Most people start house-hunting with a number in their head — what they think they can afford. Then a lender pre-qualifies them for a much bigger number, and they go shopping in a higher price range. That’s how mortgage stress is born.
After thirty-seven years in mortgage and commercial lending, I can tell you the gap between “what a lender will approve” and “what you can comfortably afford” is the single biggest source of post-closing regret. Let me walk you through how to think about this the right way.
The two numbers a lender uses
When a lender pre-qualifies you, they apply two ratios.
Front-end ratio is the monthly housing payment divided by gross monthly income. Most conventional loans cap this around 28–31%.
Back-end ratio is total monthly debt (housing plus all other minimum payments — credit cards, car loans, student loans, child support) divided by gross monthly income. Typical cap is 43–45%, though some loan programs allow 50% with strong compensating factors.
If you make $8,000 gross monthly and have $500 in existing minimum debt payments, the math at a 45% back-end ratio looks like this:
- Total debts allowed: $8,000 × 0.45 = $3,600
- Subtract existing debts: $3,600 − $500 = $3,100 maximum housing payment
- That housing payment includes principal, interest, taxes, insurance, and PMI
That $3,100/month is the lender’s “you qualify” number. The lender will then need to make an assumption on how much of that total payment is for taxes and insurance (Escrow payment), and how much will be left for the principal and interest. At today’s rates (around 6.375% on a 30-year loan), assuming a 5% down payment in a state like Texas, that translates to roughly $335,000 for a loan amount, and home price of $352,650.
The loan and home price can vary depending on which state you are in and how much you are putting down as a down payment. The one constant regardless of down payment or state is the total payment you can qualify for. That number stays the same.
You can run this exact math through the Pre-Qualification Calculator — it backs into the maximum home price your DTI will support.
Why the lender’s number is too high
The lender’s calculation is based on what you can pay, not what you should pay. It uses gross income (before taxes), but you live on net income (after taxes). And it doesn’t account for:
- Retirement contributions (401k, IRA)
- Health insurance premiums
- Childcare
- Saving for next year’s emergency
- The fact that homeowner’s insurance and property taxes go up every year, but the lender only checks today’s number
A real-world rule I’ve used with clients: take the lender’s max housing payment and multiply by 0.85. If you can comfortably afford 85% of what they’ll let you borrow, you’ll have breathing room for life.
The 30/40/20 sanity check
Here’s the framework I give my engineering and STEM clients who like rules:
- 30% of gross income maximum on housing (PITI)
- 40% of gross income maximum on housing plus all debt
- 20% of net income going to retirement, emergency funds, and other goals
If your numbers stay inside that envelope, you can absorb a major appliance failing, a medical bill, or a job change without the mortgage collapsing the rest of your finances.
What moves the math significantly
Three things change your affordable price range:
Down payment. Going from 5% down to 20% down doesn’t just reduce your loan balance — it eliminates PMI entirely. On a $400,000 home, that’s $150–$250/month back in your pocket every month, forever. Use those savings to qualify for more home or keep the smaller home and bank the cash.
Term length. A 15-year loan has a higher monthly payment but cuts total interest dramatically. 30 years is easier on cash flow but doubles total interest paid. There’s a strong case for 30-year amortization with voluntary extra principal payments — you keep the flexibility of the lower required payment but accelerate your payoff.
State you live in. Property tax and homeowners’ insurance rates vary wildly. Texas property tax is roughly twice California’s. Florida insurance is two to three times most of the rest of the country. The same $400,000 home can cost $300+/month more in Texas than in Tennessee.
Don’t skip the income-needed test
If you’re already targeting a specific home, run the math the other way: how much income do you need to qualify for that home? The Income Needed Calculator takes a sales price and tells you the income required at standard ratios. It’s a useful gut check when the price tag of your dream home is staring at you on Zillow.
The bottom line
A lender will tell you what you qualify for. Your accountant — or this calculator — will tell you what you can afford. They’re rarely the same number. Aim for the lower of the two.
Want to see your maximum buying power without giving anyone your email or phone number? Run your numbers through the Pre-Qualification Calculator. It’s accurate in all 50 states and handles the state-specific tax and insurance variables most online calculators ignore.