Debt-service ratios estimate whether gross income can support the proposed housing costs and other debt.
They are qualification tools. They are not household budgets.
Gross Debt Service ratio
Gross Debt Service, or GDS, compares qualifying housing costs with gross income.
A simplified formula is:
GDS=
Gross income
Qualifying mortgage payment+property taxes+heat+applicable condo or site costs
×100
For CMHC-insured applications, 50% of condominium fees is generally included, while 100% of applicable site or ground rent is used. CMHC currently limits GDS to 39% under its standard homeowner insurance criteria.
Total Debt Service ratio
Total Debt Service, or TDS, adds other debt obligations:
TDS=
Gross income
GDS housing costs+other debt obligations
×100
Other obligations may include:
Credit cards
Personal lines of credit
Car loans
Student loans
Other mortgages
Secured lines of credit
Court-ordered support
Other recurring credit obligations
CMHC’s current standard maximum is 44% TDS. For its calculations, unsecured revolving debts are generally assigned at least 3% of the outstanding balance as a monthly payment, while secured-line payments are assessed under its stated amortizing-payment method.
These are CMHC standards. Private insurers and individual lenders may use different inputs, limits or exception policies.
What is the qualifying payment?
The payment used in GDS and TDS may be higher than the borrower’s actual contractual payment.
For CMHC-insured mortgages, the current qualifying rate is the greater of:
Contract rate plus 2 percentage points
5.25%
CMHC currently applies maximum ratios of 39% GDS and 44% TDS under its standard purchase criteria.
For uninsured mortgages at federally regulated lenders, OSFI requires the greater of the contract rate plus the Superintendent-set buffer and the Superintendent-set floor. OSFI reviews the calibration at least annually, so the applicable rate must be rechecked before publication and before an application is submitted.
Worked example — Contract payment, qualifying payment, GDS and TDS
Illustrative assumptions
Mortgage amount: $600,000
Contract rate: 4.50%
Amortization: 25 years
Payment frequency: monthly
Canadian mortgage interest convention: nominal rate compounded semi-annually
Stress-test rate: 6.50%, because contract rate plus 2% is higher than 5.25%
Gross household income: $13,000 per month
Property taxes: $500 per month
Heating: $150 per month
Other monthly debt obligations: $700
No condominium fee
Mortgage-insurance premium excluded from this simplified example
Contractual mortgage payment
At 4.50%, the approximate monthly payment is:
$3,320.84
Qualifying mortgage payment
At 6.50%, the approximate qualifying payment is:
$4,018.94
The borrower is therefore assessed using a payment approximately:
$4,018.94−$3,320.84=$698.10
higher than the initial contractual payment.
GDS calculation
GDS=
$13,000
$4,018.94+$500+$150
×100
GDS=35.91%
TDS calculation
TDS=
$13,000
$4,018.94+$500+$150+$700
×100
TDS=41.30%
The ratios fall within CMHC’s standard 39% and 44% maximums. That does not establish approval. Credit, property, down payment, documentation, insurer acceptance and all other conditions remain relevant.
What GDS and TDS leave out
Debt-service ratios use gross income, before income tax and payroll deductions.
They may not fully capture:
Childcare
Food
Transportation
Insurance
Utilities other than the prescribed heat amount
Education
Medical expenses
Support provided voluntarily to relatives
Home maintenance
Condo special assessments
Irregular business income
Retirement saving
Lifestyle spending
Future family changes
Emergency reserves
Property repairs
A household can meet a lender’s ratio and still feel financially constrained.
Qualification maximum versus safe maximum
The lender’s maximum is designed to determine whether the loan fits a credit policy. The borrower’s safe maximum should consider after-tax cash flow and resilience.
A practical affordability test should ask:
What will remain after tax and payroll deductions?
Can the household absorb a higher payment at renewal?
What happens if one income falls temporarily?
Is there an emergency reserve after closing?
Are childcare, vehicle and commuting costs likely to change?
Does the property require significant repairs?
Is the borrower relying on overtime or bonuses?
Is the payment comfortable without repeatedly using credit?
Eligibility at the lender’s limit does not automatically establish suitability.
How other debts affect qualification
A borrower can sometimes increase mortgage qualification more efficiently by reducing debt than by making a slightly larger down payment.
For example, eliminating a $700 monthly car payment directly lowers TDS by $700. Putting the same lump sum toward the purchase may reduce the mortgage payment by much less each month.
The better decision depends on:
Interest rate
Remaining debt term
Cash reserves
Down-payment threshold
Insurance premium
Lender calculations
Need for emergency funds
A borrower should not pay off debt before the lender or broker confirms the intended treatment and the required documentary trail.
Rental-income treatment
Rental income can affect both the numerator and denominator of debt-service calculations.
Depending on the lender or insurer, rent may be:
Added partly to income
Used to offset mortgage costs
Reduced by a standardized expense factor
Taken from tax-return net income
Treated differently for the subject property and other rentals
This is one reason online affordability calculators can differ materially from an underwritten result.
Amortization and qualification
A longer amortization lowers the calculated payment but slows principal repayment and generally increases total interest.
Illustrative amortization comparison
Assume a $600,000 mortgage at 4.50%, with the rate remaining unchanged for the entire amortization solely for comparison:
| Measure | 25-year amortization | 30-year amortization |
|---|---|---|
| Monthly payment | $3,320.84 | $3,025.29 |
| Payment reduction | — | $295.55 |
| Balance after five years | $526,778 | $546,602 |
| Principal repaid in five years | $73,222 | $53,398 |
| Interest paid in first five years | $126,029 | $128,119 |
| Total interest over full amortization | $396,251 | $489,106 |
The 30-year structure reduces the initial monthly payment by approximately $296, but in this constant-rate illustration it produces approximately $92,854 more total interest and leaves approximately $19,824 more principal outstanding after five years.
Actual Canadian mortgages renew during the amortization, so the future rate will almost certainly differ. The illustration isolates the effect of amortization rather than predicting total real-world cost.
Stress-test exemptions and switches
Not every mortgage event is treated as a new mortgage with additional borrowing.
Since November 21, 2024, OSFI’s minimum-qualifying-rate requirement no longer applies to qualifying uninsured straight switches between federally regulated lenders where the borrower does not increase the loan amount or extend the remaining amortization. A lender may still review credit, property, payment history and its own underwriting requirements, and changes to the mortgage structure can remove the transaction from straight-switch treatment.
Insured transfers and insurer-related portability rules require separate analysis. The complete treatment belongs in the renewal and switch chapter.
Ratios are outputs, not explanations
When a borrower does not qualify, saying “the TDS is too high” is only the beginning.
The useful questions are:
Which input is driving the ratio?
Is the income being treated correctly?
Is a debt shown incorrectly?
Can a debt be repaid without depleting closing funds?
Is rental income being calculated under the most suitable permitted method?
Would a longer amortization be suitable?
Would a different property price solve the problem?
Is the proposed mortgage itself too large?
Does another lender have a legitimate policy difference?
The objective is not to manipulate the ratio. It is to understand the borrower’s complete financial position and choose a supportable structure.
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