function calculateHouseAffordability() {
// 1. Get Values
var annualIncome = parseFloat(document.getElementById('macAnnualIncome').value);
var monthlyDebts = parseFloat(document.getElementById('macMonthlyDebts').value);
var downPayment = parseFloat(document.getElementById('macDownPayment').value);
var interestRate = parseFloat(document.getElementById('macInterestRate').value);
var loanTermYears = parseInt(document.getElementById('macLoanTerm').value);
var dtiLimit = parseFloat(document.getElementById('macDtiRatio').value);
var taxInsRate = parseFloat(document.getElementById('macPropTaxRate').value);
// 2. Validation
var errorDiv = document.getElementById('macError');
var resultDiv = document.getElementById('macResult');
errorDiv.innerHTML = "";
resultDiv.style.display = "none";
if (isNaN(annualIncome) || annualIncome <= 0) {
errorDiv.innerHTML = "Please enter a valid Annual Gross Income.";
return;
}
if (isNaN(monthlyDebts)) monthlyDebts = 0;
if (isNaN(downPayment)) downPayment = 0;
if (isNaN(interestRate) || interestRate <= 0) {
errorDiv.innerHTML = "Please enter a valid Interest Rate.";
return;
}
if (isNaN(taxInsRate)) taxInsRate = 1.5;
// 3. Calculation Logic
// Monthly Income
var monthlyIncome = annualIncome / 12;
// Max Total Debt Payment Allowed
var maxTotalDebtPayment = monthlyIncome * dtiLimit;
// Max Mortgage Payment Allowed (PITI)
var maxPITIPayment = maxTotalDebtPayment – monthlyDebts;
if (maxPITIPayment <= 0) {
errorDiv.innerHTML = "Your current monthly debts exceed the allowable limit for a mortgage based on the selected DTI ratio.";
return;
}
// Mortgage Calculation Variables
var monthlyRate = (interestRate / 100) / 12;
var numPayments = loanTermYears * 12;
var taxInsMonthlyFactor = (taxInsRate / 100) / 12;
// Formula Derivation:
// MaxPITI = (Loan * M_factor) + ((Loan + Down) * TaxInsFactor)
// MaxPITI = Loan * M_factor + Loan * TaxInsFactor + Down * TaxInsFactor
// MaxPITI – (Down * TaxInsFactor) = Loan * (M_factor + TaxInsFactor)
// Loan = (MaxPITI – (Down * TaxInsFactor)) / (M_factor + TaxInsFactor)
var mortgageFactor = (monthlyRate * Math.pow(1 + monthlyRate, numPayments)) / (Math.pow(1 + monthlyRate, numPayments) – 1);
// Numerator
var numerator = maxPITIPayment – (downPayment * taxInsMonthlyFactor);
// Denominator
var denominator = mortgageFactor + taxInsMonthlyFactor;
var loanAmount = numerator / denominator;
// Handle case where taxes/insurance on downpayment eat up entire budget
if (loanAmount < 0) {
loanAmount = 0;
}
var maxHomePrice = loanAmount + downPayment;
// Breakdown
var principalInterest = loanAmount * mortgageFactor;
var taxInsurance = maxHomePrice * taxInsMonthlyFactor;
var totalMonthly = principalInterest + taxInsurance;
// 4. Formatting Output
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
maximumFractionDigits: 0
});
document.getElementById('resMaxPrice').innerHTML = formatter.format(maxHomePrice);
document.getElementById('resMonthlyPayment').innerHTML = formatter.format(totalMonthly);
document.getElementById('resLoanAmount').innerHTML = formatter.format(loanAmount);
document.getElementById('resPI').innerHTML = formatter.format(principalInterest);
document.getElementById('resTaxIns').innerHTML = formatter.format(taxInsurance);
document.getElementById('resDTIUsed').innerHTML = (dtiLimit * 100).toFixed(0) + "%";
// Show Results
resultDiv.style.display = "block";
}
Understanding Mortgage Affordability
Determining how much house you can afford is the critical first step in the home buying process. Unlike a standard mortgage payment calculator which simply tells you the cost of a specific loan, a Mortgage Affordability Calculator works backward from your income and existing debts to determine your maximum purchasing power.
The 28/36 Rule and DTI
Lenders typically use Debt-to-Income (DTI) ratios to determine eligibility. The most common standard is the 28/36 rule:
Front-End Ratio (28%): No more than 28% of your gross monthly income should go toward housing costs (Principal, Interest, Taxes, and Insurance).
Back-End Ratio (36%): No more than 36% of your gross monthly income should go toward all debt obligations combined (Housing + Credit Cards + Student Loans + Car Loans).
While some loan programs (like FHA) allow for higher DTI ratios (up to 43% or even 50%), staying within the conservative 36% range ensures you are not "house poor" and have financial flexibility.
Key Factors Affecting Your Buying Power
Several variables drastically change how much home you can buy:
Interest Rates: Even a 1% increase in interest rates can reduce your buying power by tens of thousands of dollars, as more of your monthly payment goes toward interest rather than principal.
Existing Debt: Every dollar spent on car loans or credit cards is a dollar less that can be used for a mortgage. Eliminating a $400 car payment can increase your home buying budget by roughly $50,000 to $70,000 depending on rates.
Property Taxes: High property tax areas reduce the loan amount you can qualify for, as taxes are part of the monthly liability lenders calculate.
Down Payment: A larger down payment not only reduces the loan amount but also reduces the Loan-to-Value (LTV) ratio, potentially eliminating Private Mortgage Insurance (PMI) and lowering your monthly costs.
How to Improve Your Affordability
If the calculator results are lower than expected, consider these strategies:
Pay down high-interest consumer debt before applying for a mortgage.
Save for a larger down payment to reduce the principal loan amount.
Look for areas with lower property tax rates.
Improve your credit score to qualify for lower interest rates.
Frequently Asked Questions
Does this calculator include PMI?
This tool estimates Taxes and Insurance based on a percentage of the home value. Private Mortgage Insurance (PMI) is usually required if your down payment is less than 20%. You should factor in an additional 0.5% to 1% of the loan amount annually if you plan to put down less than 20%.
Gross vs. Net Income?
Lenders almost always use your Gross Income (before taxes) for qualification purposes. However, for your personal budget, you should verify that the estimated payment fits comfortably within your Net Income (take-home pay).