The question "how much house can I afford?" has two very different answers: the amount a lender will approve you for, and the amount that will let you live comfortably, handle emergencies, and still make progress on your other financial goals. These numbers are rarely the same, and confusing them is one of the most common and costly mistakes first-time buyers make.
This guide explains the rules lenders use, walks you through calculating your own number manually, and explains why the right budget is often significantly lower than your pre-approval amount.
The 28/36 rule explained
The 28/36 rule is the standard used by most conventional lenders to assess affordability. It has two parts:
- The 28% front-end ratio: Your monthly housing costs (mortgage principal, interest, property taxes, homeowner's insurance, and HOA fees if applicable) should not exceed 28% of your gross monthly income.
- The 36% back-end ratio: Your total monthly debt payments (housing costs plus car loans, student loans, credit cards, and all other minimum payments) should not exceed 36% of your gross monthly income.
These are the traditional guidelines. Many lenders today will approve loans up to a 43% back-end ratio, and some programs go higher. But just because a lender will approve it doesn't mean it's the right number for you.
What is DTI (debt-to-income ratio)?
DTI is the lender's primary tool for evaluating affordability. It's calculated by dividing your total monthly debt payments by your gross monthly income (before taxes).
There are two versions:
- Front-end DTI: Housing payment only / gross monthly income
- Back-end DTI: All monthly debt payments / gross monthly income
Lenders primarily focus on back-end DTI. Most conventional loan programs cap at 43-45%. FHA loans can go up to 50% in some cases. The lower your DTI, the better rate you'll generally qualify for.
How lenders calculate your qualifying income
Lenders don't just take the number on your pay stub. Here's how different income types are treated:
- W-2 salary: Used at face value. Two recent pay stubs and two years of W-2s required.
- Hourly: Year-to-date earnings annualized, then divided by 12 for monthly income.
- Self-employed: Two-year average of net income from Schedule C or K-1 after business expenses. Business losses in any year hurt your qualifying income significantly.
- Bonuses and overtime: Averaged over two years if received consistently; excluded if it was a one-time event.
- Rental income: 75% of rental income counted (to account for vacancies), minus any rental property expenses.
- Investment/dividend income: Two-year average required; must be expected to continue.
Manual calculator walkthrough
Here is how to calculate your own number step by step, using real examples at two income levels.
Step-by-Step Calculator
Example: $90,000 gross annual income ($7,500/month)
Income and affordability at different levels
| Gross Annual Income | Max Monthly Housing (28%) | Estimated Max Home Price |
|---|---|---|
| $60,000 | $1,400 | $175,000-$210,000 |
| $80,000 | $1,867 | $240,000-$280,000 |
| $100,000 | $2,333 | $300,000-$350,000 |
| $130,000 | $3,033 | $390,000-$460,000 |
| $160,000 | $3,733 | $480,000-$570,000 |
Ranges reflect variation in rate, down payment size, and local property tax rates. These are guideline estimates only.
Why your pre-approval amount is not what you should spend
Lenders calculate the maximum they're willing to lend based on your credit profile and income. They are not calculating what leaves you with a comfortable life. Several costs are not factored into your pre-approval at all:
- Maintenance and repairs: Plan for 1-2% of the home's value per year. On a $350,000 home, that's $3,500-$7,000 annually, or $300-$580/month, that is entirely outside your mortgage payment.
- Utilities: A larger home means larger utility bills. The difference between a 1,200 sq ft apartment and a 2,400 sq ft house can be $200-$400/month in additional heating, cooling, and electricity costs.
- Lawn and exterior: Mowing, landscaping, snow removal, gutter cleaning. Budget $100-$300/month depending on the property.
- Furniture and setup costs: Many first-time buyers underestimate how much it costs to furnish a home they own vs. the apartment they rented.
- Your actual financial goals: Lenders don't care whether you're saving for retirement, paying off student loans faster, or planning to have children. Your budget should.
The standard advice: Stay at 80-90% of your pre-approval maximum. This buffer gives you room for market fluctuations, unexpected repairs, life changes, and the ability to actually enjoy living in your home rather than being house-poor.
Factors that change your purchasing power
Down payment size
A larger down payment directly reduces your loan amount and your monthly payment, and may eliminate PMI (private mortgage insurance, typically 0.5-1.5% of the loan amount annually). PMI is required when you put down less than 20% on a conventional loan. On a $350,000 loan, PMI can add $145-$435/month until you reach 20% equity.
Interest rate
Every 0.5% change in interest rate affects your purchasing power by roughly 5-6%. If rates move from 6.5% to 7%, the same monthly payment buys you roughly $25,000 less in home. Improving your credit score before applying is the most reliable way to get a lower rate.
Loan type
FHA loans accept lower credit scores (580+ for 3.5% down) and higher DTI ratios, but require both upfront and ongoing mortgage insurance that doesn't go away regardless of equity. VA loans for eligible veterans often require no down payment and no PMI. These programs expand purchasing power but change the long-term cost calculation.
For a full breakdown of how to compare loan offers across lenders, read our guide to the mortgage comparison worksheet. And for a complete look at all the costs beyond the mortgage, see our guide to closing costs explained.