Mortgage Affordability Calculator: Canada and UK Guide

Affordability calculators from TD, CIBC, and CMHC in Canada, or from lenders and comparison sites in the UK, give you a starting number — but they rarely explain the rules that govern that number. Understanding how affordability is actually calculated in your market lets you stress-test any calculator result before relying on it.

How Mortgage Affordability Works in Canada

The Two Ratios Lenders Use

Canadian lenders use two debt ratios to determine how much you can borrow. Both are calculated using your gross (before-tax) monthly income.

The Gross Debt Service (GDS) ratio measures housing costs as a percentage of income. It includes the monthly mortgage payment, property taxes, heating costs, and 50% of any condo fees. Lenders and insurers use their own applicable thresholds, so do not treat one percentage as universal.

The Total Debt Service (TDS) ratio measures all debt obligations as a percentage of income. It adds your non-housing debts (car loans, student loans, credit card minimums) to the GDS number. The applicable threshold varies by lender, insurer, borrower, and program.

If your GDS or TDS is high, approval depends on the lender, insurer, mortgage program, and full borrower profile; a single ratio is not an automatic decline rule.

The Stress Test: The Most Important Variable in Canadian Affordability

The Canadian mortgage stress test is not optional for most newly underwritten borrowers at federally regulated lenders, and it is not shown prominently on most lender calculators. The applicable qualifying rate is the higher of:

• The Bank of Canada's qualifying rate, currently 5.25%, or
• The contract rate offered by the lender plus 2%

If a lender offers you a 5-year fixed rate of 4.75%, you must prove you can afford the mortgage payments at 6.75% (4.75% + 2%). This single rule reduces the maximum borrowable amount by a meaningful margin compared to qualifying at the actual contract rate.

For a practical example: a household with a combined gross income of $120,000 and no other debts might qualify for approximately $650,000 to $700,000 at the contract rate, but the stress test reduces that to approximately $550,000 to $580,000 — depending on down payment and property taxes.

Tools like the CMHC affordability calculator and CIBC's mortgage affordability tool use the stress test rate when estimating maximum purchase price. TD's affordability calculator also applies the stress test. If the calculator you are using does not apply the stress test, the result is not meaningful for purchase planning in Canada.

High-Ratio Mortgages and CMHC Insurance

If your down payment is less than 20% of the purchase price, your mortgage is classified as high-ratio and requires mortgage default insurance — typically from CMHC, Sagen, or Canada Guaranty.

The premium is added to your mortgage balance, not paid at closing. Premium rates as of 2025:

• 5% to 9.99% down: 4.0% of the loan amount
• 10% to 14.99% down: 3.10% of the loan amount
• 15% to 19.99% down: 2.80% of the loan amount

On a $600,000 purchase with 5% down ($30,000), the insured loan amount is $570,000 and the premium is $22,800 — bringing the total mortgage balance to $592,800.

The CMHC affordability calculator accounts for this premium when showing maximum purchase price. The amortization for insured mortgages was recently adjusted: first-time buyers purchasing new builds can access 30-year amortization on insured mortgages (as of August 2024); standard insured mortgages remain limited to 25-year amortization.

High-ratio mortgages are also limited to purchase prices under $1,000,000.

How TD, CIBC, and Major Bank Calculators Differ

TD's affordability calculator uses the stress test and estimates property tax based on approximate rates by province. It assumes a standard heating cost estimate rather than requiring the user to input actual costs, which can understate or overstate the GDS for specific properties.

CIBC's affordability calculator similarly applies the stress test and produces an estimated maximum purchase price. Both calculators produce pre-qualification estimates, not approvals — lenders verify all inputs against documentation before issuing a pre-approval.

The most reliable use of any affordability calculator is as a planning range, not a commitment. Use the calculator to narrow your search price to a band, then get a formal pre-approval to confirm the actual approved amount.

How Mortgage Affordability Works in the UK

Income Multiples

UK lenders may use income multiples as one input, but affordability assessments are lender-specific and more granular than a single universal multiple. Use any multiple only as a planning estimate, not a qualification rule.

Some lenders may offer higher multiples for particular borrower profiles, but eligibility, deposit, and affordability conditions vary by lender.

The Affordability Assessment

Since the Mortgage Market Review (MMR) in 2014, UK lenders are required to conduct detailed affordability assessments rather than rely solely on income multiples. The assessment reviews:

• Gross and net income across all sources
• Fixed monthly outgoings (loan repayments, credit card commitments, regular subscriptions)
• Essential living costs (food, transport, childcare)
• Discretionary spending (estimated from bank statements)

The lender produces a "maximum loan" figure based on residual income after all costs — essentially confirming that you will have sufficient income remaining to meet the mortgage payment after your existing obligations.

Stress Testing in the UK

UK lenders assess whether the borrower can afford the mortgage if interest rates increase. The Financial Conduct Authority (FCA) removed its prescriptive stress test rule in August 2022, allowing lenders more flexibility in how they conduct their own testing. The method and any stress margin vary by lender, so do not treat a specific uplift as universal.

This is particularly relevant for buyers using a remortgage affordability calculator to assess whether they can move from a fixed rate deal to a new product. When you come to the end of a fixed rate, if you do nothing, you roll onto the lender's Standard Variable Rate (SVR) — which in 2025 averaged around 7.74%. A remortgage affordability check via a broker or comparison tool estimates what rate and loan amount you would qualify for on a new deal.

UK Deposit Size and LTV Tiers

In the UK, the deposit percentage directly affects the interest rate you are offered. Lenders tier their rates by loan-to-value (LTV), with the most competitive rates reserved for 60% LTV (40% deposit) and progressively higher rates as LTV increases.

For most first-time buyers, the practical focus is on the 85% to 95% LTV tier. Mortgage rates at 95% LTV are meaningfully higher than rates at 85% LTV. Building a slightly larger deposit — even an additional 5% — can shift you into a significantly cheaper rate band, which is worth modeling before committing to a purchase price.

Using a Remortgage Affordability Calculator

When an existing homeowner refinances (remortgages), affordability is reassessed. If your income has increased since the original mortgage, you may qualify for a larger loan or simply for a new deal at the current best rate. If your income has decreased or your commitments have grown, affordability may be tighter than the original approval.

Remortgage affordability tools from comparison sites (MoneySuperMarket, Which?) and individual lenders provide an initial estimate. The most important input is your current property value relative to the remaining mortgage balance, as this determines your LTV for the new deal.

Using Calculators as Planning Tools, Not Guarantees

Affordability calculators — whether from TD, CIBC, CMHC, or UK lenders — produce estimates based on assumed inputs. They cannot reflect the specific details of your credit file, employment history, or the lender's current underwriting policies.

Use them to establish a planning range: if three calculators consistently suggest a maximum purchase price of $550,000, it is reasonable to begin searching in the $480,000–$540,000 range and get a pre-approval to confirm. Searching up to $650,000 based on a calculator output without formal pre-approval is how buyers enter offers they cannot ultimately finance.

The Mortgage Worksheet from First Home Toolkit includes an affordability section that walks through GDS/TDS calculations (Canada) and income multiple checks (UK/AU/NZ), so you can verify calculator outputs against actual lender criteria before you start making offers.