How Much House Can You Afford? The Real Formula Lenders Use

You find a home you love. The listing says $350,000. Your gut says "that feels about right." But your gut doesn't know your debt-to-income ratio — and your lender does.

Most buyers find out what they can afford after they fall in love with a home that won't qualify. This guide shows you the exact formula lenders use, so you know your real number before you start shopping.

The Lender's Formula: Debt-to-Income Ratio (DTI)

DTI (Debt-to-Income Ratio) compares your monthly debt payments to your gross monthly income. It's the single most important number in mortgage qualification.

Lenders calculate two versions:

The 28/36 Rule (standard guideline):

• Front-end DTI ≤ 28% — housing costs stay under 28% of gross monthly income
• Back-end DTI ≤ 36% — all debt combined stays under 36% of gross monthly income

In practice, most lenders accept back-end DTI up to 43%. FHA loans allow up to 50% with strong compensating factors. But qualifying at the limit means very little breathing room — one job change or car repair and you're stretched thin.

Step-by-Step: What Can You Afford on $80,000?

Starting numbers:

• Annual gross income: $80,000 → $6,667/month
• Existing monthly debts: $400 (car + student loans)
• Down payment: $40,000 (~5%)
• Credit score: 740

Step 1 — Maximum housing payment (front-end)

Step 2 — Check back-end DTI

The front-end rule wins at $1,867 — it's the tighter constraint.

Step 3 — Subtract taxes, insurance, and PMI

Estimated for a $220,000 home with 5% down:

Step 4 — Back into the maximum loan amount

At 7% for 30 years, a $1,434/month P&I payment supports a loan of approximately $215,000.

With 5% down on a $220,000 home, the loan is $209,000 → P&I ≈ $1,391/month → total PITI ≈ $1,824/month ✓ (under $1,867 limit).

Result: approximately $220,000–$240,000 given these inputs.

Affordability by Income and Rate

These estimates assume: 5% down payment, 1.1% property tax rate, $140/month insurance, no existing monthly debts, 30-year fixed mortgage.

Estimates only. Adjust for local tax rates, existing debt, and actual down payment.

Each $200/month in existing debt cuts buying power by approximately $28,000–$30,000 at 7%.

What Banks Look At Beyond Income

Credit Score and Your Rate

Your credit score changes your interest rate — which directly changes what you can afford.

The difference between a 760 and 640 score on a $250,000 loan: $259/month — that's $93,240 over 30 years.

Down Payment and PMI

LTV (Loan-to-Value) = Loan Amount ÷ Home Value. The lower your LTV, the better your rate and the less you pay in PMI.

PMI typically costs 0.3%–1.5% of the loan per year. On $250,000 at 0.8%, that's $167/month added to your PITI.

Cash Reserves

After closing, lenders want to see 2–6 months of PITI in savings — separate from your down payment. If your PITI is $1,800/month, expect to need $3,600–$10,800 sitting in your account after closing day.

Employment History

Most lenders require 2 years of continuous employment in the same field. Self-employed borrowers need 2 years of tax returns showing stable or growing income.

How Existing Debt Cuts Your Budget

Every dollar of monthly debt reduces your maximum housing payment by exactly one dollar.

A $500/month car payment reduces your buying power by roughly $75,000 at 7% rates.

FHA vs. Conventional: Different Rules

Conventional Loans (Fannie Mae / Freddie Mac)

• Minimum credit score: 620 (best rates at 740+)
• Maximum back-end DTI: 45% (up to 50% with compensating factors)
• Minimum down payment: 3%
• PMI: Required below 20% down; cancels automatically at 80% LTV

FHA Loans

• Minimum credit score: 580 with 3.5% down; 500 with 10% down
• Maximum back-end DTI: 43% standard; up to 50% with strong factors
• Minimum down payment: 3.5%
• MIP (Mortgage Insurance Premium): Required for the life of the loan if less than 10% down

FHA allows lower scores and higher DTI, but the permanent MIP makes it more expensive long-term. If you can qualify conventional, it's usually the better deal — especially once you build equity past 20%.

VA Loans (Veterans only)

• No down payment required; no PMI
• DTI: typically up to 41%
• Funding fee: 1.4%–3.6% of loan (varies by down payment and use)

The 20% Down Payment: Worth It?

The standard advice is "put 20% down." Here's when that's right — and when it isn't.

Reasons to put less than 20% down:

1. Opportunity cost — money locked in home equity earns nothing. If investments return 7%/year and PMI costs $150/month ($1,800/year), you're paying a 0.6% premium to deploy capital elsewhere.
2. Home appreciation still accrues on the full value — whether you put 5% or 20% down, price gains apply to the whole home.
3. Getting in sooner matters — in rising markets, waiting to save 20% costs more than the PMI would have.

Reasons to put 20% down:

1. No PMI on day one — immediate savings on every payment
2. Better rate — typically 0.125%–0.25% lower at 80% LTV
3. Easier qualification — lower payment makes DTI math work more easily
4. Buffer against price drops — more equity means less risk if the market dips

Rule of thumb: If you're confident in the market and have investments earning more than your PMI costs, less than 20% can be rational. If you're risk-averse, 20% down gives you a meaningful cushion.

Warning: Pre-Approval ≠ What You Can Comfortably Afford

A lender will approve you for the maximum they're allowed to lend — not the maximum you should borrow. Getting approved for $400,000 when your comfortable budget is $300,000 is common, and it can lead to being "house poor" — owning a home that eats every spare dollar.

Run your own DTI math first. Decide on your comfortable monthly payment before you talk to a lender. Then use the pre-approval process as a ceiling check, not a shopping budget.

3 Ways to Increase What You Can Afford

1. Pay down existing debt before applying Every $100/month reduction in monthly debt obligations adds roughly $14,000–$15,000 in buying power. Paying off a car loan or credit card balance before applying can move your budget meaningfully.

2. Improve your credit score before shopping Moving from 680 to 760 can lower your rate by 0.75% or more. On a $300,000 loan, that saves $150/month — and increases your qualifying price by $20,000+. Give yourself 6–12 months to optimize before applying.

3. Shop multiple lenders Rates vary by 0.25%–0.5% across lenders for the same borrower profile. If you shop within a 14–45 day window, all mortgage inquiries count as one hard pull on your credit. The few hours spent comparing quotes can save tens of thousands over the life of the loan.

FAQ

Is the 28/36 rule enforced strictly by lenders?

No — it's a guideline. Most conventional lenders allow back-end DTI up to 43–45%, and some approve up to 50% with strong compensating factors like a large down payment, excellent credit, or significant reserves. FHA allows up to 50% in some cases. However, qualifying at the limit means very little financial flexibility — one emergency can strain your budget fast.

Do lenders use gross or net income?

Lenders use gross income (before taxes) for DTI calculations, even though your actual spending comes from net income. This is how Fannie Mae and Freddie Mac define qualifying income. Practically, this means two borrowers with the same gross income qualify for the same loan, even if one lives in a high-tax state and takes home significantly less.

What income types count for mortgage qualification?

Lenders accept W-2 wages, salary, self-employment income (2-year average from tax returns), Social Security, pension, alimony or child support (if 3+ years remain), and rental income (typically 75% of gross rent). Bonus income and RSUs are accepted with a 2-year history. Part-time income counts if it's been consistent for 2 years.

How does pre-approval affect my credit score?

Pre-approval requires a hard credit inquiry, which typically drops your score by 2–5 points temporarily. If you shop multiple lenders within a 14–45 day window, all mortgage inquiries count as a single pull under FICO's rate-shopping exception. Don't let fear of a small score dip stop you from comparing offers — the rate difference between lenders matters far more.

What's the minimum down payment for a first-time buyer?

FHA allows 3.5% with a 580+ credit score. Conventional programs like Fannie Mae HomeReady and Freddie Mac Home Possible allow 3% for qualifying first-time buyers. VA loans allow 0% for eligible veterans. Many states also offer down payment assistance through their housing finance agencies — search your state's housing authority for programs.

Can rental income from a multi-unit property help me qualify?

Yes. If you're buying a 2–4 unit property and living in one unit, lenders typically allow 75% of market rent from the other units as qualifying income. This can significantly boost your purchasing power — a duplex buyer can often qualify for a much larger loan than a single-family buyer at the same salary.

Methodology & Sources

Home affordability calculations use the standard PMT formula for mortgage payment calculations. DTI guidelines reflect the Fannie Mae Single Family Selling Guide and FHA Single Family Housing Policy Handbook 4000.1. PMI estimates reflect MGIC rate cards (2024). Property tax rates reflect the national average of approximately 1.1% as reported by the Tax Foundation (2024).

Sources: Fannie Mae Selling Guide, FHA Handbook 4000.1, Consumer Financial Protection Bureau (CFPB), Freddie Mac Primary Mortgage Market Survey.

Published: May 4, 2026 | Last updated: May 4, 2026 | By: FiscalCalc Editorial Team

All calculations are estimates for educational purposes only. Actual loan qualification depends on full underwriter review. Mortgage rates, DTI limits, and qualifying criteria vary by lender, loan type, and borrower profile. Consult a licensed mortgage professional or financial advisor before making any borrowing decisions.