The 28/36 Rule of Affordability
Lenders typically use the 28/36 rule to determine how much they are willing to lend you. This rule focuses on your Debt-to-Income (DTI) ratio, which is the percentage of your gross monthly income that goes toward paying debts.
The Mathematics of Affordability
The calculator determines your maximum home price by working backward from your income. Here is how the math works:
- Front-End Ratio (28%): We multiply your gross monthly income by 0.28. This is the maximum the lender wants you to spend on housing (Principal, Interest, Taxes, Insurance).
- Back-End Ratio (36%): We multiply your gross monthly income by 0.36 and subtract your existing monthly debts (car loans, student loans, etc.). This is the other maximum for your housing payment.
- The "Allowable" Payment: The lender will use the lower of the two numbers above.
- Reverse Amortization: Using that allowable payment, the current interest rate, and the loan term, we calculate the maximum loan amount you can support.
- Total Home Price: We add your Down Payment to the maximum loan amount to find your final purchase price.
Strategic Advice for Home Buyers
- The "Hidden" Costs of Ownership: Your mortgage is just the beginning. A good rule of thumb is the 1% Rule: set aside 1% of your home's value every year for maintenance and repairs. If you buy a $400,000 home, you should budget $4,000 a year ($333/mo) for things like leaky roofs, broken HVACs, or painting.
- Don't Spend Your Last Dollar: Lenders will tell you the maximum you can borrow, but they don't know your lifestyle. If you enjoy traveling, dining out, or have high childcare costs, you should aim for a home price 10-20% lower than what the calculator says you can afford. This "buffer" prevents you from being "house poor."
- Closing Costs are Substantial: In addition to your down payment, you will need between 2% and 5% of the home's price for closing costs (taxes, title insurance, appraisal fees, etc.). On a $300,000 home, that's an extra $6,000–$15,000 in cash you need on closing day.
- The Power of the Down Payment: Putting 20% down doesn't just lower your monthly payment; it eliminates Private Mortgage Insurance (PMI), which can cost $100–$300 a month. That's money that goes straight into your equity rather than the insurance company's pocket.
Frequently Asked Questions
Why do lenders use Gross Income instead of Net Income?
Lenders use Gross Income (before taxes) because it is a consistent, verifiable number. Since tax situations vary wildly between individuals, using Gross Income provides a standardized benchmark for the banking industry.
Can I still buy a house with a high DTI ratio?
Yes. Some loan programs (like FHA or VA) allow for back-end DTI ratios as high as 43% or even 50% in certain cases. However, borrowing this much increases your financial risk and usually comes with higher interest rates.
Does a higher interest rate lower my affordability?
Significantly. Because more of your monthly payment goes toward interest rather than principal, a 1% increase in interest rates can reduce your total "buying power" by about 10%. This is why it pays to shop around for the best mortgage rate.
Example Scenario: The $120,000 Household
Consider a couple earning a combined $120,000 per year ($10,000/mo). They have $500 in monthly car payments and $50,000 saved for a down payment.
- 28% Limit: $2,800/mo for housing.
- 36% Limit: ($3,600 - $500 debt) = $3,100/mo for housing.
- Allowable Housing Payment: $2,800 (the lower number).
Assuming a 6.5% interest rate and 1.2% property tax, the calculator shows they can afford a home worth approximately $415,000. Their mortgage would be $365,000, and their $50,000 down payment covers the rest.