function calculateAPR() {
// 1. Get input values by specific IDs
var rateInput = document.getElementById("periodicRate").value;
var frequency = document.getElementById("rateType").value;
// 2. Validate input
if (rateInput === "" || isNaN(rateInput)) {
alert("Please enter a valid numeric rate.");
return;
}
var r = parseFloat(rateInput);
var nominalApr = 0;
var effectiveApr = 0;
var dailyRate = 0;
var monthlyRate = 0;
// 3. Calculation Logic based on Credit Card standards (365 days)
// Note: Truth in Lending Act generally uses 365 days for DPR calculation.
if (frequency === "daily") {
// Input is Daily Periodic Rate (DPR)
dailyRate = r;
// Nominal APR = DPR * 365
nominalApr = r * 365;
// Monthly Rate approximation (APR / 12)
monthlyRate = nominalApr / 12;
// Effective APR (EAR) = ((1 + DPR/100)^365 – 1) * 100
// Convert percentage to decimal for math: r/100
var decimalRate = r / 100;
effectiveApr = (Math.pow((1 + decimalRate), 365) – 1) * 100;
} else if (frequency === "monthly") {
// Input is Monthly Periodic Rate (MPR)
monthlyRate = r;
// Nominal APR = MPR * 12
nominalApr = r * 12;
// Daily Rate = APR / 365
dailyRate = nominalApr / 365;
// Effective APR (EAR) = ((1 + MPR/100)^12 – 1) * 100
var decimalRate = r / 100;
effectiveApr = (Math.pow((1 + decimalRate), 12) – 1) * 100;
}
// 4. Update the DOM with results formatted to 4 decimal places
document.getElementById("nominalAPR").innerHTML = nominalApr.toFixed(2) + "%";
document.getElementById("effectiveAPR").innerHTML = effectiveApr.toFixed(2) + "%";
document.getElementById("equivDaily").innerHTML = dailyRate.toFixed(4) + "%";
document.getElementById("equivMonthly").innerHTML = monthlyRate.toFixed(4) + "%";
// Show result box
document.getElementById("resultBox").style.display = "block";
}
Understanding Credit Card Interest Calculations
Credit card interest can be confusing because issuers often present the interest rate in different formats. While the Annual Percentage Rate (APR) is the standard figure used for comparison, your monthly billing statement typically calculates interest based on a Daily Periodic Rate (DPR) or a Monthly Periodic Rate.
This calculator allows you to reverse-engineer these figures to understand exactly what your annualized cost of borrowing is based on the small percentages listed on your statement.
What is the Daily Periodic Rate (DPR)?
The Daily Periodic Rate is the interest rate charged on your balance for a single day. Credit card issuers calculate your finance charges by multiplying your average daily balance by this rate. Because the rate applies to a single day, the number is usually very small (e.g., 0.05%).
Formula: Nominal APR = Daily Periodic Rate × 365
Nominal APR vs. Effective APR
When converting periodic rates to an annual figure, there are two ways to look at the numbers:
Nominal APR: This is the standard simple interest calculation used in legal disclosures (like the Schumer Box). It assumes no compounding within the year for the rate quotation. It is simply the periodic rate multiplied by the number of periods in a year.
Effective APR (EAR): This represents the true cost of borrowing if you carry a balance throughout the year, accounting for compound interest. Since credit card interest typically compounds daily, the Effective APR is often slightly higher than the Nominal APR.
How to Find Your Rate
To use this calculator effectively, look at the "Interest Charge Calculation" section of your credit card statement. You will see columns for "Annual Percentage Rate (APR)" and often "Daily Periodic Rate." If you only see the daily rate, input that figure above to verify the annual rate.
Why Math Matters for Debt
Even a small difference in the Daily Periodic Rate can add up significantly over a year. For example, a daily rate of 0.04% results in a 14.6% APR, while a daily rate of 0.07% results in a 25.55% APR. Understanding these conversions helps you better compare credit card offers and understand the true cost of carrying a balance.