🏠 Home Affordability Calculator
How home affordability is calculated
Behind every "how much house can I afford?" answer is a straightforward chain of math. Lenders use two primary ratios — the front-end DTI and the back-end DTI — to decide whether your income can comfortably support a given monthly payment. Understanding those two numbers puts you in control of the conversation before you ever sit down with a loan officer.
The two DTI ratios that matter
DTI stands for Debt-to-Income ratio. It compares your monthly obligations to your monthly gross (pre-tax) income.
| Ratio | What it includes | Common guideline |
|---|---|---|
| Front-End DTI (Housing Ratio) | Principal + Interest + Property Taxes + Insurance + PMI (PITI) | ≤ 28–31% of gross monthly income |
| Back-End DTI (Total Debt Ratio) | All PITI + car loans + student loans + credit card minimums + any other recurring debt | ≤ 36–43% (conventional) or up to 50% (FHA) of gross monthly income |
Different loan programs set different maximum DTI thresholds. Conventional loans backed by Fannie Mae or Freddie Mac generally allow up to 43–45% back-end DTI with strong compensating factors. FHA loans can go to 50% in some cases. VA and USDA loans use their own residual-income tests. This tool uses conventional guidelines as a default; always confirm with a licensed loan officer for your specific situation.
The mortgage payment formula (P&I)
The principal and interest portion of your payment is calculated with the standard amortization formula:
Monthly P&I = L × [r(1 + r)ⁿ] / [(1 + r)ⁿ − 1]
Where:
L = Loan amount (home price minus down payment)
r = Monthly interest rate (annual rate ÷ 12 ÷ 100)
n = Number of payments (360 for a 30-year loan)
Add monthly property tax escrow, homeowner's insurance, and PMI (if applicable) to arrive at the full PITI payment that lenders compare against your income.
Worked example 1: First-time buyer, moderate income
Scenario: Single buyer, $72,000/yr income, $350/mo in debts, $20,000 down, 6.875% rate
- Gross monthly income: $72,000 ÷ 12 = $6,000
- 28% front-end limit: $6,000 × 0.28 = $1,680/mo max housing payment
- 43% back-end limit: $6,000 × 0.43 = $2,580 − $350 existing debts = $2,230/mo max housing payment from back-end
- Binding constraint: front-end at $1,680/mo
- Subtract estimated taxes ($200/mo) + insurance ($125/mo) = $405/mo in escrow → available for P&I+PMI: ~$1,275/mo
- At 6.875% on a $260,000 loan: P&I ≈ $1,707/mo — too high. Solving backwards, the affordable loan amount is roughly $195,000
- Max home price with $20,000 down: ~$215,000
Front-end DTI lands at about 27.9% — just inside the 28% guideline. Back-end DTI is approximately 33.8% — well within the 43% limit. This buyer has breathing room on back-end but is nearly at their front-end ceiling.
Worked example 2: Dual-income household, higher price point
Scenario: Two incomes totaling $145,000/yr, $800/mo in debts, $60,000 down, 7.0% rate
- Gross monthly income: $145,000 ÷ 12 = $12,083
- 31% front-end limit: $12,083 × 0.31 = $3,746/mo max PITI
- 43% back-end limit: $12,083 × 0.43 = $5,196 − $800 existing debts = $4,396/mo max PITI from back-end
- Binding constraint: front-end at $3,746/mo
- Subtract property taxes (1.1% on ~$520K = $476/mo) + insurance ($150/mo) = $626/mo → available for P&I+PMI: ~$3,120/mo
- At 7.0% on a $460,000 loan: P&I ≈ $3,061/mo — within budget
- Down payment of $60,000 on $520,000 = 11.5% → PMI applies at ~$288/mo, pushing PITI to ~$3,675/mo → front-end DTI ≈ 30.4% ✅
- Max home price: approximately $520,000
This couple is comfortably within front-end guidelines and well below back-end limits. Adding a larger down payment to clear 20% ($104,000) would eliminate the $288/mo PMI and free up significant monthly cash flow.
Worked example 3: High earner with heavy debts
Scenario: $160,000/yr income, but $2,800/mo in debts (car, student loans, credit cards), $80,000 down, 6.75% rate
- Gross monthly income: $160,000 ÷ 12 = $13,333
- 31% front-end limit: $13,333 × 0.31 = $4,133/mo max PITI
- 43% back-end limit: $13,333 × 0.43 = $5,733 − $2,800 debts = $2,933/mo max PITI from back-end
- Binding constraint: back-end at $2,933/mo — existing debts are the bottleneck
- Subtract taxes (~$370/mo on a $400K home) + insurance ($140/mo) = $510/mo → P&I budget: ~$2,423/mo
- At 6.75% on a $320,000 loan: P&I ≈ $2,075/mo → affordable. Max home price ~$400,000
- Front-end DTI: ~22% (well within limit). Back-end DTI: ~43% (at the ceiling).
This example shows why a high salary alone doesn't guarantee a large loan. Paying down $500/mo of recurring debts could shift the back-end limit enough to qualify for a significantly higher purchase price — sometimes $40,000–$80,000 more. A good loan officer will model this trade-off for you before you make any payoff decisions.
Key factors that shift your affordability number
1. Interest rate
Rate is the most powerful lever. A single percentage point difference on a $400,000 loan changes your monthly P&I by roughly $240–$270 per month — enough to push you into or out of qualifying range. Even a 0.25% rate improvement from shopping multiple lenders can meaningfully raise your ceiling.
2. Down payment size
A larger down payment reduces the loan amount, lowers your P&I, and — once you clear 20% — eliminates PMI entirely. That PMI line item can run $100–$400/month on mid-range loans, so crossing the 20% threshold is a genuine financial milestone.
3. Existing monthly debts
As the third worked example shows, existing debt is often the binding constraint even for high earners. Paying off or paying down car loans, credit cards, or personal loans before applying for a mortgage can unlock significantly more buying power. Discuss the math with a loan officer before making large payoffs — the sequencing matters.
4. Property taxes and insurance
These escrow items are real costs that reduce how much of your DTI limit is left for principal and interest. Property tax rates vary enormously by state and county — from under 0.4% in some areas to over 2.5% in others. Run your actual county rate for an accurate estimate; the national average (~1.0–1.2%) is just a starting point.
5. Loan program choice
Conventional, FHA, VA, and USDA loans all have different DTI limits, down payment requirements, and mortgage insurance structures. A VA loan with 0% down and no PMI produces a very different affordability calculation than a conventional loan with 5% down. Ask your loan officer to run all programs you qualify for — the differences can be substantial.
What this calculator does not account for
This tool gives you a solid educational estimate, but several real-world factors fall outside its scope:
- Credit score. A lower score can mean a higher interest rate, which changes every number in the chain. Lenders typically tier rates by FICO score bands.
- Employment type. Self-employed borrowers, commissioned salespeople (including many real estate agents), and those with variable income may have their qualifying income calculated differently by underwriters.
- Reserves requirement. Many loan programs require you to have a certain number of months of PITI in savings after closing. This doesn't affect the payment math but can affect how much you can actually put toward a down payment.
- HOA dues. If the property has a homeowners association, monthly dues are added into the back-end DTI calculation by most lenders.
- Local market realities. Affordability math tells you what you qualify for; market conditions in your area determine what's actually available at that price.
How real estate agents and loan officers use affordability conversations
Understanding these ratios isn't just useful for buyers — it's essential for the professionals who work with them. Agents who can explain why a buyer's pre-approval came in at $380,000 instead of $450,000 build trust and reduce frustration. Loan officers who proactively walk clients through debt-paydown scenarios before application convert more pre-qual conversations into closed loans.
If you're a real estate agent or loan officer, tools like our mortgage payment calculator and this affordability calculator are designed to sit in your listing presentations, buyer consultations, and follow-up emails — giving clients a reason to come back to you, and a way to understand their options clearly.
Keeping track of those buyer and borrower conversations — who's ready now, who needs to save more, who's a 90-day follow-up — is exactly what Closed by Friday is built for. The free plan supports up to 250 contacts with pipeline tracking and automated follow-up sequences, so no prospect falls through the cracks while they're getting their finances in order.
Frequently asked questions about home affordability
What is the 28/36 rule?
The 28/36 rule is a classic guideline that says your monthly housing costs (PITI) should not exceed 28% of your gross monthly income, and your total monthly debts (housing + all other obligations) should not exceed 36%. It's a conservative benchmark — many conventional loan programs allow up to 43% on the back-end DTI, and FHA up to 50% in some cases — but it's still a useful sanity check for buyers who want to ensure comfortable, sustainable payments, not just maximum qualification.
How much should I put down on a home?
There is no single right answer, but a few thresholds matter:
- 3–3.5%: Minimum for most conventional and FHA loans
- 10%: Reduces PMI premium significantly on many policies
- 20%: Eliminates conventional PMI entirely
- 25%+: Can unlock the best rate tiers on investment or jumbo loans
Going above 20% isn't always the right move — the opportunity cost of tying up extra cash matters too. A mortgage professional can model both scenarios for your specific situation.
Does rent I'm paying count as a debt in DTI?
Typically, no. Once you're buying, your projected PITI replaces your rent in the DTI calculation. Your existing rent payment is generally not added on top of the mortgage for DTI purposes — lenders are evaluating the new obligation, not the sum of both. However, if you plan to keep a rental property and not sell it, different rules apply. Confirm the treatment of your specific situation with your loan officer.
What credit score do I need to buy a home?
Minimum credit score requirements vary by loan type: most conventional lenders require a 620 minimum FICO; FHA loans can be available with scores as low as 580 (with 3.5% down) or even lower with a larger down payment; VA loans have no official minimum but most lenders set one between 580–620. Higher scores unlock better rate tiers. This calculator does not factor in credit score — speak with a licensed loan officer for a complete picture. Not legal or financial advice.
Should I get pre-approved before using this calculator?
This calculator is a great first step to understand where you stand before approaching a lender. Many buyers use it to identify their approximate price range, model the impact of paying down debts, or understand the DTI math before their first lender conversation. A formal pre-approval from a licensed loan officer will be more precise and is required by most sellers before accepting an offer.
Related tools and resources
For agents and loan officers building buyer consultations around these numbers:
- Mortgage Payment Calculator — Calculate exact monthly P&I for any loan amount, rate, and term
- Seller Net Sheet Calculator — Estimate seller proceeds after commissions and closing costs
- Commission Split Calculator — Model agent/broker commission splits quickly
Close more buyers — automate the follow-up
Real estate agents and loan officers use Closed by Friday to keep every buyer conversation active — from the first affordability question through closing day. Smart lists show you who's ready to move. Automated text and email sequences follow up on your behalf. Built-in AI drafts the messages. Free plan: 250 contacts, pipeline, tasks, no credit card required.
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