*Note: This calculation uses a standard 28% front-end / 36% back-end debt-to-income ratio model.
function calculateAffordability() {
// 1. Get input values
var annualIncome = parseFloat(document.getElementById('macAnnualIncome').value) || 0;
var downPayment = parseFloat(document.getElementById('macDownPayment').value) || 0;
var monthlyDebts = parseFloat(document.getElementById('macMonthlyDebts').value) || 0;
var interestRate = parseFloat(document.getElementById('macInterestRate').value) || 0;
var loanTermYears = parseFloat(document.getElementById('macLoanTerm').value) || 30;
var annualPropertyTax = parseFloat(document.getElementById('macPropertyTax').value) || 0;
var annualInsurance = parseFloat(document.getElementById('macHomeInsurance').value) || 0;
var monthlyHOA = parseFloat(document.getElementById('macHOA').value) || 0;
var errorDiv = document.getElementById('macErrorMessage');
var resultsDiv = document.getElementById('macResultsSection');
// Validation
if (annualIncome 0) {
if (monthlyRate === 0) {
// Simple division if 0% interest
maxLoanAmount = availableForPI * numberOfPayments;
} else {
// Reverse mortgage formula: Loan = Payment * ((1 – (1+r)^-n) / r)
// P = M * [ (1 – (1+r)^-n) / r ]
var discountFactor = (Math.pow(1 + monthlyRate, numberOfPayments) – 1) / (monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments));
maxLoanAmount = availableForPI * discountFactor;
}
} else {
maxLoanAmount = 0;
}
// 6. Calculate Max Home Price
var maxHomePrice = maxLoanAmount + downPayment;
// 7. Recalculate exact P&I based on the derived max loan amount for display consistency
// (Rounding issues sometimes cause slight discrepancies, so we recalc)
var finalPI = 0;
if (maxLoanAmount > 0) {
if (monthlyRate === 0) {
finalPI = maxLoanAmount / numberOfPayments;
} else {
var numerator = monthlyRate * Math.pow(1 + monthlyRate, numberOfPayments);
var denominator = Math.pow(1 + monthlyRate, numberOfPayments) – 1;
finalPI = maxLoanAmount * (numerator / denominator);
}
}
var totalMonthlyPayment = finalPI + monthlyPropertyTax + monthlyInsurance + monthlyHOA;
// 8. Display Results
// Formatting function
function formatMoney(num) {
return '$' + num.toFixed(0).replace(/(\d)(?=(\d{3})+(?!\d))/g, '$1,');
}
document.getElementById('macMaxPrice').innerText = formatMoney(maxHomePrice);
document.getElementById('macResultPI').innerText = formatMoney(finalPI);
document.getElementById('macResultTax').innerText = formatMoney(monthlyPropertyTax);
document.getElementById('macResultIns').innerText = formatMoney(monthlyInsurance);
document.getElementById('macResultHOA').innerText = formatMoney(monthlyHOA);
document.getElementById('macResultTotal').innerText = formatMoney(totalMonthlyPayment);
resultsDiv.style.display = 'block';
}
Understanding Mortgage Affordability
Buying a home is likely the largest financial decision you will make in your lifetime. Determining "how much house can I afford" is the critical first step before browsing listings or attending open houses. This Mortgage Affordability Calculator helps you estimate a realistic budget based on your income, debts, and down payment capabilities.
How Banks Calculate Your Budget
Lenders do not just look at your salary; they look at your ability to repay debt. They typically use two primary ratios to determine your maximum loan amount: the Front-End Ratio and the Back-End Ratio.
Front-End Ratio (28%): This rule suggests that your housing costs (mortgage principal, interest, taxes, and insurance) should not exceed 28% of your gross monthly income.
Back-End Ratio (36%): This is often the more limiting factor. It states that your total monthly debt payments (housing + credit cards + car loans + student loans) should not exceed 36% of your gross monthly income.
Our calculator conservatively uses the lower of these two numbers to ensure you don't overextend yourself financially. While some loan programs (like FHA or VA loans) may allow higher ratios, sticking to the 28/36 rule is a safe baseline for financial health.
Factors That Impact Your Buying Power
Several variables can drastically change your maximum home price:
Interest Rates: Even a 1% increase in interest rates can reduce your buying power by tens of thousands of dollars because more of your monthly payment goes toward interest rather than the loan principal.
Monthly Debts: High monthly obligations (like an expensive car payment) directly reduce the cash available for a mortgage. Eliminating a $400 car payment could potentially increase your home buying budget by $50,000 or more, depending on current rates.
Property Taxes & HOA: A home with a lower price tag but high property taxes or Homeowners Association (HOA) fees might actually cost you more per month than a more expensive home with lower annual fees. Always verify the specific tax rates for the area you are considering.
The Importance of the Down Payment
Your down payment does two things: it reduces the loan amount (lowering your monthly payment) and it establishes equity immediately. If you put down less than 20% of the home's value, you will likely have to pay Private Mortgage Insurance (PMI), which is an extra monthly cost not included in the standard principal and interest calculation.
Next Steps
Once you have a ballpark figure from this calculator, the next step is to get pre-approved by a lender. A pre-approval letter confirms exactly what a bank is willing to lend you, which makes you a serious buyer in the eyes of real estate agents and sellers.