Mortgage math
The balance is not the liability—the lender-assigned monthly payment is
TDS does not divide your debt balances by income. Each liability first has to be converted into a qualifying monthly payment. A $50,000 line of credit and a $50,000 installment loan can therefore produce very different TDS impacts.
Every debt has to be converted into the monthly payment the lender will count
Mortgage applications usually show both a balance and a monthly obligation. TDS uses the qualifying monthly obligation. The lender's method for turning the balance into that payment depends on the debt type.
That is why paying down the right liability can improve mortgage qualification far more than paying down another debt by the same dollar amount.
| Debt type | Common A/insured starting treatment | Where policy can differ |
|---|---|---|
| Mortgage / fixed installment loan | Required scheduled payment | Mortgage itself may need stress-test / qualifying-rate payment |
| Credit card | Common insured/A baseline: at least 3% of outstanding balance | Some lender programs use statement minimum/other internal calculation |
| Unsecured line of credit | Common insured/A baseline: at least 3% of outstanding balance | Some A/B lenders can use actual required payment under policy |
| Secured line of credit / HELOC | Common institutional method: payment based on outstanding balance amortized over 25 years at required rate | Some lenders use product-specific payments/limits or internal equivalent method |
| Car/personal installment loan | Actual required installment | Remaining term / payoff before funding can matter |
| Lease | Required monthly lease payment | Lender may require full contractual obligation |
| Child/spousal support paid | Actual legal monthly obligation | Documentation and duration matter |
| Other property mortgage | Lender rental/property method | May be captured inside a rental worksheet rather than again as separate debt |
Credit cards and unsecured lines of credit can create a much larger qualifying payment than the borrower expects
For insured qualification, current federal/insurer practice uses at least 3% of the outstanding balance for unsecured revolving credit such as credit cards and unsecured lines of credit. Many conventional A lenders use the same or similar treatment.
Example: a $20,000 revolving balance at 3% creates a $600 monthly TDS obligation, even if the statement minimum displayed to the borrower is smaller.
HopeWell broker-channel observation: some B lenders and some A-lender policies can use the actual required payment on a line of credit instead of a 3% or 1% balance-based proxy where their program permits. That difference can be material and must be checked with the live lender guide.
Secured lines of credit are usually converted differently from credit cards
A secured revolving balance should not automatically be treated as an unsecured card. A common institutional method—and the current insured baseline—is to calculate a monthly payment on the outstanding secured balance amortized over 25 years using the applicable contract/qualifying-rate method.
That 25-year treatment is not unique to one insurer; it reflects the mortgage-channel liability method HopeWell commonly encounters. Individual lenders can apply an internal method that is at least as conservative on insured business or a different policy on conventional/alternative files.
For a $50,000 outstanding secured line, the qualifying payment depends on the rate input. Use the Mortgage Payment Calculator with $50,000 principal and 25-year amortization to see the effect rather than relying on an interest-only statement payment.
Installment loans, leases and support obligations usually enter TDS at the actual required payment
A fixed car loan, personal installment loan or lease normally contributes its required monthly payment. Child or spousal support paid is also a recurring obligation and is generally entered using the actual legal payment amount.
The remaining balance matters mainly because it tells the lender how long the payment will continue and whether it can realistically be paid out before funding. Do not remove a debt from the application simply because the borrower intends to pay it off later; the lender has to accept the payout/closing condition.
HopeWell payoff-priority test: target the payment that releases the most TDS, not necessarily the biggest balance
If the goal is mortgage qualification, compare cash required to eliminate the debt with monthly TDS payment removed. A $12,000 car loan with a $650 payment can be a more important qualifying obstacle than a $20,000 revolving balance with a smaller lender-assigned payment under another policy.
This does not mean the borrower should automatically spend all savings on debt. Down payment, closing costs, emergency reserves and lender liquidity requirements can be equally important. The optimization is a whole-file decision.
| Debt | Balance | Counted payment | TDS relief if eliminated |
|---|---|---|---|
| Car loan | $12,000 | $650 | $650/month |
| Credit card | $20,000 | $600 at 3% | $600/month |
| LOC under actual-payment policy | $20,000 | $250 illustration | $250/month |
Debt consolidation can improve TDS even when total dollars owed barely change
TDS is sensitive to monthly required payments, not just total principal. Replacing several high-payment unsecured debts with one amortizing secured payment can materially reduce the monthly liability burden.
That can improve ratios, but the refinance may extend repayment, add interest, incur penalties/fees and convert unsecured debt into debt secured against the home. Use the Debt Consolidation Calculator for cash-flow economics and Maximum Mortgage Calculator for qualification.
Real file: support obligations can dominate TDS even for a high-income borrower
In a Hamilton refinance, the borrower had strong executive income but substantial alimony/support obligations and unsecured debt. The issue was not lack of income; it was the amount of monthly obligations entering TDS.
The file illustrates why a liability schedule should be built before assuming that a high salary means an easy approval.
Once every debt has a qualifying payment, TDS becomes simple arithmetic
Add the lender-assigned monthly obligations to the GDS housing costs and divide by gross qualifying income. That is the core Total Debt Service (TDS) calculation.
Paying off a car, revolving balance or support obligation can improve TDS dramatically while leaving GDS completely unchanged.
Build the next-purchase liability schedule after the equity draw
When a HELOC or second mortgage supplies purchase funds, build the liability schedule at each funding date. Record the existing first mortgage, drawn and proposed revolving debt, second-mortgage payment, surviving cards and the old property’s actual sale or retention status. Ask the purchase lender which balances, limits and repayment conditions its product uses; do not infer qualification from a low interest-only statement.
Illustration using assumed lender-accepted monthly inputs: $4,200 new-home qualifying housing plus $500 other debt over $12,000 accepted gross income is 39.2% TDS. If the lender assigns another $700 to the equity borrowing, $5,400/$12,000 is 45%. Those specified treatments show how added cash can weaken qualification. They are not a universal HELOC formula, a ratio ceiling or permission to exclude the old mortgage.
If the draw consolidates cards, identify precisely which accounts will be repaid and the accepted payout evidence. Remove their qualifying payment only when the lender accepts that treatment; add the replacement borrowing and retain any cards that survive. Do not count repaid cards as still outstanding after an accepted cure, or erase them because repayment is merely planned.
The Mississauga pre-construction file illustrates the paired decision: a proposed private equity-loan payment would have prevented bank purchase qualification, while an institutional refinance supplied the contribution with manageable obligations. No numeric policy threshold or loan term is supplied by that case.
Use TDS and the Maximum Mortgage Calculator with actual accepted inputs. Maintain a separate take-home cash budget for the period carrying both homes. The new deposit, the old-home sale payout and the monthly payment are different constraints.
Sources and methodology
Sources and verification
The insured baseline is source-checked. Conventional A/B payment treatments below are HopeWell broker-channel observations where lender methods differ.
Sagen
Covenant Underwriting
Verified August 17, 2026
Canada Mortgage and Housing Corporation
Calculating GDS / TDS
Verified October 5, 2026
Department of Justice Canada
Eligible Mortgage Loan Regulations
Verified August 18, 2026
Office of the Superintendent of Financial Institutions
Guideline B-20: Residential Mortgage Underwriting Practices and Procedures
Verified October 8, 2026