A "Large Envelope," also known by the USPS as a Flat, must meet specific physical characteristics to qualify for Flat pricing rather than the more expensive Package (Parcel) rates or the standard Letter rates.
Standard Dimensions for Flats
Minimum Height: 6.125 inches
Minimum Length: 11.5 inches
Maximum Height: 12 inches
Maximum Length: 15 inches
Maximum Thickness: 0.75 inches
Maximum Weight: 13 ounces
Uniformity and Flexibility
To qualify as a Flat, the envelope must be somewhat flexible (it must be able to bend around mail processing equipment) and must be uniformly thick. If your envelope contains a bulky item that creates a hump, it will likely be reclassified as a Package, even if the dimensions are correct.
How Postage is Calculated
The calculator uses the current 2024 representative USPS First-Class Mail Flat rates. The pricing starts at a base rate for the first ounce and increases incrementally for each additional ounce up to the 13-ounce limit. Anything over 13 ounces must be shipped via Ground Advantage or Priority Mail.
Expert Tip: If your envelope is rigid, non-rectangular, or has an uneven thickness, it will be charged as a parcel. Always ensure your documents are spread evenly within the envelope to maintain a uniform thickness under 3/4 of an inch.
function calculatePostage() {
var weight = parseFloat(document.getElementById('envWeight').value);
var thickness = parseFloat(document.getElementById('envThickness').value);
var length = parseFloat(document.getElementById('envLength').value);
var width = parseFloat(document.getElementById('envWidth').value);
var resultDiv = document.getElementById('postageResult');
var costDisplay = document.getElementById('costDisplay');
var mailTypeDisplay = document.getElementById('mailTypeDisplay');
var logicNote = document.getElementById('logicNote');
if (isNaN(weight) || isNaN(thickness) || isNaN(length) || isNaN(width)) {
alert("Please enter valid numbers for all fields.");
return;
}
resultDiv.style.display = "block";
var cost = 0;
var type = "";
var note = "";
// USPS 2024 Representative Rates (Adjusted for "Flats")
var baseFlatRate = 1.50; // Starting at 1 oz
var addlOzRate = 0.24;
// Classification Logic
if (weight > 13) {
type = "Package (Ground Advantage)";
cost = 5.00; // Average base starting price for ground advantage
note = "Weights over 13oz do not qualify for Large Envelope (Flat) rates and must be sent as a Package.";
} else if (thickness > 0.75 || length > 15 || width > 12) {
type = "Package (Parcel)";
cost = 4.75; // Standard starting package rate
note = "Dimensions or thickness exceed Flat limits. Item must be mailed as a Package.";
} else if (length < 11.5 && width < 6.125 && thickness < 0.25) {
type = "Standard Letter";
cost = 0.68 + (Math.ceil(weight – 1) * 0.24);
if (weight < 1) cost = 0.68;
note = "Your dimensions are small enough to qualify as a standard letter (if under 3.5oz).";
} else {
type = "Large Envelope (Flat)";
// Calculation for Flats
var roundedWeight = Math.ceil(weight);
if (roundedWeight <= 1) {
cost = baseFlatRate;
} else {
cost = baseFlatRate + ((roundedWeight – 1) * addlOzRate);
}
note = "Meets USPS Large Envelope (Flat) criteria. Price based on First-Class Mail rates.";
}
mailTypeDisplay.innerText = type;
costDisplay.innerText = cost.toFixed(2);
logicNote.innerText = note;
}