When a lender tells you the maximum you can borrow, it's tempting to treat that number as your budget. It's a common mistake among first-time buyers. That maximum answers only one question: will the lender get paid back? It says nothing about whether you'll still be able to save, travel or absorb a surprise once you own the place.
Borrowing capacity vs comfortable budget
ToolWhat the bank would lend you (at the stress-test rate) compared with what stays comfortable for your budget.
Lender ratios used: GDS 39% and TDS 44%, calculated at the higher of your rate + 2 points or 5.25%. Estimate only — only a pre-approval confirms your amount.
The stress test: qualifying at a higher rate than yours
To check that you could handle a rate increase, the lender calculates your payments at a qualifying rate: the higher of your contract rate plus 2 percentage points, or 5.25%. The rule applies to both insured and uninsured mortgages.
| Rate offered by the lender | Rate used to qualify you |
|---|---|
| 3% | 5.25% |
| 3.5% | 5.5% |
| 4% | 6% |
| 4.5% | 6.5% |
| 5% | 7% |
GDS and TDS ratios: what goes into each
The lender then compares your monthly costs, calculated at the qualifying rate, with your gross (pre-tax) income. CMHC's limits for an insured mortgage are:
- GDS — gross debt service (39% maximum): mortgage payment (principal and interest), municipal and school taxes, heating, and half of any condo fees.
- TDS — total debt service (44% maximum): everything in the GDS, plus your other debt payments: car loan, student loan, line of credit, minimum credit card payments.
For a first-time buyer, the TDS is often the limiting factor. Every dollar of monthly payment on another debt directly reduces how much you can borrow. Paying off a car loan before applying for a pre-approval can therefore change the result more than a raise would.
What the bank's maximum ignores
The ratios only look at housing and debt. They don't see:
- child care costs and other expenses for your kids;
- your savings for retirement or for your children's education;
- replacing your car, even if the current one is almost paid off;
- maintenance and repairs on the property;
- groceries, insurance, leisure, travel;
- the fact that the ratios are based on gross income, while you pay your bills with net income.
A household that borrows the maximum is playing by the lender's rules, but can end up with no room for everything else. That's what's sometimes called being "house poor": you own a home, but you're short on cash every month.
The calculator's "comfortable ceiling"
To estimate a more realistic budget, the mortgage affordability calculator above applies a ceiling of 28%, 32% or 36% of gross income to all your housing costs (mortgage, taxes, heating, half of condo fees) and your other debts combined. Unlike the bank's maximum, this calculation uses your actual rate, not the stress-test rate.
These thresholds are not an official rule. They're a planning convention, more cautious than the lender's ratios, that leaves room for the expenses the bank doesn't see. The right percentage depends on your situation: a household with no kids and no car can aim higher; a family paying for child care should aim lower.
A good test: for a few months, set aside the difference between your current rent and the estimated monthly cost of the property you have in mind. If that's a struggle, your comfortable budget is lower than you thought. If it goes smoothly, those savings grow your down payment.
What's next?
The calculator gives you a ballpark figure. Only a full review of your file by a lender confirms your real borrowing capacity: that's the role of a mortgage pre-approval. And whatever price you settle on, keep a separate reserve fund for surprises in your first year.
Frequently asked questions
How do lenders calculate how much I can borrow for a mortgage?
The lender calculates your payments at the qualifying rate (the higher of your rate + 2 points or 5.25%), then checks that your housing costs don't exceed 39% of your gross income, or 44% once your other debts are included.
Why shouldn't I borrow the maximum the bank approves?
Because that maximum doesn't account for your child care costs, your savings, property maintenance or your other expenses. A lower budget leaves you room for the unexpected.
What are the GDS and TDS ratios?
GDS compares your housing costs (mortgage, taxes, heating, half of condo fees) with your gross income. TDS adds your other debt payments, such as a car loan or credit card.
