Letters, Flats, and Parcels: USPS Mail Categories Explained

When sending mail through the U.S. Postal Service (USPS), you may assume that a “letter” refers to a simple sheet of paper in a standard envelope. However, in postal terms, these categories—letters, flats, and parcels—represent specific types of mailpieces based on their physical attributes, not just their general form.

In this article, we’ll break down the USPS definitions for letters, flats, and parcels, and explain how their classifications affect postage rates and delivery methods.

What is a “Letter” in Postal Terms?

A “letter” is not just any standard envelope with a sheet of paper. The USPS defines a machinable letter based on specific size and weight criteria, as follows:

• Height: Between 3.5 inches and 6.125 inches
• Length: Between 5 inches and 11.5 inches
• Aspect Ratio: Length divided by height between 1.3 and 2.5
• Thickness: Between 0.007 inch and 0.25 inch (must be at least 0.009 inch thick if height exceeds 4.25 inches or length exceeds 6 inches)
• Weight: No more than 3.5 ounces
• Shape: Rectangular, with four right-angle corners (no more than 0.125-inch radius) and parallel opposite sides

These specifications apply to machinable letters, which are processed by USPS machinery. If a mailpiece doesn’t meet these criteria, it’s considered non-machinable and subject to higher postage rates.

Defining a “Flat”

A machinable flat is a mailpiece that meets at least one of the following attributes:

• Height: More than 6.125 inches but no more than 12 inches
• Length: More than 11.5 inches but no more than 15 inches
• Thickness: More than 0.25 inch but no more than 0.75 inch
• Weight: More than 3.5 ounces but no more than 16 ounces

Flats do not have a specified aspect ratio, but they must be rectangular, defined by four right-angle corners with a radius no greater than 0.125 inches and parallel opposite sides. Interestingly, a square (with four equal sides) also qualifies as a machinable flat.

What Defines a “Parcel”?

When it comes to parcels, the classification can be more complex, as there are different categories for parcels, such as machinable, irregular, non-machinable, and lightweight parcels, among others.

A machinable parcel complies with the following criteria (in addition to the requirements for flats):

• Height: More than 12 inches but no more than 17 inches
• Length: More than 15 inches but no more than 27 inches
• Thickness: More than 0.75 inch but no more than 17 inches
• Weight: More than 16 ounces but no more than 25 pounds

Additional Factors Affecting Mail Classification

Aside from size and weight, other physical characteristics, such as material type, poly wrapping, flexibility, and uniform thickness, can influence whether a mailpiece is deemed machinable. Here’s a closer look at two important factors:

• Flexibility: A flat must pass a flexibility test, meaning it cannot be too rigid or too flimsy. To test, the mailpiece is hung from a ledge, and the amount it droops or bends is measured.
• Uniform Thickness: A machinable flat must have consistent thickness, with variations no greater than 0.25 inches, excluding the first inch from the edge of the mailpiece.

These factors help ensure that mailpieces are processed smoothly through USPS machines. If a mailpiece doesn’t meet these standards, it will fall into a higher pricing category.

Examples: How Size Affects Postage Classification

Mailpieces may fall into different categories based on size, even if they seem similar. For instance, consider the following scenarios:

• A postcard measuring 6.5 inches by 10 inches exceeds the height limit for a machinable letter, so it’s classified as a flat, which incurs higher postage.
• A postcard measuring 10 inches by 16 inches exceeds the maximum dimension for a flat, so it would be classified as a parcel.

This demonstrates how seemingly simple direct mail pieces can be categorized differently based on their physical attributes, influencing the postage cost.

Why USPS Classifications Matter

Understanding how your mailpiece is classified—whether as a letter, flat, or parcel—can save you money on postage and help avoid delays. For example, a letter may be processed much faster and at a lower cost than a flat or parcel. However, mailpieces that don’t conform to the guidelines for each category can face higher postage rates and processing challenges.

How to Ensure Your Mailpiece Meets USPS Standards

To ensure your direct mail meets USPS standards and qualifies for the lowest postage rates, follow these steps:

1. Measure Dimensions Carefully: Verify that the height, length, and thickness of your mailpiece are within the USPS guidelines for its category.
2. Consider Material and Flexibility: Ensure that your mailpiece has the proper flexibility to pass USPS tests and uniform thickness for efficient processing.
3. Use USPS Resources: The Domestic Mail Manual (DMM) provides detailed guidelines on all mail classifications. While it’s a lengthy document, it’s an essential resource for understanding postal requirements. You can find the DMM and additional resources like quick service guides and pricing sheets at USPS’s Postal Explorer website or through their Postal Pro search engine.

Conclusion

Letters, flats, and parcels aren’t just casual labels—they represent distinct categories with specific size, weight, and material requirements defined by USPS. Understanding these categories can help you save on postage costs and avoid unexpected fees. Whether you’re designing a direct mail campaign or preparing a one-time mailing, knowing the differences between these classifications is key to streamlining the process.

If you need assistance designing mailpieces for commercial use or planning a direct mail campaign, contact Phillips Printing’s mailing department at 888-ask-phil or reach out to us directly.