USPS Large Envelopes - Flats | Overview

ShippingEasy wants to help you learn about the e-commerce industry. This article describes a service provided by the United States Postal Service (USPS).

The USPS is a separate organization from ShippingEasy. For more information about USPS services, visit your local Post Office or go to

The words "envelopes" and "flats" are used interchangeably at USPS when discussing mail that is slightly larger than regular, letter-sized mail. All large envelope/flats must be rectangular or square-shaped and somewhat flexible. You can find out how USPS determines flexibility by referring to the USPS' guide.

View ShippingEasy's Postage Rates for USPS Shipments

Size Limits of Large Envelopes / Flats

For your mail to be considered a large envelope/flat and not a letter, it must exceed one or more of the letter-size minimum dimensions listed in the table below.

Dimension  Minimum
Height 6-1/8"
Length 11-1/2"
Thickness 1/4"

For your mail to be considered a large envelope/flat and not a package (parcel) it must not exceed the following criteria:

Dimension Maximum
Height 12"
Length 15"
Thickness 3/4"

Learn more about shipment dimensions.

Weight Limits of Large Envelopes / Flats

The maximum weight for large envelopes is 13 ounces. Large envelope/Flats can be shipped via First Class or Priority Mail, however, a large envelope/flat exceeding 13 ounces is classified as a Priority Mail item. 

Large Envelopes/Flats shipped via First Class are not eligible for tracking. Tracking is included when shipped via Priority Mail. Learn more.

Learn more about shipment weight.

More questions about USPS letters and flats? Check out USPS' Domestic Mail Manual.


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