How Lenders Calculate Home Affordability: DTI, 28/36 Rule & More (2026)
Quick Answer
- *The 28/36 rule: housing costs should not exceed 28% of gross income; total debts should not exceed 36%.
- *Lenders evaluate two DTI ratios — front-end (housing only) and back-end (all debts). CFPB guidelines cap back-end DTI at 43% for conventional loans.
- *The average 30-year fixed rate was 6.72% as of March 2026 (Freddie Mac), making the monthly payment on a $300K mortgage about $1,942.
- *PMI adds 0.5–1.5% annually for buyers with less than 20% down — a real cost that must factor into your affordability calculation.
Why Affordability Math Matters
The median existing home price in the US hit $407,500 in 2025, according to the National Association of Realtors. With 30-year fixed rates at 6.72% as of March 2026 (Freddie Mac Primary Mortgage Market Survey), a buyer financing 80% of that median price carries a monthly principal and interest payment of around $2,135 — before taxes, insurance, or PMI.
Getting pre-approved for a mortgage is not the same as buying a house you can comfortably afford. Lenders approve you up to a maximum. That maximum is not a budget recommendation. This guide explains exactly how lenders do the math, so you can decide what actually works for your finances.
The 28/36 Rule Explained
The 28/36 rule is the classic guideline for housing affordability. It sets two limits:
- Front-end limit (28%): Your monthly housing costs — principal, interest, property taxes, and homeowners insurance (PITI) — should not exceed 28% of your gross monthly income.
- Back-end limit (36%): Your total monthly debt payments — mortgage plus car loans, student loans, credit card minimums, and any other recurring debt — should not exceed 36% of gross monthly income.
For a household earning $90,000/year ($7,500/month gross), these thresholds work out to:
| Rule | Limit | Monthly Amount |
|---|---|---|
| 28% front-end | Housing (PITI) only | $2,100 |
| 36% back-end | All debts combined | $2,700 |
If you have $500/month in existing debt (car payment + student loan), the 36% back-end cap leaves $2,200 for housing. The binding constraint in this case is the back-end limit, not the front-end one.
Keep in mind: the 28/36 rule is a guideline, not a hard regulatory floor. Many modern lenders approve loans with higher ratios. The rule is a solid personal finance target, not necessarily what the bank will require.
Front-End DTI vs Back-End DTI
Debt-to-income ratio (DTI) is the core metric lenders use to underwrite mortgages. There are two versions, and both matter.
Front-End DTI (Housing Ratio)
Front-end DTI = Monthly PITI ÷ Gross Monthly Income
PITI includes: principal + interest + property taxes + homeowners insurance. If you have an HOA fee, some lenders add that here too. PMI is also typically included for buyers putting less than 20% down.
Back-End DTI (Total Debt Ratio)
Back-end DTI = (Monthly PITI + All Other Monthly Debts) ÷ Gross Monthly Income
Other debts include minimum payments on credit cards, car loans, student loans, personal loans, child support, and alimony. It does not include utilities, groceries, subscriptions, or living expenses.
According to CFPB guidelines, conventional lenders prefer a back-end DTI no higher than 43%. Most FHA loans allow up to 50% DTI with compensating factors — things like a high credit score (720+) or substantial cash reserves after closing.
How Lenders Actually Qualify Buyers
The pre-approval process is more detailed than just running DTI ratios. Here is what lenders actually examine:
- Gross income documentation: W-2s, pay stubs, tax returns (2 years), and for self-employed buyers, profit/loss statements. Lenders use gross income — before taxes — not net take-home pay.
- Credit score and credit report: Conventional loans typically require 620+. Scores above 740–760 unlock the best rates. Lenders pull all three bureaus and use the middle score.
- Debt obligations: Every recurring debt on your credit report. Even a $50/month credit card minimum counts. Lenders use the minimum payment shown on your credit report, not your actual payment habits.
- Assets and reserves: Down payment funds plus 2–6 months of mortgage payments in reserves (varies by lender). Gift funds from family are allowed with documentation.
- Employment history: Lenders prefer 2 years of stable employment in the same field. Job changes within the same industry are generally acceptable. Gaps raise questions.
- Property appraisal: The home must appraise at or above the purchase price. If it appraises low, you either renegotiate, pay the difference in cash, or walk away.
Income Required at Different Home Prices
The table below shows the annual gross income needed to afford various home prices under the 28% front-end rule, assuming a 6.72% rate, 30-year term, 20% down, and estimating 1.2% annually for property taxes and insurance combined.
| Home Price | Loan Amount (80%) | Monthly P&I | Est. Monthly PITI | Income Required (28% rule) |
|---|---|---|---|---|
| $250,000 | $200,000 | $1,294 | $1,544 | $66,200/yr |
| $350,000 | $280,000 | $1,811 | $2,161 | $92,600/yr |
| $407,500 | $326,000 | $2,107 | $2,515 | $107,800/yr |
| $500,000 | $400,000 | $2,587 | $3,087 | $132,300/yr |
| $700,000 | $560,000 | $3,622 | $4,322 | $185,200/yr |
The $407,500 row is the national median (NAR, 2025). At 6.72% with 20% down, buying the median home requires roughly $108K in annual household income just to stay within the 28% guideline — before accounting for existing debts. For buyers carrying a car payment and student loans, the required income climbs further.
What Happens If You Put Less Than 20% Down
According to the National Association of Realtors (2025), the average down payment was 13% for all buyers and just 8% for first-time buyers. Most buyers are not putting 20% down.
Putting less than 20% down has two affordability consequences:
- Higher loan balance: More principal means a larger monthly payment.
- PMI: Private mortgage insurance is required by conventional lenders when your down payment is below 20%. PMI typically costs 0.5–1.5% of the loan amount annually (Urban Institute, 2024). On a $300,000 loan, that is $1,500–$4,500/year, or $125–$375/month added to your payment.
| Down Payment | Loan Amount | Monthly P&I | Est. PMI/month | Total Monthly Payment |
|---|---|---|---|---|
| 20% ($70,000) | $280,000 | $1,811 | $0 | ~$2,161 |
| 10% ($35,000) | $315,000 | $2,037 | ~$263 | ~$2,650 |
| 5% ($17,500) | $332,500 | $2,150 | ~$332 | ~$2,832 |
| 3.5% ($12,250) | $337,750 | $2,184 | ~$338 | ~$2,872 |
On a $350,000 home, the difference between 20% down and 3.5% down is roughly $700/month. That is a meaningful gap in affordability, and it directly affects your DTI calculation.
PMI is not permanent. Once your equity reaches 20% — through appreciation, payments, or both — you can request cancellation. Under the Homeowners Protection Act, lenders must automatically cancel PMI when your loan-to-value ratio hits 78%.
How to Improve Your Home Affordability
There are four real levers, and none of them are magic. They require time or money — usually both.
1. Pay Down Existing Debt
Your back-end DTI is the most common constraint. Paying off a $400/month car loan frees up $400 in debt capacity — which translates to a larger mortgage at the same DTI. A $400/month freed up at 6.72% for 30 years supports roughly $62,000 more in loan principal.
2. Increase Your Down Payment
More down means a smaller loan, lower monthly payment, no PMI (at 20%+), and often a better rate. The National Association of Realtors reports first-time buyers average 8% down — moving to 20% on a $350,000 home means saving an additional $42,000, but it reduces your monthly payment by roughly $500 and eliminates PMI entirely.
3. Boost Your Gross Income
Both DTI ratios use gross income in the denominator. A raise, second income, or documented side income (typically requires 2 years of tax returns to count) directly expands your DTI ceiling. A $10,000/year income increase adds $833/month to gross income, which under the 28% rule adds $233/month in housing capacity — roughly $30,000 more in purchasing power.
4. Improve Your Credit Score
Your credit score determines your interest rate. Moving from a 680 to a 760 score can reduce your mortgage rate by 0.5–1 percentage point. On a $300,000 loan, that is roughly $100–$200/month lower payment and $36,000–$72,000 less in total interest over 30 years. Strategies: pay down revolving balances below 30% utilization, dispute errors, avoid opening new accounts before closing.
Run the numbers for your situation
Use our free Home Affordability Calculator →Also see: Mortgage Calculator · How Much House Can I Afford?
Frequently Asked Questions
What is the 28/36 rule for buying a house?
The 28/36 rule is a guideline used by lenders and financial planners to set limits on housing debt. The “28” means your front-end DTI — monthly mortgage payment (principal, interest, taxes, insurance) — should not exceed 28% of your gross monthly income. The “36” means your total debt obligations, including the mortgage plus all other recurring debts, should not exceed 36% of gross monthly income. Many conventional lenders use this as a starting benchmark, though modern underwriting often allows higher ratios with strong compensating factors.
How much house can I afford on a $100,000 salary?
On a $100,000 annual salary (about $8,333/month gross), the 28% front-end rule allows up to $2,333/month for housing costs. At a 6.72% mortgage rate (March 2026, Freddie Mac) with a 20% down payment and 30-year term, that monthly payment supports a home price of roughly $340,000–$360,000. With a 10% down payment and PMI included, the affordable price drops to around $280,000–$310,000. Use our Home Affordability Calculator for a precise estimate based on your debt load, credit score, and down payment.
What is debt-to-income ratio for a mortgage?
Debt-to-income ratio (DTI) is your total monthly debt payments divided by your gross monthly income, expressed as a percentage. Lenders use two versions: front-end DTI (housing costs only) and back-end DTI (all debts combined). According to CFPB guidelines, conventional lenders prefer a back-end DTI no higher than 43%. FHA loans allow up to 50% DTI with compensating factors such as a high credit score or significant cash reserves. The lower your DTI, the more favorable your loan terms.
How do I improve my home affordability?
The four main levers are: (1) increase your down payment to eliminate PMI and reduce the loan balance; (2) pay off existing debts to lower your back-end DTI; (3) boost your gross income, since both DTI ratios use gross income as the denominator; and (4) improve your credit score, which can reduce your rate by 0.5–1%. Shopping multiple lenders also typically saves 0.25–0.5% on the rate, which adds up to real money over 30 years.
What is PMI and can I avoid it?
PMI (private mortgage insurance) is required by most conventional lenders when your down payment is less than 20%. It protects the lender if you default — not you. PMI typically costs 0.5–1.5% of the loan amount annually (Urban Institute, 2024), which on a $300,000 loan is $1,500–$4,500/year. You can avoid PMI by putting 20% or more down, using an 80-10-10 piggyback loan structure, or qualifying for a VA or USDA loan. Once your equity reaches 20%, you can request PMI cancellation; lenders must cancel it automatically at 22% equity.
How does my credit score affect how much house I can afford?
Your credit score affects both whether you qualify for a mortgage and the interest rate you receive. Scores above 760 typically unlock the best rates. Below 620 generally disqualifies buyers from conventional loans (FHA allows down to 500 with 10% down, or 580 with 3.5% down). The rate difference between a 620 and a 760 score can be 1–1.5 percentage points. On a $350,000 30-year mortgage, that gap means roughly $200–$300 more per month and over $70,000 more in total interest paid over the life of the loan.