What Qualifies As a Large Mailing Envelope? 

To be eligible for large mailing, your envelope must be rectangular and over three-1/2″ tall and five-1/2″ wide. This means that an envelope that measures 3″ by 6″ would not qualify because the short side is shorter than three-1/2″. Similarly, a 6-1/2″ x 9″ envelope would not be eligible because the “shortest” edge is longer than six-1/2″. When you are unsure of whether your envelope qualifies for large mailing, take it to your post office for a measurement.

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Conformer envelopes 

Conformer envelopes are durable, heavy-duty envelopes designed for shipping important or heavy items. They are more affordable than boxes, so they are ideal for sending important documents or marketing materials. Conformer envelopes are strong enough to pierce regular padded envelopes, so you won’t have to worry about adding additional shipping costs. 

Conformer envelopes are usually made of stiff paperboard that is strong enough to protect oversized items. They also have a plug seal, so you can be sure your package will be secure. When mailing your package, remember to write your address clearly and in the center. The address should be written in permanent ink, such as a Sharpie, so it will not smear or fade. 

Non-machinable envelopes 

Non-machinable large mailing envelopes are those that cannot be mated by a machine. They are also known as hand-canceled envelopes. While the post office should never leave a non-machinable envelope unattended, it should be put in a stack to be delivered to the correct location. Do not place the envelope in a mail slot or “outgoing” box. Most likely, it will end up in a container and not be delivered. 

To be non-machinable, the piece must meet certain requirements. First, it must have an aspect ratio that is less than 1.3. Second, it must not have a spine or closure device that will make it nonmachinable. Third, the piece cannot weigh more than 40 pounds around an 11-inch-diameter turn. Finally, it must have no clasps, strings, or other similar closure devices. 

Letter-size mail that weighs more than one pound 

When you mail a letter that weighs more than a pound, you should use a postal scale to weigh it. Home scales are not sensitive enough to weigh a letter. You should also note that USPS rounds up the weight when it calculates postage. Therefore, you should weigh your letter with a scale and use a rounded-up number. 


When mailing large items, the dimensions of a large mailing envelope are critical. This type of envelope has a length and width that exceeds the maximum dimensions of two letter sizes and must be at least 12 inches high and 15 inches long. This gives ample room for the material to be shipped. The dimensions are also important for postage purposes, as larger envelopes require higher postage for First Class mail. 

There are many things to consider when choosing the right envelope for your business. First, consider the type of product you will be mailing. Most often, envelopes are made from writing or text-weight paper. Choosing the right weight is crucial for several reasons. A thick envelope demonstrates a nice quality, but it can add extra weight to the mailing, which can result in extra postage. Therefore, you should decide on the weight of the envelope based on the intended use and potential mailing weight. Also, consider the anatomy of the envelope, including the top fold, side seam, throat, and seam overlap. 


First-Class mail is sent in large mailing envelopes, also known as flats. These are used to carry legal documents, magazines, or marketing materials. They are designed to hold standard-size paper (8 1/2 x 11) without folding. These envelopes are mailed like other kinds but do not need to be addressed at a post office. They can be dropped in the mailbox or handed to the mail carrier. 

USPS rules require that a large mailing envelope not exceed a certain size. A standard letter-size envelope is 6 1/8″ high, 11 1/2″ long, and 1/4″ thick. The USPS has a specific size and weight requirements for large mailing envelopes. An envelope that exceeds these limits is considered a parcel, which increases the price of postage.