Mortgage Math

Gross Debt Service (GDS) measures how much of the borrower's gross qualifying income is required for the subject property's qualifying housing costs. Total Debt Service (TDS) starts with those same housing costs and adds the monthly obligations assigned to the borrower's other debts.

In plain English, GDS asks 'How much of the accepted income goes to this home?' TDS asks 'How much goes to this home plus everything else the lender counts as debt?' Read the glossary definitions for Gross Debt Service ratio (GDS) and Total Debt Service ratio (TDS).

Use the same time period on both sides of each equation—monthly costs divided by monthly income, or annual costs divided by annual income.

Two lenders can use the same GDS/TDS formula and still produce different answers because the inputs can differ. One may average variable income differently, another may use a more generous rental worksheet, and another may assign a larger qualifying payment to a line of credit.

That is why HopeWell treats residential mortgage math as a five-step chain: qualifying income → rental treatment → liability treatment → qualifying mortgage payment → GDS/TDS. The ratio comes last, not first.

Assume a household has $150,000 of accepted annual income, or $12,500 per month. Qualifying housing costs are $4,400 per month. GDS is therefore 35.2%.

Now add a $750 car payment, $300 student-loan payment and $350 qualifying revolving-credit payment. Total counted costs become $5,800, so TDS becomes 46.4%. Nothing about the home changed; the difference is the other debt.

Use the Maximum Mortgage Calculator to see how paying off a debt, changing income or changing the qualifying rate moves the maximum mortgage.

For mortgages that must meet the federal insured/insurable eligibility rules, the standard maximums are 39% GDS and 44% TDS. The lender still underwrites the file, but it cannot simply ignore the applicable insurance eligibility limits.

Conventional uninsured lending has more lender-specific room. HopeWell has seen strong A-lender files considered above 39/44 where equity, credit, income quality and the overall file justified an exception. Alternative lenders commonly work around 50/50, and some published programs can go higher. Private lenders usually do not rely on rigid bank-style GDS/TDS ceilings, but they still examine bank statements, income and recurring obligations to judge whether the proposed payment is realistic.

Those differences are explored in Gross Debt Service (GDS) and Total Debt Service (TDS).

A borrower can pass GDS and TDS but still be limited by LTV, down payment, property value, insurer eligibility, lender maximums or a specific program rule. Conversely, a low-LTV borrower can still be limited by income or debt.

HopeWell's practical approach is to find the binding constraint: the first number that actually prevents the desired mortgage. Improving a non-binding number may make the file look better without increasing the amount available.

For example, paying off a car loan can materially improve TDS but does nothing to GDS or LTV. A higher appraisal can improve LTV but does not change TDS. Adding stable qualifying income improves both GDS and TDS.

Use the page that owns the number you are trying to solve instead of forcing every decision into one calculator.

In an Aurora refinance, corporate income analysis helped support the path back to an A lender. In a North York rental-portfolio refinance, lender rental treatment changed the qualification picture.

These examples do not create universal lender rules. They show the practical point: the same borrower facts can produce different qualification results when the permitted calculation method changes.

GDS and TDS use gross qualifying income and do not capture every real household expense. Childcare, food, transportation, income taxes, insurance, savings goals, irregular repairs and lifestyle costs can sit outside the ratios.

Use lender math to understand approval capacity. Then use a separate household cash-flow test to decide whether using the full approved amount is actually sensible.