The Federal National Mortgage Association (Fannie Mae) is a government-sponsored organization that purchases mortgages from lenders and sells them to investors. Two income-to-debt ratios established by Fannie Mae are standard requirements for conventional mortgages. The first requirement is that monthly mortgage principal and interest payments (P&I), plus insurance and property taxes, cannot exceed 28% of the buyer's gross monthly income (some exceptions may apply to increase this limit to 33%). The second requirement limits total monthly debt payments (housing, credit cards, car payments, etc.) to 36% of gross monthly income. In addition to these requirements, you may have to pay 10% to 20% down on the total purchase price to qualify for a conventional mortgage.