How to Calculate Interest Rate from Payment
.calculator-widget {
font-family: 'Segoe UI', Roboto, Helvetica, Arial, sans-serif;
max-width: 800px;
margin: 0 auto;
background: #ffffff;
border: 1px solid #e0e0e0;
border-radius: 8px;
box-shadow: 0 4px 12px rgba(0,0,0,0.05);
overflow: hidden;
}
.calc-header {
background: #2c3e50;
color: #ffffff;
padding: 20px;
text-align: center;
}
.calc-header h2 {
margin: 0;
font-size: 24px;
font-weight: 600;
}
.calc-body {
padding: 30px;
display: flex;
flex-wrap: wrap;
gap: 30px;
}
.calc-inputs {
flex: 1;
min-width: 300px;
}
.calc-results {
flex: 1;
min-width: 300px;
background: #f8f9fa;
padding: 20px;
border-radius: 6px;
border: 1px solid #e9ecef;
display: flex;
flex-direction: column;
justify-content: center;
align-items: center;
text-align: center;
}
.input-group {
margin-bottom: 15px;
}
.input-group label {
display: block;
margin-bottom: 5px;
font-weight: 500;
color: #333;
font-size: 14px;
}
.input-group input, .input-group select {
width: 100%;
padding: 10px;
border: 1px solid #ccc;
border-radius: 4px;
font-size: 16px;
box-sizing: border-box;
}
.input-group .helper-text {
font-size: 12px;
color: #666;
margin-top: 3px;
}
button.calc-btn {
width: 100%;
padding: 12px;
background: #27ae60;
color: white;
border: none;
border-radius: 4px;
font-size: 16px;
font-weight: bold;
cursor: pointer;
transition: background 0.3s;
margin-top: 10px;
}
button.calc-btn:hover {
background: #219150;
}
.result-value {
font-size: 36px;
font-weight: 700;
color: #2c3e50;
margin: 10px 0;
}
.result-label {
font-size: 16px;
color: #7f8c8d;
text-transform: uppercase;
letter-spacing: 1px;
margin-bottom: 5px;
}
.result-detail {
margin-top: 15px;
font-size: 14px;
color: #555;
width: 100%;
border-top: 1px solid #ddd;
padding-top: 15px;
}
.result-row {
display: flex;
justify-content: space-between;
margin-bottom: 8px;
}
.article-section {
padding: 30px;
border-top: 1px solid #eee;
background: #fff;
color: #333;
line-height: 1.6;
}
.article-section h3 {
color: #2c3e50;
margin-top: 0;
}
.article-section p {
margin-bottom: 15px;
}
.article-section ul {
margin-bottom: 15px;
padding-left: 20px;
}
.error-msg {
color: #e74c3c;
font-size: 14px;
margin-top: 10px;
display: none;
text-align: center;
}
@media (max-width: 600px) {
.calc-body {
flex-direction: column;
}
}
function calculateAffordability() {
// 1. Get DOM elements and values
var annualIncomeInput = document.getElementById("annualIncome");
var monthlyDebtsInput = document.getElementById("monthlyDebts");
var downPaymentInput = document.getElementById("downPayment");
var interestRateInput = document.getElementById("interestRate");
var loanTermInput = document.getElementById("loanTerm");
// 2. Parse values
var annualIncome = parseFloat(annualIncomeInput.value);
var monthlyDebts = parseFloat(monthlyDebtsInput.value);
var downPayment = parseFloat(downPaymentInput.value);
var interestRate = parseFloat(interestRateInput.value);
var loanTerm = parseInt(loanTermInput.value);
// 3. Validation
var errorDiv = document.getElementById("errorMsg");
if (isNaN(annualIncome) || isNaN(monthlyDebts) || isNaN(downPayment) || isNaN(interestRate) || annualIncome <= 0) {
errorDiv.style.display = "block";
return;
} else {
errorDiv.style.display = "none";
}
// 4. Logic Implementation
var monthlyIncome = annualIncome / 12;
// Calculate Max Payment based on 28% Front-End Ratio
var maxPaymentFront = monthlyIncome * 0.28;
// Calculate Max Payment based on 36% Back-End Ratio
var maxPaymentBack = (monthlyIncome * 0.36) – monthlyDebts;
// The allowable payment is the lower of the two, but cannot be negative
var allowablePITI = Math.min(maxPaymentFront, maxPaymentBack);
if (allowablePITI < 0) allowablePITI = 0;
// Estimation: Reserve 15% of the allowable payment for Taxes and Insurance (T&I)
// This leaves 85% for Principal and Interest (P&I)
var allowablePI = allowablePITI * 0.85;
// Calculate Max Loan Amount based on allowable P&I
// Formula: Loan = (Payment * ( (1+r)^n – 1 )) / ( r * (1+r)^n )
var monthlyRate = interestRate / 100 / 12;
var numPayments = loanTerm * 12;
var maxLoan = 0;
if (monthlyRate === 0) {
maxLoan = allowablePI * numPayments;
} else {
var compoundFactor = Math.pow(1 + monthlyRate, numPayments);
maxLoan = (allowablePI * (compoundFactor – 1)) / (monthlyRate * compoundFactor);
}
// Max Home Price = Loan Amount + Down Payment
var maxHomePrice = maxLoan + downPayment;
// 5. Update UI
// Helper function for currency formatting
var formatter = new Intl.NumberFormat('en-US', {
style: 'currency',
currency: 'USD',
maximumFractionDigits: 0
});
document.getElementById("maxHomePrice").innerHTML = formatter.format(maxHomePrice);
document.getElementById("maxMonthlyPayment").innerHTML = formatter.format(allowablePITI);
document.getElementById("maxLoanAmount").innerHTML = formatter.format(maxLoan);
// Determine which ratio limited the loan
var limitingFactor = (maxPaymentBack < maxPaymentFront) ? "36% (Debt Limited)" : "28% (Income Limited)";
document.getElementById("dtiUsed").innerHTML = limitingFactor;
}