Buying a home involves more math than any other financial decision most people will ever make. Mortgage payments, affordability limits, closing costs, property taxes, PMI, equity, refinancing break-even analysis — every one of these matters, and getting any of them wrong can cost you tens of thousands of dollars.
I've organized this in the order you'll actually need it — from "how much can I afford?" all the way through ongoing homeownership math. Each section links to the specific calculator that does the number-crunching, plus a deep-dive article if you want the full breakdown.
🎯 How to use this guide: Work through each section in order. Each covers the key formula, a quick example, common mistakes, and the calculator to run your own numbers. You can also jump to any section directly.
Start here. Not at open houses, not on Zillow. Affordability sets the ceiling for everything else — your neighborhood options, your commute, and how much financial breathing room you'll have after closing day.
Lenders use two ratios. The front-end ratio caps your housing costs (principal, interest, taxes, insurance, HOA) at 28% of gross monthly income. The back-end ratio caps all monthly debt payments at 36–43% of gross income, depending on the loan type.
| Annual Income | Max Monthly Housing (28%) | Estimated Home Price | Notes |
|---|---|---|---|
| $60,000 | $1,400 | $210,000–$250,000 | Assumes 5–10% down, 7% rate |
| $80,000 | $1,867 | $280,000–$330,000 | DTI assumes moderate other debts |
| $100,000 | $2,333 | $350,000–$420,000 | 20% down eliminates PMI |
| $125,000 | $2,917 | $440,000–$520,000 | Higher DTI tolerance at this level |
| $150,000 | $3,500 | $530,000–$620,000 | Property tax varies wildly by state |
Note: Estimates assume 30-year fixed, 2026 average rates. Your actual range depends on credit score, debts, and local taxes.
The biggest mistake I see: using gross income without thinking about what actually hits your bank account. A $100,000 salary might net $65,000 after taxes, retirement contributions, and health insurance. At that take-home, $2,333/month in housing feels a lot heavier than the 28% rule implies.
Your monthly payment is not just principal and interest. The real number includes property taxes (usually escrowed), homeowner's insurance, PMI if you put less than 20% down, and HOA fees if applicable. This total — PITI (Principal, Interest, Taxes, Insurance) — is the number that actually matters.
The mortgage payment formula for principal and interest is:
M = P × [r(1+r)n] / [(1+r)n – 1]
Where P = loan amount, r = monthly interest rate (annual rate ÷ 12), n = total payments (years × 12).
Example: a $350,000 loan at 7% over 30 years gives you $2,329/month in P&I. Add property taxes ($350/mo), insurance ($150/mo), and PMI ($145/mo at 10% down), and you're at $2,974/month — 28% more than P&I alone. That gap is where budget surprises happen.
The rate makes an enormous difference. That same $350,000 at 6% instead of 7% costs $2,098/month in P&I — saving $231/month and $83,160 over 30 years. Shopping rates across 3–5 lenders is the single highest-return activity in the entire homebuying process. Don't skip it.
Putting 20% down avoids PMI (Private Mortgage Insurance), which runs 0.5–1.5% of the loan amount annually. On a $350,000 loan, that's $1,750–$5,250/year, or $146–$437/month. And here's what frustrates people: PMI protects the lender, not you. You're paying for insurance that benefits someone else.
But waiting to save 20% isn't free either. If home prices are rising 3–5% annually, a $400,000 home becomes $420,000–$440,000 in a year. The math sometimes favors buying now with 5–10% down, paying PMI temporarily, and building equity while prices appreciate around you.
| Down Payment | Amount on $400K | Loan Amount | Est. Monthly PMI | PMI Duration |
|---|---|---|---|---|
| 3% (FHA min) | $12,000 | $388,000 | $220–$390 | Loan life (FHA) |
| 5% | $20,000 | $380,000 | $190–$320 | Until 20% equity |
| 10% | $40,000 | $360,000 | $150–$270 | Until 20% equity |
| 20% | $80,000 | $320,000 | $0 | N/A |
With conventional loans, PMI drops automatically at 78% loan-to-value, or you can request removal at 80%. FHA loans are a different story — mortgage insurance stays for the life of the loan if you put less than 10% down. The only way out is refinancing into a conventional loan once you have 20% equity.
Closing costs are due on the day you finalize the purchase, and they're bigger than most first-time buyers expect. Budget 2–5% of the purchase price, though the exact number depends on your state, lender, and negotiating leverage.
The big pieces: origination fees (0.5–1% of loan), appraisal ($400–$700), title search and insurance ($1,000–$3,000), escrow/attorney fees ($500–$2,000), recording fees ($100–$500), and prepaid items (property tax, insurance, per-diem interest). Transfer taxes are the wild card — some states charge nothing, others hit you for 1–2% of the purchase price.
You'll get a Loan Estimate within 3 business days of applying. Compare these across lenders — origination fees, discount points, and third-party fees are where the biggest differences show up. Three days before closing, you'll receive the Closing Disclosure. Compare it line by line to the Loan Estimate and flag anything that changed.
This decision is bigger than most people realize, and the math is dramatic.
| Metric | 30-Year at 7.0% | 15-Year at 6.25% | Difference |
|---|---|---|---|
| Loan Amount | $350,000 | $350,000 | — |
| Monthly P&I | $2,329 | $3,000 | +$671/mo |
| Total Interest | $488,281 | $189,958 | –$298,323 |
| Total Cost | $838,281 | $539,958 | –$298,323 |
The 15-year saves nearly $300,000 on a $350,000 loan. That's not a typo. But the $671/month higher payment cuts into your flexibility. The question worth asking: could you invest that $671/month at returns that beat the 6.25% guaranteed "return" of paying off the mortgage faster? That depends on your risk tolerance and time horizon.
My favorite approach: take the 30-year for the lower required payment, then make extra principal payments whenever cash flow allows. You get the safety net of the lower minimum with the option to accelerate. The Extra Payment Calculator shows exactly how extra payments shorten your timeline.
Property taxes are the cost that blindsides new homeowners. The national average is about 1.1% of assessed value, but the range is wild: New Jersey averages 2.47%, Texas 1.80%, California 0.71%, Hawaii 0.29%. On a $400,000 home, that's anywhere from $1,160 to $9,880 per year depending on where you buy.
On top of taxes: homeowner's insurance ($1,200–$3,000/year), maintenance (1–2% of home value annually), utilities, and HOA fees if applicable. A good rule of thumb: budget 1–4% of your home's value annually for all non-mortgage costs. It adds up faster than most people expect.
If you're comparing cities, the Cost of Living Calculator helps normalize the picture. A $400,000 home in Houston and a $400,000 home in Portland feel very different once you factor in income tax, property tax, insurance, and utility costs.
Not a simple question, despite what everyone's uncle says at Thanksgiving. The breakeven where buying beats renting depends on how long you stay, home price appreciation, rent increases, and what you'd earn investing the down payment instead.
Quick rule: if you're staying fewer than 5 years, renting usually wins once you account for closing costs, transaction fees, and the early years of a mortgage where most of your payment goes to interest. Beyond 5–7 years, buying tends to pull ahead as you amortize the fixed costs and start building real equity.
Local markets change the math dramatically. In cities where the price-to-rent ratio exceeds 20 (home price > 20x annual rent for a comparable place), renting is often the smarter financial move. Where the ratio is below 15, buying usually wins. Always run the numbers for your specific situation.
Once you own, two big financial decisions come up: when (or whether) to refinance and how to tap home equity.
Refinancing replaces your current mortgage with a new one — usually for a lower rate, a different term, or to switch from adjustable to fixed. The break-even math is simple: closing costs divided by monthly savings. $6,000 in costs with $200/month savings = 30 months to break even. Only pull the trigger if you're staying past that point.
Home equity access has two main options: HELOCs (variable rate, revolving credit line) and cash-out refinances (new fixed-rate mortgage for more than you owe). HELOCs work better for ongoing projects or short-term needs; cash-out refinances make more sense when rates are favorable and you need a lump sum. I compare both in detail in the HELOC vs cash-out refinance guide.
If you're looking at real estate as an investment, the math changes completely. It's no longer "can I afford the payment?" It's "does this property generate positive cash flow and a solid return?"
Three metrics matter most. Cap rate (Net Operating Income ÷ Purchase Price) measures the property's yield independent of financing — 5–8% is typical for residential. Cash-on-cash return measures your actual return on money invested (down payment + closing costs) — I'd target 8–12%. GRM (Gross Rent Multiplier = Purchase Price ÷ Annual Rent) is a quick screening tool — below 15 is generally favorable.
The most common mistake new landlords make: forgetting vacancy (budget 5–8%), maintenance (8–12% of rent), property management (8–10% if hired), and capital expenditures (5–10% for roof, HVAC, appliances). These expenses can eat 25–40% of gross rent. The property that looks great on a napkin often looks very different on a spreadsheet.
🏠 Start with the Home Affordability Calculator — Run your numbers →
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