A mortgage rate buydown is a financing technique where the interest rate on a loan is lowered for a specific period (temporary buydown) or the life of the loan (permanent buydown) by paying an upfront fee. This calculator focuses on Temporary Buydowns (like the popular 2-1 buydown), where the seller or builder typically deposits a lump sum into an escrow account to subsidize the borrower's monthly payments for the first few years.
Understanding the Calculation Logic
The calculation of a temporary rate buydown is strictly mathematical. It represents the difference between what the borrower pays at the reduced rate and what the lender is owed at the full note rate. This difference is not "erased"; it is prepaid.
To calculate the total cost of the buydown (the required subsidy), follow these steps:
Calculate Base Payment: Determine the monthly principal and interest payment using the full loan amount, the full note rate, and the full term (usually 30 years).
Calculate Reduced Payments: For each year of the buydown, calculate the payment as if the interest rate were lower (e.g., 2% lower in Year 1). Note: The amortization remains based on the 30-year term and the original loan balance.
Find the Monthly Delta: Subtract the reduced payment from the base payment to find the monthly shortfall.
Annualize: Multiply the monthly shortfall by 12 to get the annual subsidy requirement for that year.
Sum Total Cost: Add the annual requirements for all years of the buydown period. This total is the amount that must be collected at closing.
Common Buydown Structures
The 2-1 Buydown
This is the most common temporary buydown. The rate is reduced by 2% in the first year and 1% in the second year. By year three, the rate returns to the full note rate.
Year 1: Rate is Note Rate minus 2%.
Year 2: Rate is Note Rate minus 1%.
Year 3-30: Rate is the full Note Rate.
The 1-0 Buydown
A simpler structure where the rate is reduced by 1% for the first year only. This requires a smaller upfront subsidy than a 2-1 buydown, making it easier for sellers to agree to during negotiations.
The 3-2-1 Buydown
An aggressive strategy used in high-interest environments. The rate drops by 3% in year one, 2% in year two, and 1% in year three. The upfront cost for this is significantly higher because it covers three years of substantial payment subsidies.
Why Calculate the Buydown Cost?
Understanding the specific cost is crucial for negotiations. If you are a buyer asking for a 2-1 buydown, you are essentially asking the seller for a specific dollar amount credit. By using the Rate Buydown Calculator above, you can determine exactly how much that credit needs to be. If the calculator shows a total cost of $12,500, you know you need to ask the seller for at least that amount in concessions.
function calculateBuydown() {
// 1. Get Inputs using var
var principal = parseFloat(document.getElementById('loanPrincipal').value);
var baseRate = parseFloat(document.getElementById('noteRate').value);
var years = parseFloat(document.getElementById('loanDuration').value);
var type = document.getElementById('buydownType').value;
var resultDiv = document.getElementById('rbResult');
var scheduleBody = document.getElementById('scheduleBody');
// Validation
if (isNaN(principal) || isNaN(baseRate) || isNaN(years) || principal <= 0 || baseRate <= 0) {
alert("Please enter valid positive numbers for Loan Amount and Interest Rate.");
return;
}
// 2. Constants for Calculation
var months = years * 12;
var baseMonthlyRate = baseRate / 100 / 12;
// 3. Calculate Base Payment (Full Note Rate)
// PMT = P * r * (1+r)^n / ((1+r)^n – 1)
var basePmt = principal * (baseMonthlyRate * Math.pow(1 + baseMonthlyRate, months)) / (Math.pow(1 + baseMonthlyRate, months) – 1);
// 4. Define Buydown Structure
var schedule = [];
var totalSubsidyCost = 0;
// Logic for different types
var reductions = [];
if (type === '2-1') {
reductions = [2, 1]; // Year 1 down 2%, Year 2 down 1%
} else if (type === '1-0') {
reductions = [1]; // Year 1 down 1%
} else if (type === '1-1') {
reductions = [1, 1]; // Year 1 down 1%, Year 2 down 1%
} else if (type === '3-2-1') {
reductions = [3, 2, 1];
}
scheduleBody.innerHTML = ''; // Clear previous
// 5. Loop through reduction years
for (var i = 0; i < reductions.length; i++) {
var reduction = reductions[i];
var currentRate = baseRate – reduction;
var currentMonthlyRate = currentRate / 100 / 12;
// Calculate Reduced Payment
// IMPORTANT: Amortization is still based on the full term (30 years usually), just at lower rate
var reducedPmt = principal * (currentMonthlyRate * Math.pow(1 + currentMonthlyRate, months)) / (Math.pow(1 + currentMonthlyRate, months) – 1);
var monthlySavings = basePmt – reducedPmt;
var annualSubsidy = monthlySavings * 12;
totalSubsidyCost += annualSubsidy;
// Add row to table
var row = "