What Size Are USPS Flat Rate Boxes?

If you’re shipping via USPS Flat Rate, you’ll need to know what size boxes are available and which one will best fit your needs. In this blog post, we’ll break down the dimensions of the different Flat Rate boxes so you can make the best decision for your shipment.

Checkout this video:


USPS flat rate boxes come in a variety of sizes. The most popular flat rate box is the medium flat rate box. The USPS website offers a tool to help you determine which size flat rate box is right for your shipment.

Here are the dimensions for the various sizes of USPS flat rate boxes:

-Medium Flat Rate Box: 12-1/4″ x 12-1/4″ x 6″
-Large Flat Rate Box: 12-1/4″ x 12-1/4″ x 12-1/2″
-APO/FPO Large Flat Rate Box: 12″ x 12″ x 5-1/2″
-Legal Flat Rate Envelope: 9-1/2″ x 4-3/8″”
-Padded Flat Rate Envelope: 10″ 7/8″” x 5″ 1/2″”

Flat Rate Boxes

USPS Flat Rate Boxes come in a variety of sizes. The most popular size is the Large Flat Rate Box, which is available in two different versions. There is also a Medium Flat Rate Box, a Small Flat Rate Box, and a Flat Rate Envelope. Let’s take a look at the dimensions of each of these boxes.

Small Flat Rate Box

The Small Flat Rate Box has a rectangular shape and offers a flat-rate shipping charge, no matter the weight or destination of the package. This size is perfect for small items that weigh less than 70 pounds.

Medium Flat Rate Box

The Medium Flat Rate Box – 1.63 Cubic Feet holds up to 70 pounds and costs $13.20 for retail customers, $11.70 for commercial base customers, and $12.30 for commercial plus customers to ship anywhere in the United States when using a USPS Flat Rate service.

Large Flat Rate Box

The large flat rate box from the United States Postal Service is 13-5/8″ x 11-7/8″ x 3-3/8″ and is available for $19.95. This size box can accommodate bigger items such as lamp shades and small electronics.

Extra Large Flat Rate Box

The largest USPS flat rate box is the Extra Large Flat Rate Box. It measures 12″ x 12″ x 5-1/2″, and it’s perfect for large, heavy items that won’t fit in a smaller box. The cost to ship an Extra Large Flat Rate Box is $45.95, regardless of where it’s being shipped within the United States.

How to Choose the Right Size Box

USPS Flat Rate Boxes come in a variety of sizes. The right size box will depend on the items you are shipping. If you are unsure what size box to use, the USPS website has a handy tool that will help you choose the right size box based on the items you are shipping.

Consider the Contents You’re Shipping

If you’re shipping items like books, DVDs, or other smaller items that can be tightly packed together, a small box might work well. However, if you’re shipping larger items or items that need more room to move around (such as clothing), a larger box would probably be a better choice.

Determine the Dimensions of Your Contents

Choosing the right size box is an important part of packing for a move or sending a package. The U.S. Postal Service® (USPS®) has multiple shipping boxes and envelopes to fit your mailpiece needs. While most packages will fit into one of our five standard box sizes, some items are not well suited for shipment in anything other than their original packaging or require special handling.

Determine the Dimensions of Your Contents
To find the right box size for your contents, first determine the dimensions (length, width, and height) of each item you plan to ship. Once you have those dimensions, select a shipping box that is large enough to accommodate your contents without being too large or too small. Remember that it’s always better to err on the side of choosing a larger box rather than a smaller one – you can always fill empty space in a larger box with packing material to keep your contents from moving around during transit.

If you’re unsure which USPS shipping option is best for your needs, please visit our Shipping Services page or contact us for assistance.


The most popular USPS Flat Rate Boxes are the Medium and Large boxes. The Medium Box can accommodate items up to 70 lbs, while the Large Box can accommodate items up to 130 lbs.