How Lenders Decide What You Can Afford
Mortgage lenders don't just look at your income. They calculate two debt-to-income (DTI) ratios to determine your maximum loan amount:
Front-End DTI: Housing Ratio (28%)
Formula: (Housing payment) ÷ (Gross monthly income)
Housing payment includes: Principal + Interest + Property taxes + Homeowners insurance + HOA fees (if any)
Lender limit: 28% (some lenders allow up to 31%)
Example: If you earn $5,000/month gross, your max housing payment is $1,400 (28% of $5,000).
Back-End DTI: Total Debt Ratio (36–43%)
Formula: (All monthly debt) ÷ (Gross monthly income)
All monthly debt includes: Housing payment + car loans + student loans + credit card payments + personal loans + alimony
Lender limit: 36% (conventional), 43% (FHA, VA), 35% (Canada stress test)
Example: If you earn $5,000/month gross and pay $400 in car loans + $200 in student loans, your max total debt is $1,750 (35% of $5,000). Max housing payment is $1,750 – $600 = $1,150.
What Determines Your Maximum Home Price?
Your maximum home price is limited by whichever is lower:
- Front-end limit: Maximum housing payment based on 28% of income
- Back-end limit: Maximum housing payment after accounting for existing debts
The Math Behind It
Once we know your maximum housing payment, we work backwards to find the home price:
- Maximum payment = Front-end or back-end limit (whichever is lower)
- Payment = Principal + Interest (P&I) + Property taxes + Insurance
- Solve for home price
Factors That Improve Your Affordability
Higher Income
A $10K annual income increase raises your debt ceiling by ~$230/month. Over a 30-year mortgage at 6%, that supports an additional home price of ~$40K.
Lower Existing Debts
Pay off your car loan or credit cards before buying. Each $100/month in reduced debt payments frees up ~$170 in home buying power.
Larger Down Payment
A 20% down payment (vs. 10%) reduces your loan amount, lowering the P&I portion of your payment. This leaves more room in your DTI for taxes and insurance.
Better Credit & Lower Interest Rate
A 0.5% lower interest rate reduces your P&I payment by ~$100–$200/month (depending on loan size), freeing up that much DTI capacity.
Lower Property Taxes or Insurance
Buying in a lower-tax state or a low-cost insurance area reduces your monthly payment. Moving from New Jersey (high tax) to Texas (low tax) can enable you to afford a $50K+ higher home price.
Common Mistakes
Ignoring the Back-End Ratio
You might hit the 28% front-end limit on housing, but if you have $400/month in other debts, you hit the back-end limit much sooner. Always check both.
Forgetting Taxes and Insurance
Many buyers focus only on P&I (principal + interest). But taxes and insurance are real costs that lenders include in DTI. A $400K home in New Jersey costs $300–$400 more per month in taxes alone vs. Texas.
Maxing Out Your Budget
Just because you can afford 28% of income doesn't mean you should spend it. Financial experts recommend 20–25% for flexibility. A lower housing cost leaves room for emergencies, vacations, and savings.
Assuming Perfect Credit
If your credit score is below 650, you won't qualify for prime rates. Check your credit before applying. You might need to improve it first.