Our example's $1,770 payment plus a typical ~$600 of monthly tax and insurance is $2,370, which under the 28% rule wants a gross income around $101,000. If the lender approves more than the rule suggests — and they often will — remember that approval is their risk assessment, not your budget.

The Costs the Listing Never Shows

Budget for all of these or the first year will: closing costs of 2–5% of the loan ($5,600–$14,000 here), due at signing; PMI if you put down less than 20% (roughly 0.3–1.5% of the loan per year until ~20% equity); property tax (commonly 0.5–2.5% of home value annually, by location); homeowners insurance; and maintenance, where the standard planning figure is 1% of home value per year — $3,500 annually that renters never see. A house payment that "equals your rent" is not a house that costs the same as renting.

The 15 vs 30-Year Decision

The same $280,000 loan over 15 years costs $2,439.10 a month — $669 more — but total interest drops from ~$357,000 to ~$159,000: a saving of nearly $198,000, before counting that 15-year loans usually carry lower rates too. The catch is that the higher payment is mandatory. The flexible alternative: take the 30-year loan and voluntarily pay extra. Adding just $200 a month to our example clears the loan in 22 years 9 months and saves about $101,000 — most of the 15-year benefit, with an escape hatch if income wobbles. Whichever route you take, flag extra payments as principal-only with your servicer, or some will quietly apply them to next month's bill.

Why the Early Years Feel Like Running in Place

Interest is charged on the remaining balance, so early payments are mostly interest: in month one of the example, about $1,517 of the $1,770 payment is interest and only ~$253 reduces the debt. After five full years of paying (~$106,000 out the door), the balance has fallen by only ~$16,000. Two consequences for first-time buyers specifically: selling within a few years means transaction costs (6–10% to exit) can exceed the equity your payments built — which is why the common advice is don't buy unless you'll stay ~5 years — and refinancing later restarts this interest-heavy phase unless you refinance into a shorter remaining term.

Shop the Rate Like It's Your Job (Briefly, It Is)

On this loan, the difference between 6.5% and 7.0% is about $93 a month — roughly $33,500 over the term. Getting quotes from three or more lenders, and letting each know you're comparing, is plausibly the highest hourly wage you will ever earn. Compare offers by APR, not headline rate, since APR folds in the mandatory fees.

The Pre-Signing Checklist

Before committing: run your real numbers through the mortgage calculator (payment, total interest, and the amortization table); stress-test the payment against the 28% rule using honest tax and insurance estimates; confirm cash for closing costs plus a post-purchase emergency fund — a new homeowner with empty accounts is one boiler away from the credit card; and run the rent-vs-buy comparison for your actual market and time horizon, because in some cities, at some prices, the spreadsheet honestly says keep renting. The goal isn't to talk you out of the house. It's to make sure the day you sign, no number on the page is a surprise.

How Much the Rate Really Matters: The Full Table

Because half a percent sounds trivial and isn't, here is the same $280,000 loan across the realistic 2026 rate range: at 5.5%, $1,589.81 a month and ~$292,000 lifetime interest; at 6.0%, $1,678.74 and ~$324,000; at 6.5%, $1,769.79 and ~$357,000; at 7.0%, $1,862.85 and ~$391,000; at 7.5%, $1,957.80 and ~$425,000. The span from top to bottom of that table is $368 a month and $133,000 over the term — more than many people's entire retirement savings, determined by the rate environment and your credit profile. You can't control the first, but the second is workable: utilization down, no new credit applications in the months before applying, errors disputed off your report. The credit score guide on this site covers the mechanics; the table above is the motivation.

And a closing word on the down payment trade-off: putting down 10% instead of 20% on this house means borrowing $315,000, adding PMI of roughly $80–$390 a month until you reach ~20% equity, and paying interest on an extra $35,000 for years. Sometimes that trade is right — markets where prices outrun saving speed exist — but it should be a calculated decision, not a default. Run both versions in the calculator and compare total five-year cost before deciding which buyer you are.