Why the mortgage affordability calculator matters
Affordability is decided by monthly payment ratios rather than the price you have in mind, which is why two buyers with the same salary get very different approvals. This page turns that decision into a handful of inputs you can defend in a budget review: volume, unit cost, rate of adoption, and time. The output is a planning baseline, not a promise — it tells you whether the idea deserves a vendor quote, a pilot, or a pass.
- • Biggest swing factor: existing monthly debt, which comes straight off the allowance
- • Second-order factor: the interest rate, which sets how much payment buys
- • Often ignored: the down payment you bring
What actually changes the answer
existing monthly debt, which comes straight off the allowance moves this number first, then the interest rate, which sets how much payment buys. Run a conservative case and an upside case before you commit. If the maths only works in the upside case, treat it as a time-boxed test with a kill date rather than a line in next year's plan.
What to do with the result
Get pre-approved before you shop. This gives you the range; a lender gives you the letter.
Related guides
Long-form playbooks on the same topic, written by the RevenueLab editorial team.
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Read the guideFAQ
What does the mortgage affordability calculator work out?
It applies Max payment = min(36% × monthly income − debts, 28% × monthly income); Loan = payment × [(1 − (1 + i)^−n) ÷ i]; Price = loan + down payment to the values you enter for gross annual income, existing monthly debt payments, down payment / cash available, interest rate, loan term (years). Affordability is decided by monthly payment ratios rather than the price you have in mind, which is why two buyers with the same salary get very different approvals.
How accurate is this mortgage affordability calculator?
Uses the standard 28/36 ratios. Actual underwriting also weighs credit score, reserves, employment history and property taxes. Replace the defaults with your own invoice, usage export, payroll data, statement, or vendor quote before making a commitment — the maths is exact, so the answer is only as good as the inputs you feed it.
Which input should I stress-test first?
existing monthly debt, which comes straight off the allowance. Re-run with a pessimistic value for it; if the decision flips, that assumption is the thing you need real data on before signing anything. After that, check the interest rate, which sets how much payment buys and the down payment you bring.
Which scenario should I start from?
Start with the preset closest to your situation — cautious budget, today's numbers, stronger position — then edit the sliders. Presets are realistic starting points, not benchmarks to match, and every change updates the result instantly.
What should I do after running the numbers?
Get pre-approved before you shop. This gives you the range; a lender gives you the letter. A useful planning benchmark to compare against: Lenders commonly cap total debt at 36–43% of gross income.
Can I share or save this calculation?
Yes. Your inputs are written into the page URL, so copying the link shares the exact scenario you are looking at — the person who opens it sees the same numbers. You can also export the inputs and results to CSV or PDF from the result card and keep it with the rest of your workings.
How this calculator is built
Independently maintained
Written by Sam Doshi and the RevenueLab editorial team. We don't sell the data feeds this tool is built on.
Sourced from primary data
Benchmarks come from public AdSense / Stripe / IRS disclosures and reader-submitted data — never third-party "$X per view" claims. Full methodology.
Last editorial review
Reviewed on a rolling quarterly cycle. Dated reviews are published on the methodology record for each calculator.
Editorial standards
See our editorial policy and disclaimer. Results are estimates, not advice.