The way most lenders look at how much you can be approved for is by looking at your Gross Debt Service Ratio (GDS) and your Total Debt Service Ratio (TDS).
The GDS is the maximum percentage of your gross income that is allocated to paying the costs of carrying your home. This ratio includes your principal and interest mortgage payment, property taxes, heating and/or condo fees. To qualify your monthly carrying costs cannot exceed 32% of your gross monthly income.
The TDS is the maximum percentage of your gross income that can be used to pay your GDS plus all other debts. This ratio includes everything from the GDS as well as any other loans, credit cards or lines of credit. To qualify it cannot exceed 40% of your gross monthly income.