GDS and TDS Explained: How Canadian Lenders Actually Calculate What You Can Afford

Ask a Canadian mortgage lender how much you can borrow, and the real answer runs through two ratios most first-time buyers have never heard of: GDS and TDS, not simply your income or the house price. Together, they're the actual mechanism lenders use to decide whether a mortgage is affordable, and understanding what counts toward each one is one of the more practically useful things a buyer can learn before house-hunting, not after an application gets declined. It's worth noting these ratios are separate from how the interest itself is calculated. Canadian fixed-rate mortgages compound semi-annually rather than monthly, which is its own quirk covered in our comparison of mortgage compounding across countries.

GDS: your housing costs alone

The Gross Debt Service (GDS) ratio measures what share of your gross (pre-tax) monthly income goes toward housing costs specifically. For an insured mortgage (one with less than 20% down, requiring mortgage default insurance), the typical ceiling is 39% of gross monthly income.

What counts toward GDS:

• Mortgage principal and interest
• Property taxes
• Heating costs (usually a standardized estimate, not your actual bill)
• 50% of condo fees, if applicable

What doesn't count: anything unrelated to the home itself, such as car payments, credit cards, or student loans. GDS is narrowly about the cost of housing.

TDS: housing costs plus everything else you owe

The Total Debt Service (TDS) ratio takes the same housing-cost numerator as GDS and adds every other debt obligation you're carrying (credit card minimum payments, car loans, lines of credit, student loans, and so on), then measures that total against gross monthly income. The typical ceiling here is 44%.

TDS is usually the binding constraint for anyone carrying meaningful non-housing debt, because it has to accommodate both the mortgage and everything else. Two applicants with identical income and an identical mortgage request can be approved and declined respectively, purely based on how much car and credit card debt each one is carrying, even though their GDS ratios (housing costs alone) would be identical.

A worked comparison

Take two applicants, each with $8,000 in gross monthly household income, each applying for a mortgage with housing costs (principal, interest, taxes and heating) of $2,800 a month.

• Applicant A, no other debt: GDS = $2,800 ÷ $8,000 = 35% (passes the 39% ceiling). TDS = same $2,800 ÷ $8,000 = 35% (comfortably passes the 44% ceiling too).
• Applicant B, carrying a $650/month car payment and $200/month in minimum credit card payments: GDS is identical at 35%, since car and credit card debt don't factor into GDS at all. But TDS = ($2,800 + $650 + $200) ÷ $8,000 = 45.6%, over the typical 44% ceiling, even though the housing cost itself is unchanged.

Applicant B, on paper, "can afford the house" by GDS standards, but fails on TDS because of debt that has nothing to do with the house itself. This is precisely why paying down a car loan or a credit card balance before applying can sometimes unlock mortgage approval, or a larger approved amount, even when the housing costs stay exactly the same.

Why these specific ceilings, and who sets them

GDS and TDS ceilings of 39% and 44% are the standard limits used for insured mortgages (CMHC, Sagen, or Canada Guaranty-backed, for down payments under 20%). Individual lenders can, and sometimes do, apply somewhat different (occasionally more flexible) ceilings for conventional (uninsured, 20%+ down) mortgages, particularly for applicants with strong credit and stable income. The 39%/44% figures are best treated as the standard reference point, not an absolute rule that applies identically at every lender for every mortgage type.

What this means practically, before you apply

• Run both ratios yourself before approaching a lender. If your TDS is close to or over 44%, focus on paying down existing debt (even modestly) before applying — the effect on your approved amount can be more significant than it looks, since it's a ratio, not a fixed dollar threshold.
• Remember GDS caps housing costs specifically, so a larger down payment (which lowers your mortgage principal and interest) or choosing a home with lower property taxes both directly improve your GDS ratio.
• A car loan or line of credit you're planning to pay off soon still counts against TDS today.If you can reasonably pay it down or off before applying, do it beforehand rather than after — the ratio is assessed at the point of application, not on your future intentions.

Do the math before the lender does

GDS and TDS are the actual formula behind "how much mortgage can I get," not bureaucratic hoops, and unlike a black-box credit decision, both are fully calculable in advance with numbers you already have: your income, your housing-cost estimate, and your existing debt payments. Working them out yourself before applying tells you, with real precision, whether you're likely to be approved for the amount you want, and exactly what to pay down first if you're not.