Mortgage Affordability Calculator
Work backwards from your income to the house price an underwriter will actually approve, using the 28/36 debt-service rules that govern manual underwriting.
Mortgage Affordability Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Results are a model, not a quotation — an institution's own figures govern.
- Every input is an assumption; change one and the answer changes with it.
- Rounding is applied only at the display layer, so totals may differ by a cent from a statement that rounds each line.
In short: Lenders cap housing costs at about 28% of gross monthly income and total debt at 36%. On $9,000 a month with $650 of existing debt, the binding limit is $2,520 of housing cost, which supports a purchase price of roughly $389,238 with 20% down at 6.5%.
Formula
No closed form exists for price because PITI contains both an annuity term and terms proportional to price, so the engine solves it numerically by bisection.
Worked Example
- Apply the 28% test. $9,000 × 0.28 = $2,520.00.
- Apply the 36% test. $9,000 × 0.36 = $3,240.00, less $650 of debt = $2,590.00.
- Take the lower. $2,520.00 binds, so the 28% housing test is the constraint here.
- Invert the payment. Solve PITI(price) = $2,520.00 numerically.
- Read the result. Maximum price $389,237.89, loan $311,390.31.
Analyst note. Clearing $650 of monthly debt would lift the 36% figure to $3,240 but leave the 28% test binding at $2,520 — so it would not raise this borrower's capacity at all. Knowing which constraint binds tells you whether paying down debt actually helps.
Strengths & Limits Of This Model
Where this engine is strong
- Runs entirely in your browser — no figure you type is transmitted or stored.
- Shows the full working, so every number can be traced and challenged.
- Free, unmetered and free of affiliate incentives.
Where it stops
- Generalised assumptions cannot capture every individual circumstance.
- Jurisdiction-specific rules and mid-year changes may not be reflected.
- A model output is not a substitute for a professional review of your position.
Practical Use Cases
Setting a realistic price band before viewing
Working income-first produces a number you can defend to an underwriter rather than one you hope they will accept. Once you have a target, price the exact payment with the Mortgage Payment Calculator.
Deciding whether to clear debt first
The binding-constraint output shows whether existing obligations are actually limiting you. If the 28% test binds, paying down a car loan will not increase your approval — check the ratios directly with the Debt To Income Calculator.
Testing sensitivity to interest rates
The same income supports materially less house as rates rise. Re-run at several rates to see the band move, and compare competing offers using the Loan Comparison Calculator.
Methodology & Editorial Standards
Affordability applies the conventional 28/36 underwriting ratios: housing costs capped at 28% of gross monthly income, and total debt service capped at 36%. The binding constraint is whichever produces the lower housing allowance, and it is reported explicitly because it determines whether reducing other debt would help. Because full PITI contains an annuity term in the loan plus terms proportional to price, no closed-form inversion exists; the engine solves for price by 200-iteration bisection, which converges far below one cent. PMI at 0.55% annually is included whenever the deposit is under 20%. Qualified Mortgage rules permit ratios up to 43%, and some programmes to 50% with compensating factors, so this is a conservative baseline rather than an absolute ceiling. The engine implements the standard published formula for this calculation. Inputs are validated for domain and sign before evaluation, and any undefined case returns an em-dash rather than a spurious value.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Mortgage Affordability Calculator — 20 Expert FAQs
20 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How much house can I afford on $9,000 a month?
About $389,238 with 20% down at 6.5% over 30 years, assuming $650 of other debt and typical taxes and insurance. The binding limit is the 28% housing test, which caps the payment at $2,520.
What is the 28/36 rule?
Housing costs should not exceed 28% of gross monthly income, and total debt service should not exceed 36%. On $9,000 monthly that is $2,520 and $3,240 respectively, before deducting existing obligations from the second figure.
Which ratio actually limits my approval?
Whichever produces the lower housing allowance, which this calculator reports directly. Borrowers with substantial car or student payments are usually limited by the 36% test; those with little other debt are limited by the 28% test.
Will paying off my car loan let me buy more house?
Only if the 36% total-debt test is the binding constraint. If the 28% housing test already binds, clearing other debt changes nothing about your approval, though it does improve your actual cash position.
Do lenders really cap at 36%?
Not always. Qualified Mortgage rules allow up to 43% in most conforming cases, and some programmes stretch to 50% with strong compensating factors such as large reserves or a high credit score. Treat 28/36 as the comfortable baseline rather than a hard ceiling.
Should I borrow the maximum I am approved for?
Generally no. Approval is calculated on gross income, but you live on net income after tax. A payment at 28% of gross can be 36% or more of what actually reaches your account, which leaves little room for anything else.
Why does the calculator use gross rather than net income?
Because underwriting does. Every debt-to-income ratio in mortgage lending is computed on gross income, so using net figures here would produce a number that does not match what a lender will tell you.
How does the interest rate change what I can afford?
Substantially. Each percentage point costs roughly $226 monthly on a $336,000 loan, which converts directly into a lower affordable price at the same income. The same household supported around 30% less house in 2023 than in 2021 purely through rate movement.
Does a bigger deposit increase the price I can afford?
Yes, in two ways. It reduces the loan needed for a given price, and once it reaches 20% it removes PMI from the payment entirely, freeing that amount for principal and interest instead.
What counts as other monthly debt?
Minimum payments on credit cards, car loans, student loans, personal loans, and any court-ordered support. Utilities, insurance, groceries and other living expenses are excluded from the ratio, even though they obviously affect your real budget.
Is the maximum price the same as what I should offer?
No. The maximum is a ceiling derived from underwriting rules, not a recommendation. Buying below it preserves the capacity to absorb rate resets on other debt, maintenance surprises and income interruption.
Why is the calculation solved numerically?
Because price appears both inside the annuity term through the loan amount and outside it through property tax and PMI, so the equation has no clean algebraic inversion. The engine bisects to a precision far finer than a cent.
Is this mortgage affordability calculator free to use?
Yes. It is free, requires no account, and has no usage limits. ApexConverter is funded by contextual advertising, never by selling user data.
Is my data sent to a server?
No. The engine runs as Vanilla JavaScript inside your browser under our Zero-Server Client-Side Execution model. Your figures are computed locally and are never transmitted, logged, or stored.
How accurate is this calculator?
It applies the standard closed-form formula in IEEE-754 double precision, rounding only at the display layer. The engine is reconciled against an independent reference implementation before release.
Does it work on mobile?
Yes. The interface is mobile-first with numeric keypad hints and is tested down to a 320-pixel viewport with no horizontal scrolling.
Can I use it offline?
Largely, yes. Because computation is client-side, the page continues to calculate without a network connection once it has loaded.
Which currency does it use?
Amounts display in US$ accounting format, but the underlying mathematics is currency-agnostic. The result is identical in any currency, so simply read the figures in your own.
Why does a result show an em-dash?
An em-dash indicates the calculation is not defined for the inputs given — typically a division by zero or a value outside the valid domain. We show a dash rather than a misleading number.
How do I report an error?
Email apexconverter.praxiscalc@gmail.com with the tool URL, your exact inputs, the output received and the output you expected. Verified mathematical errors are patched within 72 hours.