Direct Mail, Marketing

Mailpiece Specifications

As you begin to plan and design a direct mail campaign, there are some essential guidelines to understand and follow to ensure deliverability and cost-effectiveness. If followed, you can ensure your mailpiece will be deliverable and that you will get the best rate at the post office. First, we’ll cover today include the difference between letters, flats, and parcels. Each of these mail at progressively higher costs, so knowing the specifications for each is vital to controlling costs. We’ll also discuss making your mailpiece machinable, which also drastically reduces the mailing costs associated with your campaign.

Processing category: Postcard, Letter, Flat, or Parcel


If you are looking to get your prospects’ immediate attention without the need to open an envelope, a postcard is a right choice. These mail pieces are cost-effective, easy to design, and effective marketing tools. To qualify as a postcard, your mail piece must meet the following requirements:

  • Rectangular 
  • At least 3.5″ high x 5″ long x 0.007″ thick 
  • No more than 4.25″ high x 6″ long x 0.016″ thick

Failure to meet these requirements will result in your mail piece being classified and charged as a letter. Also, be careful with sticker and/or other attachments to your postcard. These additional items, though small, may disqualify your piece for the postcard rate. More information on postcards can be found here.

Tips from USPS

  • If you’re planning to mail a postcard, First-Class Mail gives you the best value for your postage dollars. There is no lower postcard price in USPS Marketing Mail.
  • 0.007 inches? How do I measure that? As a guide, an index card is thick enough. If in doubt, contact a Mailpiece Design Analyst (MDA) at the Post Office near you. MDAs have tools for precisely measuring thickness and can tell you if your mailpiece is thick enough.
  • Make sure your mailpiece meets the minimum thickness requirement. Thin, flimsy pieces tend to get caught in mail processing equipment. If your mailpiece gets damaged in the equipment, then your message doesn’t reach your customers.
  • What is high? What is long? Length is the side parallel to the address. Height is the side that is perpendicular to the length.


Letters are cheaper than both flats and parcels but are typically more costly than postcards. If you can design your mailpiece as a letter effectively, you’ll be able to run your campaign at a lower cost while still reaching your prospects’ mailbox. Here are the dimensions stipulated by the USPS for classification as a letter:

  • Length
    • Min 5″
    • Max 11.5″
  • Height
    • Min 3.5″
    • Max 6.125″
  • Thickness
    • Min 0.007″
    • Max 0.25″
  • Weight 3.5 ounces

To be eligible for mailing at a price for letters, a piece must be:

  • Rectangular
  • At least 3.5″ high x 5″ long x 0.007″ thick. 
  • No more than 6.125″ high x 11.5″ long x 0.25″ thick.

If your letter has one or more nonmachinable characteristics, you will pay a nonmachinable surcharge. More information on the USPS standard for letters can be found here.


Flats are how the USPS refers to large envelopes, magazine, etc. To be a flat, a mailpiece exceeds one or more of the maximum dimensions of a letter. For example, a flat must:

  • Have one dimension that is greater than 6.125″ high OR 11.5″ long OR 0.25″ thick.
  • Be no more than 12″ high x 15″ long x 0.75″ thick.

While flats can hold a considerable amount of material, costs can rise quickly as weight increases at the rate for flats. Here are all the specifications for flats from the USPS:

  • Length
    • Min 11.5″
    • Max 15″
  • Height
    • Min 6.125″
    • Max 12″
  • Thickness
    • Min 0.25″
    • Max 0.75″

If the rate for flats is cost-prohibitive to your campaign, look into the possibility of folding your flat-sized piece to fit in a letter-sized envelope. Doing this will significantly reduce the cost associated with the mailing. Find more information about flats from USPS here.


If your mailpiece doesn’t qualify as a postcard, letter, or flat, then it’s a parcel. Often thought of as big boxes, parcels can be big or small. A parcel is a designation for items that don’t fit in any of the other categories; these can sometimes be very small. Generally speaking, a commercial parcel should measure:

  • At least 3″ high x 6″ long x .25″ thick. 
  • Except for USPS Retail Ground and Parcel Select, no mailpiece may measure more than 108″ in length and girth combined. Length is the measurement of the longest dimension, and girth is the distance around the thickest part (perpendicular to the length). Maximum weight is 70 pounds.

If you are sending a parcel, you can still reduce costs by utilizing machinable parcels. For your parcel to be machinable, they must fit the following specifications:

  • No more than 27″ long x 17″ width x 17″ high.
  • No more than 25 pounds (35 pounds for Parcel Select and Parcel Return Service, except books and other printed matter which cannot exceed 25 pounds).

Find more information on parcels here.


By understanding the guidelines set forth by the USPS for postcards, letters, flats, and parcels, you can understand how to design your mailpiece cost-effectively. For help with mailpiece design, contact our design team at ANS by email Have a great day.

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