Understanding how much house you can afford is a crucial first step in the home-buying process. A mortgage affordability calculator helps you estimate your maximum loan amount based on your income, debts, and down payment. This tool considers key factors that lenders use to assess your borrowing capacity.
Key Factors in Mortgage Affordability
Income: Your gross monthly income (before taxes) is a primary determinant of how much you can borrow. Lenders typically look at your debt-to-income ratio (DTI).
Debts: Existing monthly debt payments, such as car loans, student loans, and credit card minimum payments, significantly impact your DTI.
Down Payment: The larger your down payment, the less you need to borrow, which can reduce your monthly payments and potentially qualify you for better loan terms.
Interest Rate: Even a small difference in interest rate can have a large impact on your monthly payment and the total cost of the loan over time.
Loan Term: The length of your mortgage (e.g., 15 or 30 years) affects your monthly payment. Shorter terms mean higher monthly payments but less interest paid overall.
Property Taxes and Homeowners Insurance: These costs, often referred to as PITI (Principal, Interest, Taxes, Insurance), are part of your total monthly housing expense and are factored into affordability.
Use this calculator to get a preliminary estimate of your mortgage affordability. Remember that this is a guide, and your actual loan approval amount may vary based on lender specific criteria and a full underwriting process.
function calculateAffordability() {
var grossMonthlyIncome = parseFloat(document.getElementById("grossMonthlyIncome").value);
var monthlyDebtPayments = parseFloat(document.getElementById("monthlyDebtPayments").value);
var downPayment = parseFloat(document.getElementById("downPayment").value);
var annualInterestRate = parseFloat(document.getElementById("interestRate").value);
var loanTermYears = parseFloat(document.getElementById("loanTermYears").value);
var annualTaxes = parseFloat(document.getElementById("estimatedAnnualTaxes").value);
var annualInsurance = parseFloat(document.getElementById("estimatedAnnualInsurance").value);
var resultDiv = document.getElementById("result");
resultDiv.innerHTML = ""; // Clear previous results
// Input validation
if (isNaN(grossMonthlyIncome) || grossMonthlyIncome <= 0 ||
isNaN(monthlyDebtPayments) || monthlyDebtPayments < 0 ||
isNaN(downPayment) || downPayment < 0 ||
isNaN(annualInterestRate) || annualInterestRate <= 0 ||
isNaN(loanTermYears) || loanTermYears <= 0 ||
isNaN(annualTaxes) || annualTaxes < 0 ||
isNaN(annualInsurance) || annualInsurance < 0) {
resultDiv.innerHTML = "Please enter valid positive numbers for all fields.";
return;
}
// Lender's typical maximum DTI (front-end ratio, considering PITI)
var maxPITIAsPercentageOfIncome = 0.28; // Common threshold for housing costs
var maxTotalDebtAsPercentageOfIncome = 0.36; // Common threshold for total debt including PITI
var monthlyTaxes = annualTaxes / 12;
var monthlyInsurance = annualInsurance / 12;
// Calculate maximum allowable monthly housing payment (PITI)
var maxHousingPayment = grossMonthlyIncome * maxPITIAsPercentageOfIncome;
// Calculate maximum allowable total monthly debt payment (including PITI)
var maxTotalMonthlyDebt = grossMonthlyIncome * maxTotalDebtAsPercentageOfIncome;
// Max allowed monthly payment for principal and interest (PI)
// This accounts for the fact that PITI includes taxes and insurance
var maxPILoanPayment = Math.min(maxHousingPayment, maxTotalMonthlyDebt – monthlyDebtPayments) – monthlyTaxes – monthlyInsurance;
if (maxPILoanPayment <= 0) {
resultDiv.innerHTML = "Based on your inputs, your estimated maximum affordable loan principal and interest payment is $0 or negative. You may need to increase income, reduce debt, or lower housing cost expectations.";
return;
}
// Mortgage calculation formula (to find loan amount from monthly payment)
var monthlyInterestRate = (annualInterestRate / 100) / 12;
var numberOfPayments = loanTermYears * 12;
// Formula for loan amount M = P [1 – (1 + r)^-n] / r where P is monthly payment, r is monthly rate, n is number of months
// Rearranged to find P: P = M * r / [1 – (1 + r)^-n]
// We want to find the Loan Amount (Principal) given the maxPILoanPayment (M)
var maxLoanAmount = maxPILoanPayment * (Math.pow(1 + monthlyInterestRate, numberOfPayments) – 1) / (monthlyInterestRate * Math.pow(1 + monthlyInterestRate, numberOfPayments));
// The total home price would be the loan amount plus the down payment
var estimatedMaxHomePrice = maxLoanAmount + downPayment;
// Display results
resultDiv.innerHTML = "Based on your inputs:";
resultDiv.innerHTML += "Estimated maximum monthly PITI payment: $" + maxHousingPayment.toFixed(2) + "";
resultDiv.innerHTML += "Estimated maximum total monthly debt payment: $" + maxTotalMonthlyDebt.toFixed(2) + "";
resultDiv.innerHTML += "Estimated maximum monthly Principal & Interest (PI) payment: $" + maxPILoanPayment.toFixed(2) + "";
resultDiv.innerHTML += "Estimated maximum mortgage loan amount: $" + maxLoanAmount.toFixed(2) + "";
resultDiv.innerHTML += "Estimated maximum affordable home price (Loan + Down Payment): $" + estimatedMaxHomePrice.toFixed(2);
}