A client once asked me a question before we had even talked about neighbourhoods or property types. She wanted to know: “How do lenders actually decide what I qualify for?”
It’s exactly the right question to start with — and one that most buyers don’t get a clear answer to until they’re sitting across from a mortgage professional. The short answer involves two ratios and a stress test. Understanding how they work changes how you approach your budget before you start searching.
The Two Ratios That Determine Your Mortgage
Lenders use two key ratios to evaluate whether your income can support the mortgage you’re applying for. Both are straightforward once you see how they’re built.
Housing costs as a percentage of gross monthly income.
- Mortgage payment
- Property taxes
- Heating costs
- 50% of condo fees (if applicable)
All debt obligations as a percentage of gross monthly income.
- Everything in GDS
- Car loan payments
- Credit card minimums
- Student loans & lines of credit
The GDS ratio looks only at the home you’re buying. The TDS ratio adds everything else you owe on top. Both must fall within their limits — and the more restrictive one becomes the actual ceiling on your mortgage.
Gross Debt Service — Your Housing Ceiling
GDS measures what percentage of your gross monthly income goes toward the costs of the specific home you’re buying. The general guideline is that this should not exceed 39 percent.
So if your gross household income is $10,000 per month, your total qualifying housing costs — mortgage payment, property taxes, heating, and half your condo fees — need to land at $3,900 or below to meet the GDS threshold.
Your mortgage professional translates that monthly payment into a purchase price using current rates and your amortization period. The exact number is always specific to your situation — which is exactly why getting pre-approved before you start searching is so important. You can also run a quick estimate using the mortgage calculator on our website to get a feel for the numbers before you meet with a lender.
Total Debt Service — Where Existing Debt Shrinks Your Budget
TDS is where a lot of buyers get surprised. The general guideline is that all your debt payments combined should not exceed 44 percent of your gross monthly income.
That sounds like room to breathe — until you see how quickly existing debt uses it up. A car payment, a student loan, and a modest credit card balance add up faster than most people expect.
| Debt Type | Monthly Payment | Impact on TDS |
|---|---|---|
| Car loan | $500 / month | − $500 from housing room |
| Student loan | $300 / month | − $300 from housing room |
| $10,000 credit card balance | $300 / month (3% of balance) | − $300 from housing room |
| Total existing debt payments | $1,100 / month | Reduces mortgage room by $1,100 |
That $1,100 in existing monthly debt payments directly reduces how much mortgage you can carry. On a $10,000/month gross income with a 44% TDS ceiling ($4,400), subtract $1,100 in existing debt, and your effective housing ceiling under TDS drops to $3,300 — potentially lower than the GDS ceiling.
Paying down credit card balances before applying for a mortgage has a meaningful impact on your TDS ratio. Even reducing a $15,000 balance to $5,000 cuts $300/month from your calculated TDS obligations — which translates to real additional mortgage room.
The Stress Test — Qualifying at a Higher Rate
On top of the GDS and TDS ratios, there’s the mortgage stress test — and this one catches buyers off guard more than anything else in the qualification process.
When a lender evaluates your application, they don’t qualify you at your actual contract rate. They qualify you at a higher rate to make sure you could still handle the payments if rates were to rise.
The practical effect: you’ll qualify for a smaller mortgage than your payment at the actual contract rate would suggest. How much smaller depends on current rates. A mortgage professional will show you the exact number for your specific situation.
The 30-Year Amortization for First-Time Buyers
If you’re a first-time buyer purchasing a newly built home, there’s one tool that directly improves your qualification picture: access to a 30-year amortization on CMHC-insured mortgages, rather than the standard 25-year maximum for insured mortgages.
A 30-year amortization means lower monthly payments on the same mortgage amount. Lower monthly payments improve your GDS ratio — which may allow you to qualify for more than you would under a 25-year amortization.
This option is available to first-time buyers purchasing newly built homes with as little as 5 percent down. The down payment and CMHC insurance structure will be covered in full in the next guide. The key point here is that this amortization option exists and directly affects how much you can qualify for — so if you’re a first-time buyer looking at new construction, your mortgage professional needs to know that.
The Comfortable Number vs. the Maximum Number
One more thing worth saying clearly: the number a lender approves you for and the number you’re actually comfortable paying every month are often different. Sometimes meaningfully different.
GDS and TDS are ceilings — not targets. A household running at 38% GDS is technically within the threshold. Whether those monthly payments feel manageable depends on your income stability, fixed expenses, family situation, savings goals, and a dozen things that don’t show up in the ratio calculation.
The buyers who make the best long-term decisions are the ones who calculate two numbers before they start searching: what they qualify for, and what they’re comfortable paying each month. Starting with both in mind is far better than discovering the gap after you’ve already fallen in love with a property. A good place to start is the mortgage calculator on our website — plug in different payment amounts and see how they translate before you ever speak to a lender.
Important: The GDS and TDS guidelines described here reflect broadly accepted Canadian mortgage qualification principles. Specific lenders may apply different thresholds. This blog is educational — not mortgage advice. Always work with a licensed mortgage professional to understand exactly what you qualify for in your specific situation.
Knowing how these ratios work before you sit down with a lender means you show up to that conversation informed — and you’re far less likely to be surprised by the number they come back with. That’s worth something before you start your search.
Frequently Asked Questions
What is the GDS ratio for a mortgage in Canada?
The Gross Debt Service (GDS) ratio measures your housing costs as a percentage of your gross monthly income. Housing costs include your mortgage payment, property taxes, heating costs, and 50% of condo fees if applicable. The general guideline is that GDS should not exceed 39% of gross monthly income.
What is the TDS ratio for a mortgage in Canada?
The Total Debt Service (TDS) ratio measures all your debt obligations as a percentage of your gross monthly income. This includes everything in the GDS calculation plus car payments, credit card minimums (typically 3% of the outstanding balance), student loans, lines of credit, and any other regular debt payments. The general guideline is that TDS should not exceed 44% of gross monthly income.
How does the mortgage stress test work in Canada?
The stress test requires you to qualify at a higher rate than your actual contract rate — specifically, the higher of your contract rate plus 2%, or the regulatory minimum qualifying rate set by federal regulators. You don’t pay this higher rate. You simply have to demonstrate you could manage the payments at that level. The result is that you qualify for a somewhat smaller mortgage than your payment at the actual contract rate would suggest.
What debts are included in the TDS ratio calculation?
TDS includes all housing costs from the GDS calculation, plus: car loan monthly payments, credit card minimums (typically calculated at 3% of the outstanding balance per month), student loan payments, line of credit payments, and any other regular debt obligations. Credit card balances in particular can have a bigger impact than buyers expect — a $15,000 balance adds $450/month to your TDS calculation.
What is the 30-year amortization option for first-time buyers?
First-time buyers purchasing newly built homes can access CMHC-insured mortgages with a 30-year amortization rather than the standard 25-year maximum for insured mortgages. A longer amortization means lower monthly payments on the same mortgage amount — which directly improves your GDS ratio and may allow you to qualify for more. Your mortgage professional can confirm whether your specific purchase qualifies.
Is it better to qualify for the maximum mortgage or stay below it?
The GDS and TDS limits are ceilings — not targets. Qualifying for a certain amount doesn’t mean that amount is comfortable for your lifestyle, savings goals, or family situation. The buyers who tend to make the best long-term decisions calculate two numbers before they start searching: what they qualify for, and what they’re actually comfortable paying every month. Those two numbers are often different, and knowing both matters before you fall in love with a property.
Sales Representative · Zolo Realty Brokerage · Licensed in Ontario (RECO #5035461)
Toronto real estate agent specializing in first-time buyers and families across the GTA. 80+ five-star reviews · 100+ clients · $25M+ in completed transactions.