Know What You’re Mailing: Letters vs. Flats vs. Parcels

By Tom Ryan, Director, Global Product Marketing, Pitney Bowes

When you’re preparing mail to send to customers, there are a number of considerations you probably take into account - ensuring accurate address information or incorporating color as a way to stand out from other mail. But, it’s also important to consider what type of mail you’re sending to drive the most impact to the recipient, as well as your bottom line.

The U.S. Postal Service® offers three basic types of mail: letters, flats and parcels. Knowing which category your mailpiece falls into is more important than you might think. Not only does the classification determine postage, but it helps ensure your mailings are timely and cost-effective.

It can be confusing to keep the categories straight; the main point to remember is your mailing isabout the size, not the content.


To be classified as a letter, the mailpiece must be rectangular in shape and fall within the following dimensions:

  • Between 3½ inches and 6⅛ inches long
  • Between 5 inches and 11½ inches wide
  • At least 0.007 inches thick, but no more than ¼ inch thick

Most envelopes you think of as letters are, in fact, letters. Meeting the 0.007-inch-thick minimum is typically not a problem. A quick tip -  you can use an index card as a benchmark for thickness. When in doubt, contact a Mailpiece Design Analyst (MDA) near you to get a precise measurement.

Mailing a First-Class Mail® letter that weighs up to 1 ounce costs just 49 cents. Though the cost of a retail stamp has risen, customers can save 3 cents on letters up to 3.5 ounces by taking advantage of metered mail and online stamps.


Flats, also known as large envelopes, are too big in at least one dimension to be classified as a letter, but still thin enough not to be considered a parcel. If the measurements of your mail piece fit these categories, it’s a flat:

  • More than 6⅛ inches long, more than 11 ½ inches wide or more than ¼ inch thick
  • But, no larger than 12 inches long, 15 inches wide and ¾ inch thick

The base cost for a First-Class flat under one ounce is 98 cents. Even though larger envelopes allow you to put more material inside, be mindful about what you’re adding. The heavier the mailpiece, the higher the postage, especially for First-Class Mail.


Anything that’s too large to be a letter or flat is considered a parcel. As long as the piece exceeds the measurements for flats, it’s a parcel, even if it’s in an envelope. All parcels must be at least 3 inches high, 6 inches long and ¼ inch thick.

There are also guidelines about how large parcels can be. With the exception of USPS Retail GroundTM and Parcel SelectTM, a parcel cannot be greater than 108 inches in length and girth combined, where length is its longest dimension and girth is the dimension perpendicular to the length.

In addition, the maximum weight of a parcel is 70 pounds. At just $2.67 for First-Class mail parcels up to 4 ounces, though, parcels offer some of the best value for your postage dollars.

There’s a lot to know. Keep these guidelines as a cheat sheet above your desk for reference all year long.

Whether you’re sending letters, flats or parcels, do more and save more with Pitney Bowes SendPro. Learn more about our SendPro family of solutions here