The percentage arrived at by dividing your monthly shelter costs (principal, interest, property taxes, heating and half of condo fees) PLUS all other monthly debt obligations by your gross monthly income and multiplying by 100. This is used by all lenders as the \"upper limit\" yardstick by which to measure the ability of a borrower (or borrowers) to make mortgage payments. For example, most lenders require that this ratio be no more than 40% for a particular application, with some as low as 37%. 40% is also the maximum qualifying TDS in most applications for default insurance.