This calculator helps you determine how long it will take to pay off a loan when making fixed weekly payments. By inputting the total loan amount, the annual interest rate, and your desired weekly payment, you can get an estimate of your loan's payoff timeline.
How it Works: The Math Behind the Calculator
The calculation is based on an iterative process that simulates each weekly payment. At the start of each week, interest is accrued on the remaining balance, and then the weekly payment is applied. This continues until the loan balance reaches zero.
The core formula used for calculating the remaining balance after one payment, considering weekly compounding, is:
Annual Interest Rate: The yearly rate charged by the lender.
Weekly Interest Rate: Calculated by dividing the annual rate by 52 (the number of weeks in a year). So, Weekly Interest Rate = Annual Interest Rate / 52 / 100.
Weekly Payment: The fixed amount you plan to pay each week.
Payoff Time: The total number of weeks required to pay off the loan.
The calculator iterates through this process, adding one week at a time, until the loan balance is fully repaid. The total number of weeks is then converted into years and months for easier understanding.
When to Use This Calculator
Debt Management: If you have multiple debts, you can use this to strategize which ones to pay off aggressively.
Budgeting: Understand the commitment of a particular weekly payment amount.
Loan Comparison: See how different payment amounts affect your payoff timeline and the total interest paid.
Extra Payments: While this calculator assumes a fixed weekly payment, you can simulate paying off a loan faster by increasing the 'Desired Weekly Payment' field.
Important Note: This calculator provides an estimate. Actual loan payoff times can vary due to factors like payment processing times, exact number of days in months/years, and any fees associated with the loan. Always consult your loan agreement for precise details.
function calculatePayoff() {
var loanAmount = parseFloat(document.getElementById("loanAmount").value);
var annualInterestRate = parseFloat(document.getElementById("interestRate").value);
var weeklyPayment = parseFloat(document.getElementById("weeklyPayment").value);
var resultDiv = document.getElementById("result");
var resultValueDiv = document.getElementById("result-value");
var resultDetailsP = document.getElementById("result-details");
resultValueDiv.innerHTML = "–";
resultDetailsP.innerHTML = "";
if (isNaN(loanAmount) || loanAmount <= 0) {
alert("Please enter a valid loan amount.");
return;
}
if (isNaN(annualInterestRate) || annualInterestRate < 0) {
alert("Please enter a valid annual interest rate.");
return;
}
if (isNaN(weeklyPayment) || weeklyPayment <= 0) {
alert("Please enter a valid weekly payment.");
return;
}
var weeklyInterestRate = (annualInterestRate / 100) / 52;
var remainingBalance = loanAmount;
var weeks = 0;
var totalInterestPaid = 0;
// Check if weekly payment is less than the interest accrued in the first week
if (weeklyPayment 0) {
var interestForThisWeek = remainingBalance * weeklyInterestRate;
// Ensure payment is sufficient to cover interest
if (weeklyPayment > interestForThisWeek) {
remainingBalance = remainingBalance – weeklyPayment + interestForThisWeek;
totalInterestPaid += interestForThisWeek;
weeks++;
} else {
// This case should ideally be caught by the initial check, but as a failsafe:
alert("Payment is insufficient to cover interest for this week. Cannot calculate payoff.");
return;
}
// Safety break for extremely long calculations or potential infinite loops
if (weeks > 5000) { // Approximately 96 years, a reasonable limit
alert("Calculation exceeded a reasonable number of weeks. Please check your input values.");
return;
}
}
var years = Math.floor(weeks / 52);
var remainingWeeks = weeks % 52;
var payoffString = "";
if (years > 0) {
payoffString += years + (years === 1 ? " year" : " years");
if (remainingWeeks > 0) {
payoffString += ", ";
}
}
if (remainingWeeks > 0) {
payoffString += remainingWeeks + (remainingWeeks === 1 ? " week" : " weeks");
}
if (payoffString === "") { // Case where loan is paid off in less than a week (unlikely with valid inputs)
payoffString = "less than 1 week";
}
resultValueDiv.innerHTML = payoffString;
resultDetailsP.innerHTML = "Total interest paid: $" + totalInterestPaid.toFixed(2);
}