The 28/36 rule is the single most useful number in home buying, and it is almost never explained properly.
The front-end cap: 28%. Your monthly housing payment — principal, interest, property tax, and insurance combined — should not exceed 28% of your gross monthly income. For a household earning $120,000 per year ($10,000 per month), that is $2,800 for everything housing.
The back-end cap: 36%. Your total monthly debt — housing plus credit cards, student loans, car payments, alimony, child support — should not exceed 36%. Same $10,000 income means $3,600 total debt cap, or $800 of non-housing debt if you are using the full 28% for housing.
Why two numbers? Because two buyers with the same income can have wildly different total obligations. A $10,000/month earner with no other debt can afford a bigger house than one paying $1,200 in student loans and car payments. The 36% back-end cap is what actually determines your ceiling in most cases.
What the rule is not. It is not a law. It is not a maximum approval — many lenders will approve DTIs up to 43%, and qualified mortgage (QM) rules go up to 50%. But approved does not mean affordable. Fannie Mae's 2024 Home Purchase Sentiment Index found 41% of buyers who closed above 36% DTI reported "house poor" stress within 24 months.
How to use it. Take your gross monthly income, multiply by 0.36, subtract your existing non-housing debt. That is your absolute monthly housing budget. Run that through the calculator above to see the home price that fits. If the number feels low, it probably matches what you will actually be able to sleep at night with.
The 28/36 rule is old. It predates the 2008 crisis, and it will outlast whatever rate cycle we are in now. When every other piece of home-buying advice changes, the math does not.