What it means
The ratio is a mortgage underwriting screen rather than an accounting measure, and it is used most heavily in Canada, where it is written into insured lending rules. Similar front end housing ratios appear in other markets under different names, generally with limits in the high twenties to low thirties.
Income in the calculation is gross, meaning before tax, which is why the percentage limits look generous relative to what a household actually takes home. A 32% ratio on gross income can absorb well over 40% of net pay once tax and contributions are deducted.
The costs counted are deliberately narrow: principal and interest on the mortgage, property taxes, heating, and typically 50% of condominium or maintenance fees. Car loans, credit cards and student debt are excluded here, which is exactly why lenders run the total debt service ratio alongside it.
Because the mortgage payment is the biggest term, small changes in interest rates move the ratio sharply. This is why stress testing exists: many regulators require the ratio to be recalculated at a rate above the contract rate, so borrowers still qualify if rates rise.
Failing the test does not always end an application. Lenders may allow a higher ratio for borrowers with a large deposit, a strong credit record or a co-signer, though insured loans usually have hard ceilings that cannot be exceeded.
In practice
Real-world examples.
Example
A couple with combined gross income of $150,000 want a property whose total housing costs would be $4,400 a month. Their GDS is $4,400 / $12,500 = 35.2%, above the usual ceiling, so they either increase the deposit or look at cheaper properties.
Example
A first time buyer passes the ratio on the contract rate of 4.9% but fails when the lender recalculates at the required stress test rate two percentage points higher. The bank reduces the approved loan amount rather than declining the application outright.
Example
A self employed applicant with variable income is assessed on the average of two years of declared earnings rather than the most recent year. The lower averaged figure pushes his GDS from 30% to 34% and he adds a co-signer to bring the file back inside policy.
Formula
Calculation
GDS = (monthly mortgage principal and interest + property tax + heating + 50% of condo fees) / gross monthly income
A household earns $120,000 a year, so gross monthly income is $120,000 / 12 = $10,000. The proposed mortgage payment is $2,100 a month, property tax works out at $350 a month and heating is estimated at $150 a month, with no condominium fee.
Total housing costs are $2,100 + $350 + $150 = $2,600 a month, which is $2,600 x 12 = $31,200 a year. The ratio is $2,600 / $10,000 = 26%, comfortably inside a 32% limit and leaving room for the lender's stress test at a higher assumed interest rate.Case study
Seen in the real world.
The following is an illustrative and fictional example. The Ashcombe household, an invented family of four, had a gross annual income of $96,000, which is $96,000 / 12 = $8,000 a month. They had found a house needing a $420,000 mortgage, and at 5.5% over a 25 year term that payment came to about $2,579 a month.
Adding property tax of $300 a month and heating of $140 a month took total housing costs to $2,579 + $300 + $140 = $3,019, so the ratio was $3,019 / $8,000 = 37.7%. The lender's ceiling was 32%, so the application failed on this test alone before any of their other debts were considered.
Working backwards showed them what was possible. A 32% limit allowed $8,000 x 32% = $2,560 of housing costs, and after the fixed $440 of tax and heating that left $2,120 for the mortgage payment, which supports a loan of roughly $345,000 on the same terms. In this fictional case the family raised their deposit by $75,000 over the following year and requalified with a ratio just under the limit.
Watch out
Common mistakes.
- Calculating the ratio on take home pay instead of gross income, which makes the percentage look far worse than the lender's version.
- Leaving out property tax and heating because they are not paid to the lender, when both are counted in the standard formula.
- Assuming that passing the GDS test means the loan is approved, when the total debt service ratio, credit history and deposit size all still apply.
Questions
People also ask.
What is the usual limit?
Around 32% for insured lending, with some lenders stretching towards 39% for strong applicants on uninsured loans.
How is GDS different from TDS?
GDS counts housing costs only, while the total debt service ratio adds car loans, credit card minimums, student loans and other monthly commitments.
Does rental income from the property count?
Often yes, with lenders typically including 50% to 80% of the expected rent in income or netting it against housing costs, depending on their policy.
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