Your mortgage pre-approval letter might say you can borrow $650,000. But should you? The number a lender is willing to give you and the number you can comfortably afford are often very different — and confusing the two is one of the most common financial mistakes first-time homebuyers make.

In this guide, we'll walk you through how lenders calculate affordability, how to calculate your own budget using real cash flow, and why the 28/36 rule is a better starting point than any pre-approval letter.

How Lenders Decide What You Can Borrow

Mortgage lenders primarily look at two numbers: your gross income and your debt-to-income (DTI) ratio. They don't care much about your actual take-home pay, your retirement contributions, your childcare costs, or how much you spend on groceries.

The standard guideline most lenders follow is:

  • Front-end DTI: Your housing costs (principal, interest, taxes, insurance, PMI, HOA) should not exceed 28% of your gross monthly income.
  • Back-end DTI: All your debts — housing plus car loans, student loans, credit cards — should not exceed 36–43% of your gross monthly income.

On a $100,000 annual salary ($8,333/month gross), this means lenders may approve you for a monthly housing payment up to $2,333 using the 28% rule, or as high as $3,583 using a 43% total DTI ceiling.

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The Problem with the Pre-Approval Number

Lenders calculate using your gross income. But you don't live on your gross income — you live on your net (take-home) pay. On a $100,000 salary, after federal taxes, FICA, and state taxes, you might take home $72,000–$78,000 per year depending on your state. That's roughly $6,000–$6,500 per month.

A $2,333 housing payment that looks like 28% of gross suddenly becomes 36–39% of your actual take-home. That's before your 401(k) contribution, health insurance premium, childcare, or any savings at all.

The 28/36 Rule: A Better Starting Point

The 28/36 rule has been the benchmark for conservative home-buying for decades, and it still holds up. Here's how to apply it:

  1. Take your gross annual income and divide by 12.
  2. Multiply by 0.28 — this is your maximum monthly housing payment.
  3. Subtract estimated property tax, insurance, PMI (if applicable), and HOA.
  4. Whatever is left is your principal and interest budget.
  5. Plug that number into a mortgage calculator to see the maximum loan amount it supports at current rates.

Running the Real Numbers: An Example

Let's take a real scenario. Sarah earns $95,000/year. She has a $350/month car payment and $150/month in student loans. She has $60,000 saved for a down payment.

  • Gross monthly income: $7,917
  • 28% front-end limit: $2,217/month (housing only)
  • Estimated tax + insurance: $450/month
  • Available for P&I: $1,767/month
  • At 6.75% for 30 years: supports a loan of approximately $272,000
  • With $60K down: max home price ≈ $332,000

Her lender may approve her for a $450,000 home using a 43% back-end DTI. But using the 28% rule, she arrives at a much more conservative — and sustainable — $332,000 budget.

Hidden Costs Buyers Forget to Budget

Beyond the monthly mortgage payment, homeownership carries real ongoing costs that renters don't pay. Budget for:

  • Maintenance and repairs: 1–2% of the home's value per year. On a $350,000 home, that's $3,500–$7,000/year.
  • Utilities: Often higher than in an apartment, especially if you're moving to a larger space.
  • Closing costs: 2–5% of the purchase price, due at closing.
  • Moving costs, new furniture, immediate repairs.

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What About Down Payment?

Down payment affects both your loan amount and your monthly payment in two ways. First, more down means a smaller loan. Second, if you put down at least 20% on a conventional loan, you eliminate PMI — which typically runs 0.5–1.5% of your loan amount annually.

On a $350,000 loan, PMI of 0.8% adds $233/month to your payment. That's nearly $2,800/year that disappears once you hit 20% equity.

Our advice: if you can comfortably reach 20% down, it's usually worth it to wait. If you can't, FHA (3.5% down) or conventional loans with 5–10% down are reasonable options — just factor the PMI cost into your budget.

The Bottom Line

The right home price is the one that leaves your financial life intact. You should still be able to:

  • Contribute to your 401(k) or Roth IRA
  • Maintain a 3–6 month emergency fund
  • Pay for childcare, transportation, and normal life expenses
  • Have breathing room for home repairs

Use the 28% rule as your ceiling, run your real cash-flow numbers, and use our mortgage calculator to stress-test different scenarios before you fall in love with a house that stretches you too thin.