Before you start browsing listings, there is one number you need to nail down: how much house you can actually afford. Falling in love with a home outside your budget is one of the fastest ways to make the homebuying process painful. This guide walks through the formulas lenders use, the rules of thumb financial advisors recommend, and how to build your own honest number.
The Two Ways to Calculate Affordability
There are two lenses on this question: what lenders will approve and what you can comfortably afford. These are often not the same number. Lenders will sometimes approve you for more than you should spend. Your job is to find the lower figure.
The Lender’s Formula: Debt-to-Income Ratio
Lenders qualify you based on your debt-to-income ratio (DTI). They look at two numbers:
- Front-end DTI: Your proposed monthly housing payment (principal, interest, taxes, insurance, and HOA if applicable) divided by your gross monthly income. Most lenders want this at 28% or lower for conventional loans; FHA allows up to 31%.
- Back-end DTI: All monthly debt payments including the new mortgage, car loans, student loans, and minimum credit card payments. Most lenders cap this at 43% to 45% for conventional; FHA allows up to 57% with compensating factors.
To estimate the maximum mortgage payment a lender would approve, multiply your gross monthly income by 0.28 (front-end) or 0.43 (back-end, after subtracting existing debts). Take the lower number.
Example:
Gross monthly income: $7,500
Front-end limit (28%): $2,100/month
Existing monthly debts (car + student loans): $600
Back-end limit (43%): $7,500 × 0.43 = $3,225 − $600 = $2,625/month
Binding limit: $2,100/month (the lower figure)
The 28/36 Rule
A stricter version favored by many financial planners is the 28/36 rule: spend no more than 28% of gross income on housing and no more than 36% on total debt. This leaves more room for savings, emergencies, and retirement contributions.
Using the same example above, the 36% back-end limit would be $7,500 × 0.36 = $2,700 minus $600 existing debts = $2,100 max housing payment. In this case both rules produce the same number, but if the borrower had more existing debt, the 36% cap would bite harder than the lender’s 43%.
From Monthly Payment to Purchase Price
Once you know your maximum monthly housing payment, you can work backward to a purchase price. A simplified formula for the principal and interest portion of a 30-year mortgage:
Loan amount = Monthly P&I payment ÷ (Interest rate / 12) × [1 − (1 + r)^−360]^−1
For practical purposes, use a mortgage calculator. At 7.0% interest on a 30-year loan:
| Monthly P&I Budget | Approximate Loan Amount |
|---|---|
| $1,500 | ~$225,000 |
| $1,800 | ~$270,000 |
| $2,100 | ~$315,000 |
| $2,500 | ~$375,000 |
Add your down payment to the loan amount to get the purchase price. Remember to reserve 1% to 2% of your monthly housing budget for property taxes and insurance — these are real costs that reduce the P&I you can afford.
The Down Payment Variable
Your down payment directly affects how much home you can afford. A larger down payment means a smaller loan, lower monthly payments, and potentially no mortgage insurance. Here is how down payment changes the math on a $350,000 purchase:
| Down Payment | Loan Amount | Monthly P&I (7.0%) | Monthly PMI (~0.6%) | Total Monthly |
|---|---|---|---|---|
| 3% ($10,500) | $339,500 | $2,260 | $170 | $2,430+ |
| 10% ($35,000) | $315,000 | $2,096 | $158 | $2,254+ |
| 20% ($70,000) | $280,000 | $1,863 | $0 | $1,863+ |
Don’t Forget These Hidden Costs
The mortgage payment is not the only housing expense. Budget for:
- Property taxes: Vary widely by location. In high-tax states like New Jersey or Illinois, property taxes can add $500 to $1,500 per month on a mid-range home.
- Homeowners insurance: Typically $100 to $200/month, more in coastal or high-risk areas.
- HOA fees: Can range from $50 to $1,000+/month for condos and planned communities.
- Maintenance and repairs: Budget 1% to 2% of the home’s value per year. On a $350,000 home, that is $3,500 to $7,000 annually.
- Utilities: Owning often means higher utility costs than renting — heating, cooling, water, and trash.
The Practical Affordability Check
Rather than starting with maximum qualification, start with your actual monthly budget:
- List your take-home (after-tax) income
- List all fixed monthly expenses (car, student loans, insurance, subscriptions, childcare)
- Subtract what you want to save each month (retirement, emergency fund, other goals)
- Whatever is left is your available spending — housing competes with groceries, dining, hobbies, and travel
- From your remaining budget, decide what feels comfortable for housing
This bottom-up approach often yields a number that is meaningfully lower than the lender’s maximum — and a payment you can actually live with without feeling house-poor.
A Common Mistake: Confusing Pre-Approval Amount With Budget
Lenders approve you for the maximum they are willing to lend, not the amount that fits your lifestyle. It is not unusual for a lender to pre-approve someone for $450,000 when the buyer’s actual comfortable budget is $300,000. Use the pre-approval as a ceiling, not a target.
Salary-to-Home-Price Rules of Thumb
Simple rules to sanity-check your number:
- 2x to 3x gross annual income: Conservative rule. On a $100,000 salary, that’s $200,000 to $300,000.
- 4x to 5x gross income: Stretching into typical urban market territory. Manageable if other debts are minimal.
- More than 5x: Proceed carefully. High sensitivity to rate increases, job disruptions, or unexpected repairs.
Bottom Line
The honest answer to “how much house can I afford” is the lower of: what a lender will approve and what your monthly budget can comfortably handle without sacrificing savings, retirement, or quality of life. Run both calculations before you start shopping, and treat the lender’s number as a ceiling rather than a goal.
If the payment at your target price feels tight, the better move is to wait, save more, and improve your credit score rather than buy at the edge of your capacity. A home should build wealth — not create financial stress every month.