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Mortgage Affordability Calculator
Applies the front-end (housing) and back-end (total debt) debt-to-income limits to gross income, subtracts taxes and insurance to get the affordable principal-and-interest payment, and converts it into a maximum loan amount and home price for the given rate and term.
When to use
You want to know how much house or mortgage an income supports under standard lender ratios, before applying for a loan.
Do not use when: You already have a loan amount and want its payment (use mortgage-payment) or want to check ratios for an existing payment (use debt-to-income). Lender rules vary; informational only, not financial advice.
Formula
monthly_income = annual_gross_income / 12; housing = min(monthly_income × front_end/100, monthly_income × back_end/100 − monthly_debt_payments); P&I = housing − monthly_taxes_insurance; max_loan = P&I × (1 − (1 + r/12)^−(12·term)) / (r/12); max_price = max_loan + down_payment
Classic 28/36 qualifying ratios (front-end housing, back-end total debt) as used in conventional US underwriting; actual approvals also depend on credit score, reserves, loan program and the appraisal. Informational mathematics only; not financial advice.
Inputs
| Parameter | Type | Unit | Required | Description |
|---|---|---|---|---|
annual_gross_income | number | yes | Household income before taxes, per year. Range: > 0, ≤ 1000000000 | |
monthly_debt_payments | number | default 0 | Minimum payments on car loans, student loans, credit cards and other debts (excluding the new housing payment). Range: ≥ 0, ≤ 10000000 | |
down_payment | number | default 0 | Cash available for the down payment; added to the loan to get the home price. Range: ≥ 0, ≤ 10000000000 | |
interest_rate_percent | number | % per year | yes | Annual mortgage rate; the monthly rate is rate/12. Range: ≥ 0, ≤ 30 |
term_years | number | years | default 30 | Amortization period in years. Range: ≥ 1, ≤ 40 |
front_end_ratio_percent | number | % | default 28 | Maximum housing payment (principal, interest, taxes, insurance) as a percent of gross monthly income. Range: ≥ 1, ≤ 100 |
back_end_ratio_percent | number | % | default 36 | Maximum total debt payments including housing as a percent of gross monthly income. Range: ≥ 1, ≤ 100 |
monthly_taxes_insurance | number | default 0 | Property tax, homeowner's insurance, HOA and mortgage insurance per month, which count toward the housing payment. Range: ≥ 0, ≤ 1000000 |
Outputs
| Output | Type | Unit | Description |
|---|---|---|---|
front_end_limit | number | monthly income × front_end_ratio. | |
back_end_limit | number | monthly income × back_end_ratio − monthly_debt_payments. | |
max_monthly_housing_payment | number | The lower of the two limits (PITI). | |
limiting_ratio | string | Which ratio sets the limit. | |
max_principal_and_interest | number | max_monthly_housing_payment − monthly_taxes_insurance. | |
max_loan_amount | number | P&I × (1 − (1 + i)^−n) / i with i = rate/12 and n = term × 12. | |
max_home_price | number | max_loan_amount + down_payment. | |
income_multiple | number | max_loan_amount / annual_gross_income. |
Example
90,000 income, 500 debts, 50,000 down, 6 %, 30 years, 300 taxes/insurance: {"annual_gross_income":90000,"monthly_debt_payments":500,"down_payment":50000,"interest_rate_percent":6,"term_years":30,"monthly_taxes_insurance":300} → {"front_end_limit":2100,"back_end_limit":2200,"max_monthly_housing_payment":2100,"limiting_ratio":"Front-end ratio (28 % of gross income)","max_principal_and_interest":1800,"max_loan_amount":300224.91,"max_home_price":350224.91,"income_multiple":3.34}
120,000 income, 1,200 debts, 80,000 down, 5.5 %, 25 years, 450 taxes/insurance: {"annual_gross_income":120000,"monthly_debt_payments":1200,"down_payment":80000,"interest_rate_percent":5.5,"term_years":25,"monthly_taxes_insurance":450} → {"front_end_limit":2800,"back_end_limit":2400,"max_monthly_housing_payment":2400,"limiting_ratio":"Back-end ratio (36 % of gross income minus other debts)","max_principal_and_interest":1950,"max_loan_amount":317544.33,"max_home_price":397544.33}
GET https://tttkmbb.com/api/v1/calculate/mortgage-affordability?annual_gross_income=90000&monthly_debt_payments=500&down_payment=50000&interest_rate_percent=6&term_years=30&monthly_taxes_insurance=300
Machine access
- API:
GET https://tttkmbb.com/api/v1/calculate/mortgage-affordability(query parameters) orPOSTwith a JSON body{"inputs": {...}} - Schema: https://tttkmbb.com/api/v1/calculators/mortgage-affordability · Markdown: https://tttkmbb.com/investing/mortgage-affordability.md · JSON definition: https://tttkmbb.com/investing/mortgage-affordability.json
- MCP: server
https://tttkmbb.com/mcp, toolrun_calculator with calculator_id="mortgage-affordability" - OpenAPI operationId:
calculate_mortgage_affordability - Freshness:
static. Authentication: none. Rate limit: fair use (see rate limits).
Sources
FAQ
Where do 28 and 36 come from?
They are the traditional conventional-mortgage guidelines: housing costs up to 28 % of gross income and all debt up to 36 %. Fannie Mae allows up to 36 % (45 % with compensating factors) for manually underwritten loans and up to 50 % through automated underwriting.
Is the result what a lender will approve?
No. It is the payment the ratios allow; lenders also weigh credit history, employment, cash reserves and the property, and some use different ratios.
Related calculators
- Mortgage Payment Calculator — Full payment for a chosen loan amount.
- Debt-to-Income Ratio Calculator — Check the ratios for an existing payment.
- Loan Payment Calculator — Level payment on any amortizing loan.