Ready to Buy? Do the Math (Because Lenders Will). The 28/36 rule can help you estimate how much home you can afford. 28% of your gross monthly income is the maximum lenders want to see going toward housing costs. That includes principal, interest, taxes and insurance. 36% is the ceiling for all your monthly debt payments combined, including your new mortgage. Know where you stand before you make an offer. [callout] Search "28/36 rule" to find an online calculator. Source: Bankrate