# Mortgage payment and affordability calculator documentation

Inputs, formulas, assumptions and API/MCP examples for the mortgage payment and affordability calculator. Reproduce a calculation and cite its sources.

Canonical calculator: https://klarfort.com/tools/mortgage-calculator/
Content updated: 2026-09-07
Assumptions reviewed: 2026-09-07

## Overview
See how a home purchase fits your monthly cash flow after tax, living costs and planned saving. The housing estimate includes the mortgage, property tax, insurance, maintenance and service charges you enter.

Explore the effect of a higher interest rate or a period without household income. The affordability result is a cash-flow estimate using your assumptions, not a lender decision.

## How to use
1. Enter the home price, deposit, mortgage rate and term in months.
2. Add take-home household income, non-housing living costs and the monthly saving you want to protect.
3. Enter ownership costs, cash available before purchase, closing costs and your reserve target.
4. Set the loan-to-value limit, rate increase and months without income, then compare the base and stress results.

## Understanding the result
Monthly housing cost combines the mortgage payment with entered ownership costs. A negative monthly surplus means those costs exceed the budget left after living costs and protected saving.

The affordable-price estimate holds ownership costs at their entered values and also applies the deposit and loan-to-value limit. Check reserve shortfall separately, because it does not reduce the displayed borrowing estimate.

## Questions
### What does loan-to-value mean?
Loan-to-value, or LTV, is the mortgage balance divided by the home price. For example, a 320,000 mortgage on a 400,000 home has an 80% LTV. The limit here is one you enter.

### How does the job-loss scenario work?
It sets all entered household income to zero for the chosen number of months. Mortgage and ownership costs plus non-housing living costs continue, drawing on cash left after the deposit and closing costs.

### Is the affordable price a mortgage approval amount?
No. It uses your after-tax cash flow, entered ownership costs, deposit and chosen LTV limit. It does not assess credit, lender rules, tax eligibility or the availability of a mortgage.

## Default worked example
Monthly housing cost: 2,654.02 USD. Monthly housing costs include the loan and the ownership costs entered. Affordability is a cash-flow scenario. Assumptions as of 2026-09-07.

## Formula
Housing cost = mortgage + property tax/12 + insurance/12 + maintenance + service charges.

- Affordability uses entered after-tax household income, living costs and protected savings, without lender eligibility rules.
- The affordable-price estimate holds ownership costs at their entered values and separately caps price by the entered deposit and LTV.
- Stress testing applies the higher rate to the same original principal and term.
- The job-loss scenario assumes all entered household income stops and essential expenses continue.
- Reserve shortfall is reported separately; it does not silently change borrowing.

## Inputs
- currency: Currency; default "USD"
- homePrice: Home price; default 400000
- downPayment: Deposit; default 80000
- annualRate: Annual interest rate (%); default 5
- months: Duration in months; default 300
- afterTaxMonthlyIncome: After-tax monthly household income; default 7000
- monthlyLivingCosts: Monthly non-housing living costs; default 2500
- monthlySavings: Protected monthly saving; default 500
- annualPropertyTax: Annual property tax entered; default 2400
- annualInsurance: Annual home insurance; default 1200
- maintenancePercent: Annual maintenance (% of price); default 1
- monthlyServiceCharges: Monthly service charges; default 150
- cashSavings: Cash before the purchase; default 120000
- closingCosts: Upfront closing costs; default 10000
- reserveMonths: Required reserve months; default 6
- jobLossMonths: Months with no household income; default 3
- maxLoanToValuePercent: Maximum loan-to-value (%); default 80
- stressRateIncrease: Stress interest-rate increase (percentage points); default 2

## Interfaces
Input schema: https://klarfort.com/tools/mortgage-calculator/schema.json
Worked request and response: https://klarfort.com/tools/mortgage-calculator/examples.json
Public table: https://klarfort.com/tools/mortgage-calculator/table.json
JSON API: POST https://klarfort.com/tools/api/v1/mortgage-calculator
MCP endpoint: POST https://klarfort.com/tools/mcp
MCP tool name: mortgage-calculator

## JSON request body
```json
{
  "currency": "USD",
  "homePrice": 400000,
  "downPayment": 80000,
  "annualRate": 5,
  "months": 300,
  "afterTaxMonthlyIncome": 7000,
  "monthlyLivingCosts": 2500,
  "monthlySavings": 500,
  "annualPropertyTax": 2400,
  "annualInsurance": 1200,
  "maintenancePercent": 1,
  "monthlyServiceCharges": 150,
  "cashSavings": 120000,
  "closingCosts": 10000,
  "reserveMonths": 6,
  "jobLossMonths": 3,
  "maxLoanToValuePercent": 80,
  "stressRateIncrease": 2
}
```

## Usage and credit
Free for personal, commercial and any other lawful use with credit to KlarFort at https://klarfort.com/. Third-party data terms still apply.

Computed with KlarFort Calculators, https://klarfort.com/

Place a visible credit near the reused material or in the credits for your work or integration. One clear credit per work or integration is sufficient; no separate credit per API request is required. The credit_url field contains the homepage. The citation_url field is an optional calculator reference. Full permission: https://klarfort.com/tools/docs/license/

## Sources
KlarFort calculator methodology: https://klarfort.com/tools/methodology/ (1)