*Calculations based on standard 28%/36% qualifying ratios.
function calculateHouseAffordability() {
// Get Inputs
var income = parseFloat(document.getElementById("grossAnnualIncome").value);
var monthlyDebts = parseFloat(document.getElementById("monthlyDebts").value);
var downPayment = parseFloat(document.getElementById("downPayment").value);
var interestRate = parseFloat(document.getElementById("interestRate").value);
var termYears = parseFloat(document.getElementById("loanTerm").value);
var annualTax = parseFloat(document.getElementById("propertyTax").value);
var annualInsurance = parseFloat(document.getElementById("homeInsurance").value);
// Validation
var errorDiv = document.getElementById("errorMsg");
var resultsDiv = document.getElementById("resultsArea");
if (isNaN(income) || income <= 0 || isNaN(interestRate) || isNaN(downPayment)) {
errorDiv.textContent = "Please enter valid numbers for Income, Interest Rate, and Down Payment.";
errorDiv.style.display = "block";
resultsDiv.style.display = "none";
return;
}
if (isNaN(monthlyDebts)) monthlyDebts = 0;
if (isNaN(annualTax)) annualTax = 0;
if (isNaN(annualInsurance)) annualInsurance = 0;
errorDiv.style.display = "none";
// Constants for DTI (Standard Conventional Loan)
var FRONT_RATIO = 0.28; // 28% of gross income for housing
var BACK_RATIO = 0.36; // 36% of gross income for housing + debts
var monthlyIncome = income / 12;
var monthlyTaxAndIns = (annualTax + annualInsurance) / 12;
// Calculate Max Allowable Monthly Housing Payment based on Front Ratio
var maxPaymentFront = monthlyIncome * FRONT_RATIO;
// Calculate Max Allowable Monthly Housing Payment based on Back Ratio
var maxPaymentBack = (monthlyIncome * BACK_RATIO) – monthlyDebts;
// The bank takes the lower of the two
var maxTotalMonthlyPayment = Math.min(maxPaymentFront, maxPaymentBack);
// If debts are too high, affordability might be 0
if (maxTotalMonthlyPayment <= monthlyTaxAndIns) {
document.getElementById("maxHomePrice").textContent = "$0";
document.getElementById("maxLoanAmount").textContent = "$0";
document.getElementById("monthlyPayment").textContent = "$0";
document.getElementById("dtiUsed").textContent = "N/A";
resultsDiv.style.display = "block";
return;
}
// Calculate Max P&I (Principal & Interest) Payment
var maxPI = maxTotalMonthlyPayment – monthlyTaxAndIns;
// Calculate Loan Amount from P&I
// Formula: P = L[r(1+r)^n]/[(1+r)^n – 1]
// Reverse: L = P * [(1+r)^n – 1] / [r(1+r)^n]
var monthlyRate = (interestRate / 100) / 12;
var numPayments = termYears * 12;
var maxLoan = 0;
if (monthlyRate === 0) {
maxLoan = maxPI * numPayments;
} else {
var mathPower = Math.pow(1 + monthlyRate, numPayments);
maxLoan = maxPI * (mathPower – 1) / (monthlyRate * mathPower);
}
var maxHomeValue = maxLoan + downPayment;
// Determine which DTI ratio limited the loan
var dtiDescription = (maxPaymentFront < maxPaymentBack) ? "Front-End (28%)" : "Back-End (36%)";
// Update UI
resultsDiv.style.display = "block";
document.getElementById("maxHomePrice").textContent = "$" + Math.floor(maxHomeValue).toLocaleString();
document.getElementById("maxLoanAmount").textContent = "$" + Math.floor(maxLoan).toLocaleString();
document.getElementById("monthlyPayment").textContent = "$" + Math.floor(maxTotalMonthlyPayment).toLocaleString();
document.getElementById("dtiUsed").textContent = dtiDescription;
}
How Much House Can You Afford?
Determine your buying power before starting your home search with our specialized Mortgage Affordability Calculator. Unlike simple payment calculators, this tool works backward from your income and debts to establish a realistic purchase price budget.
Understanding the 28/36 Rule
Lenders typically use two primary ratios to determine how much money they are willing to lend you. This calculator incorporates these industry standards automatically:
Front-End Ratio (28%): Your estimated monthly housing costs (mortgage principal, interest, taxes, and insurance) should not exceed 28% of your gross monthly income.
Back-End Ratio (36%): Your total monthly debt payments (housing costs plus credit cards, student loans, car loans, etc.) should not exceed 36% of your gross monthly income.
Key Factors That Impact Your Affordability
Several variables can significantly shift your maximum home price:
Interest Rates: Even a 1% increase in rates can reduce your buying power by tens of thousands of dollars because more of your monthly payment goes toward interest rather than principal.
Monthly Debts: High monthly obligations (like an expensive car payment) directly reduce the amount available for a mortgage. Eliminating a $400 monthly debt could increase your purchasing power by roughly $50,000 or more, depending on current rates.
Down Payment: A larger down payment increases your price range dollar-for-dollar and lowers the loan amount needed, reducing monthly interest costs.
Using the Results
If the calculator shows a maximum home price of $450,000, this is your ceiling based on strict lender guidelines. However, financial experts often recommend buying below your maximum qualification to ensure you have budget flexibility for maintenance, savings, and lifestyle expenses.