function calculatePostage() {
var weight = parseFloat(document.getElementById('weightOz').value);
var length = parseFloat(document.getElementById('lengthIn').value);
var height = parseFloat(document.getElementById('heightIn').value);
var thickness = parseFloat(document.getElementById('thicknessIn').value);
var resultDiv = document.getElementById('result');
var classEl = document.getElementById('mailClassRes');
var rateEl = document.getElementById('rateRes');
var noteEl = document.getElementById('noteRes');
var warnEl = document.getElementById('dimensionsWarning');
if (isNaN(weight) || isNaN(length) || isNaN(height) || isNaN(thickness)) {
alert("Please enter valid numbers for all fields.");
return;
}
resultDiv.style.display = "block";
warnEl.innerHTML = "";
// Define Limits
var minL = 5, minH = 3.5, minT = 0.007;
var maxLetterL = 11.5, maxLetterH = 6.125, maxLetterT = 0.25;
var maxFlatL = 15, maxFlatH = 12, maxFlatT = 0.75;
// Pricing (Approximate Base Retail Rates 2024/2025)
var letterBase = 0.73;
var letterAddOz = 0.24;
var flatBase = 1.50;
var flatAddOz = 0.24;
var nonMachinableSurcharge = 0.46; // For letters
// Logic Variables
var mailType = "";
var cost = 0;
var notes = "Standard";
var isNonMachinable = false;
// Check Minimums
if (length < minL || height < minH || thickness < minT) {
classEl.innerText = "Below Minimum Standards";
rateRes.innerText = "N/A";
noteRes.innerText = "Item is too small to be mailed.";
return;
}
// — LETTER CHECK —
// Must be rectangular
// Aspect ratio check (Length divided by Height)
var aspectRatio = length / height;
var fitsLetterDims = (length <= maxLetterL && height <= maxLetterH && thickness <= maxLetterT);
var fitsLetterWeight = (weight <= 3.5);
if (fitsLetterDims && fitsLetterWeight) {
mailType = "First-Class Mail Letter";
// Check Machinability
if (aspectRatio 2.5) {
isNonMachinable = true;
notes = "Non-Machinable (Aspect Ratio)";
}
// (There are other non-machinable criteria like rigidity, but we can't test that here easily)
// Calculate Letter Cost
// First oz is base, extra oz is add-on
var chargeableWeight = Math.ceil(weight); // USPS rounds up
if (chargeableWeight < 1) chargeableWeight = 1;
var additionalOz = chargeableWeight – 1;
cost = letterBase + (additionalOz * letterAddOz);
if (isNonMachinable) {
cost += nonMachinableSurcharge;
}
classEl.innerText = mailType;
rateRes.innerText = "$" + cost.toFixed(2);
noteRes.innerText = notes;
return;
}
// — LARGE ENVELOPE (FLAT) CHECK —
var fitsFlatDims = (length <= maxFlatL && height <= maxFlatH && thickness <= maxFlatT);
var fitsFlatWeight = (weight 3.5oz
var exceedsLetter = (length > maxLetterL || height > maxLetterH || thickness > maxLetterT || weight > 3.5);
if (fitsFlatDims && fitsFlatWeight && exceedsLetter) {
mailType = "First-Class Mail Large Envelope (Flat)";
notes = "Must be flexible & uniform thickness";
// Calculate Flat Cost
var chargeableWeight = Math.ceil(weight);
if (chargeableWeight 13 && weight = 16) {
notes += " Over 1lb (Priority Mail or Ground required).";
}
classEl.innerText = mailType;
rateRes.innerText = "Starts at ~$5.00+";
noteRes.innerText = "Price varies by Destination Zone";
warnEl.innerText = "Note: Since this item exceeds Flat dimensions or weight, it is classified as a Package. Package rates depend on the destination ZIP code.";
}
Understanding USPS Envelope Classifications
Choosing the correct mail class is crucial for ensuring your mail is delivered without postage due and at the lowest possible cost. The United States Postal Service (USPS) categorizes mail based primarily on physical dimensions (Length, Height, Thickness) and weight. This calculator helps determine if your item qualifies as a standard Letter, a Large Envelope (Flat), or a Package.
1. Standard Letters
To qualify for the most affordable First-Class Mail Letter rate, your envelope must meet strict physical standards. It must be rectangular, meaning the length (the dimension parallel to the address) must be at least 1.3 times the height but no more than 2.5 times the height.
Min Dimensions: 5″ x 3.5″ x 0.007″
Max Dimensions: 11.5″ x 6.125″ x 0.25″
Max Weight: 3.5 ounces
If a letter is square, rigid, or has an uneven surface (like a clasp), it may be subject to a Non-Machinable Surcharge.
2. Large Envelopes (Flats)
Items that exceed the dimensions of a standard letter but are still flat and flexible are categorized as Large Envelopes or "Flats". This is common for sending 8.5×11 documents without folding them.
Max Dimensions: 15″ x 12″ x 0.75″
Max Weight: 13 ounces
Requirement: Must be flexible and uniformly thick.
If a Large Envelope is rigid (contains a hard object like a pen or stiff cardboard), it automatically bumps up to the Package rate, which is significantly more expensive.
3. Packages (Ground Advantage / Priority)
If your mailpiece is thicker than 0.75 inches, heavier than 13 ounces (for First-Class), or is rigid/non-flexible, it is classified as a Package. Package pricing is based on weight and the distance (Zone) it travels. As of 2024, standard retail packages usually ship via USPS Ground Advantage.
How to Measure Your Envelope
Accurate measurement is key to getting the right postage rate:
Length: Measure the side that is parallel to the address text.
Height: Measure the side perpendicular to the length.
Thickness: Measure the bulk of the envelope. If it bulges, measure the thickest point.
Frequently Asked Questions
What is the "Aspect Ratio" rule?
USPS machines process mail horizontally. To be machine-readable, the length divided by height must be between 1.3 and 2.5. Square envelopes (ratio of 1.0) fail this test and require extra postage (non-machinable surcharge).
How many stamps do I need?
A standard 1 oz letter requires 1 "Forever" stamp. Each additional ounce usually costs roughly 24 cents extra. Large Envelopes start at a higher rate (roughly equivalent to 2 stamps) and increase with weight.