Getting pre-approved for a mortgage feels like an answer to "how much house can I afford," but pre-approval numbers are based on what a lender is willing to risk lending you — not necessarily what fits comfortably into your actual monthly budget. Those are two different questions with two different answers.
Lenders primarily use your debt-to-income ratio (DTI) to determine your maximum loan: the percentage of your gross monthly income that goes toward debt payments, including the new mortgage. Most lenders cap total DTI around 36-43%, though some programs allow more with strong compensating factors like excellent credit or a large down payment. That ceiling is designed to protect the lender from default risk, not to reflect what leaves you with a comfortable cushion for savings, emergencies, and everyday life.
How Affordability Is Calculated
The calculation works backward from your income: take your gross monthly income, multiply by your target DTI (say 36%), then subtract your existing monthly debts (car payments, student loans, credit cards) to find your maximum monthly payment for principal and interest. From there, that monthly payment amount is run through the standard loan formula at your expected rate and term to solve for the maximum loan amount, and your down payment is added on top to get your maximum home price.
Max Home Price = Max Loan Amount (solved from max P&I payment) + Down Payment
A Worked Example
With a $100,000 annual income ($8,333/month), $400 in existing monthly debts, a 6.5% rate, 30-year term, and a 36% DTI target, your maximum debt payment is $3,000/month. Subtracting the $400 in existing debts leaves $2,600/month available for principal and interest, which supports a loan of roughly $411,000. Add a $40,000 down payment, and your maximum affordable home price comes out to about $451,000.
That's the lender's ceiling, though — many financial planners recommend targeting a DTI closer to 25-28% for housing alone, which in this example would support a home price closer to $340,000-$370,000, leaving meaningfully more breathing room in the monthly budget.
Common Mistakes to Avoid
- Borrowing to the maximum approved amount: just because you qualify doesn't mean it's comfortable — leave room for savings, retirement contributions, and unexpected expenses.
- Forgetting property tax and insurance in the affordability math: these add real dollars on top of principal and interest and reduce what you can actually afford to borrow.
- Not accounting for future income changes: a DTI that's comfortable now may not be if income drops or a partner stops working.
- Skipping the down payment size in the equation: a larger down payment both lowers your loan amount and can improve your rate, both increasing what you can afford.
Bottom Line
Your real affordability number is often more conservative than your pre-approval letter. Use a House Affordability Calculator with your actual income, debts, and a DTI target you're personally comfortable with — not just the lender's maximum — to find a home price that fits your life, not just your credit profile.