Use this calculator to estimate the postage cost for your domestic First-Class Mail Large Envelope (also known as a Flat) based on its weight and dimensions. Please note that these rates are illustrative and subject to change by the USPS.
Understanding Large Envelope (Flat) Postage
Large envelopes, often referred to as "Flats" by the United States Postal Service (USPS), are a common and cost-effective way to send documents, magazines, large photos, and other flexible items that are too big or thick for a standard letter envelope but not bulky enough to be considered a package.
Key Characteristics of a Large Envelope:
Dimensions: To qualify as a large envelope, your mailpiece must meet specific size requirements:
Length: More than 11.5 inches but no more than 15 inches.
Height: More than 6.125 inches but no more than 12 inches.
Thickness: More than 0.25 inches but no more than 0.75 inches.
If your item is smaller than the minimums, it might qualify as a letter. If it exceeds the maximums, it will be classified and charged as a package.
Flexibility: Large envelopes must be uniformly thick and flexible enough to bend. If an item is rigid, contains lumpy contents, or has clasps/string, it may incur a "non-machinable" surcharge because it cannot be processed by automated sorting equipment.
Weight: Postage for large envelopes is primarily determined by weight, measured in ounces. The heavier the envelope, the higher the cost.
How Postage is Calculated:
The base rate for a large envelope covers the first ounce. Each additional ounce (or fraction thereof) adds a fixed amount to the total cost. A non-machinable surcharge is added if the envelope doesn't meet the flexibility and uniformity criteria.
Using the Calculator:
Simply enter the weight, length, height, and thickness of your large envelope into the respective fields. If your envelope is rigid, has clasps, or is unevenly thick, check the "Non-Machinable" box. The calculator will then provide an estimated postage cost and indicate if your item falls within the large envelope dimensions or if it might be classified differently.
Important Considerations:
Rates Change: USPS postage rates are subject to change. The rates used in this calculator are illustrative and based on recent First-Class Mail Large Envelope pricing. Always verify current rates on the official USPS website or at a post office.
Domestic Mail: This calculator is designed for domestic (within the U.S.) First-Class Mail Large Envelopes. International postage has different rates and rules.
Accuracy: While this calculator provides a good estimate, the final postage cost is determined by the USPS based on their official measurements and classifications.
Example Scenarios:
Let's look at a few examples using the calculator's logic:
Standard Large Envelope:
Weight: 3.5 ounces
Length: 12 inches
Height: 9 inches
Thickness: 0.2 inches
Non-Machinable: No
Calculation: Base rate (1 oz) + (2.5 additional oz * per oz rate) = $1.35 + (3 * $0.24) = $1.35 + $0.72 = $2.07
Result: Estimated Postage: $2.07. Classification: Large Envelope.
Heavier Large Envelope:
Weight: 8 ounces
Length: 14 inches
Height: 11 inches
Thickness: 0.5 inches
Non-Machinable: No
Calculation: Base rate (1 oz) + (7 additional oz * per oz rate) = $1.35 + (7 * $0.24) = $1.35 + $1.68 = $3.03
Result: Estimated Postage: $3.03. Classification: Large Envelope.
Non-Machinable Large Envelope:
Weight: 2 ounces
Length: 13 inches
Height: 10 inches
Thickness: 0.3 inches
Non-Machinable: Yes
Calculation: Base rate (1 oz) + (1 additional oz * per oz rate) + Non-Machinable Surcharge = $1.35 + (1 * $0.24) + $0.40 = $1.35 + $0.24 + $0.40 = $1.99
Result: Estimated Postage: $1.99. Classification: Large Envelope (with non-machinable surcharge).
Item Exceeding Large Envelope Dimensions:
Weight: 5 ounces
Length: 16 inches (exceeds max 15)
Height: 10 inches
Thickness: 0.3 inches
Non-Machinable: No
Result: Estimated Postage: N/A. Classification: Likely a Package. (A warning message would appear).
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function calculatePostage() {
var envelopeWeight = parseFloat(document.getElementById("envelopeWeight").value);
var envelopeLength = parseFloat(document.getElementById("envelopeLength").value);
var envelopeHeight = parseFloat(document.getElementById("envelopeHeight").value);
var envelopeThickness = parseFloat(document.getElementById("envelopeThickness").value);
var isNonMachinable = document.getElementById("isNonMachinable").checked;
var resultDiv = document.getElementById("postageResult");
var postageCost = 0;
var classification = "Large Envelope";
var warnings = [];
// Clear previous results and error classes
resultDiv.innerHTML = "";
resultDiv.classList.remove("error");
// Validate inputs
if (isNaN(envelopeWeight) || envelopeWeight <= 0 ||
isNaN(envelopeLength) || envelopeLength <= 0 ||
isNaN(envelopeHeight) || envelopeHeight <= 0 ||
isNaN(envelopeThickness) || envelopeThickness 0.25″
var maxLength = 15;
var maxHeight = 12;
var maxThickness = 0.75;
var isLetterCandidate = false;
var isPackageCandidate = false;
// Check if it's too big for a large envelope
if (envelopeLength > maxLength || envelopeHeight > maxHeight || envelopeThickness > maxThickness) {
isPackageCandidate = true;
classification = "Likely a Package";
warnings.push("Dimensions exceed maximums for a Large Envelope. It will likely be classified and charged as a Package.");
}
// Check if it's too small for a large envelope (and might be a letter)
// This logic is a bit tricky because a user might intentionally send a small item as a flat.
// We'll warn if it meets letter criteria, but still calculate as a flat if it doesn't exceed max flat dimensions.
if (envelopeLength <= 11.5 && envelopeHeight <= 6.125 && envelopeThickness <= 0.25 && envelopeWeight <= 3.5) { // Max weight for letter is 3.5 oz
isLetterCandidate = true;
warnings.push("Dimensions and weight suggest this might qualify as a standard Letter, which could be cheaper.");
} else if (envelopeLength < minLength && envelopeHeight < minHeight && envelopeThickness < minThickness) {
// If it's smaller than typical flat minimums, but not necessarily a letter (e.g., too heavy for letter)
warnings.push("Dimensions are smaller than typical Large Envelope minimums. Ensure it meets all other Flat criteria.");
}
if (isPackageCandidate) {
resultDiv.innerHTML = "Classification: " + classification + "" + warnings.join("");
resultDiv.classList.add("error");
return; // Stop calculation if it's a package
}
// Calculate postage for Large Envelope
postageCost = baseRateFirstOunce; // Cost for the first ounce
if (envelopeWeight > 1) {
var additionalOunces = Math.ceil(envelopeWeight – 1); // Round up for additional ounces
postageCost += (additionalOunces * additionalOunceRate);
}
if (isNonMachinable) {
postageCost += nonMachinableSurcharge;
classification += " (with Non-Machinable Surcharge)";
}
var resultHTML = "Estimated Postage: $" + postageCost.toFixed(2) + "";
resultHTML += "Classification: " + classification;
if (warnings.length > 0) {
resultHTML += "Important Notes:" + warnings.join("");
}
resultDiv.innerHTML = resultHTML;
}