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All Things Shipping | Sep 15, 2020

USPS Flat Rate Box Sizes, Prices, and How it Works

USPS Flat Rate Box Sizes, Prices, and How it Works

USPS Priority Mail Flat Rate boxes are an easy-to-use, cost-effective shipping option. You can easily ship like the experts; just know the USPS Flat Rate box sizes, prices, and how it works before you buy.

How Does USPS Flat Rate Shipping Work?

The USPS Priority Mail Flat Rate service will ship your package anywhere in the U.S. in 1-3 business days for a flat fee. To ship with this service, follow these easy steps:

  1. Order free Flat Rate boxes from the USPS. You have to use the branded Flat Rate boxes to get the Flat Rate shipping price, and you can order them online or pick them up at your local Post Office. 
  2. Sign up for a free Shippo account to access the best rates on USPS Flat Rate shipping and print your labels from the comfort of your office or home.
  3. Bonus! Determine if a USPS Flat Rate box is the cheapest option for the items you’re looking to ship, since it’s not always the most affordable choice.

What are the 2020 USPS Flat Rate Box Sizes and Prices?

As you’ll see below, Shippo offers discounted shipping rates when compared to what you’d find at your local Post Office. No volume or coding requirements. To sign up for a free Shippo account, visit www.goshippo.com/register/.

2020 USPS Priority Mail Flat Rate

Shippo vs. Retail Rates

Products Dimensions Shippo Rate Retail Rate Savings Using Shippo vs. Retail

% Savings with Shippo

Flat Rate Envelope 12 ½” x 9 ½” $7.15 $7.75 $0.60 8%
Legal Flat Rate Envelope 15” x 9 ½” $7.45 $8.05 $0.60 7%
Padded Flat Rate Envelope 12 ½” x 9 ½” $7.75 $8.40 $0.65 8%
Small Flat Rate Box Outside:

8 11/16” x 5 7/16” x 1 ¾”

Inside:

8 ⅝” x 5 ⅜” x 1 ⅝”

$7.65 $8.30 $0.65 8%
Medium Flat Rate Box – 1 (Top Loading) Outside:

11 ¼” x 8 ¾” x 6”

Inside:

11” x 8 ½” x 5 ½”

$13.20 $15.05 $1.85 12%
Medium Flat Rate Box – 2 (side-loading) Outside:

14” x 12” x 3 ½”

Inside:

13 ⅝” x 11 ⅞” x 3 ⅜”

$18.30 $21.10 $2.80 13%
Large Flat Rate Boxes Outside:

12 ¼” x 12 ¼” x 6”

Inside:

12” x 12” x 5 ½”

$16.80 $19.60 $2.80 14%

Is Priority Mail the Same as Flat Rate?

USPS Priority Mail is one of the most-popular service levels the Postal Service offers, and for good reason. It’s cost effective and encompasses three widely-used shipping options: Priority Mail Flat Rate (which we discuss above), Priority Mail weight-based, and Priority Mail Cubic

To ship via Flat Rate, you need to use the Flat Rate branded boxes. To use Cubic and weight-based options, you will need to use your own packaging or USPS Priority Mail boxes. 

Which is Cheaper: Flat Rate or Priority Mail?

Generally speaking, the USPS Flat Rate boxes are a great option for heavy items that are traveling long distances. We ran the numbers to find out exactly when you should use a Flat Rate box and when you should use your own packaging and ship via USPS Priority Mail weight-based or Cubic instead. 

When to Use USPS Flat Rate Envelopes

  • USPS Flat Rate Envelopes are almost ALWAYS the cheapest option.
  • However, use your own envelope for packages weighing exactly 1 lb. and traveling locally.
  • And if your package weighs less than 1 lb., use your own envelope and ship via USPS First Class Package Service. 

Use your own envelope (NOT a Flat Rate envelope) for the following scenarios:

Flat Rate Envelope (12 1/2″ x 9 1/2″) – $7.15

Qualifications for Using Own Packaging Cost to Ship Savings
Weighs 1 lb. and traveling less than 50 miles (Zone 0,1,2) $7.02 $0.13

Legal Flat Rate Envelope (15″ x 9 1/2″) – $7.45

Qualifications for Using Own Packaging Cost to Ship Savings
Weighs 1 lb. and traveling less than 300 miles (Zone 0-3) $7.02 – $7.35 $0.10+

Padded Flat Rate Envelope (12 1/2″ x 9 1/2″) – $7.75

Qualifications for Using Own Packaging Cost to Ship Savings
Weighs 1 lb. and traveling less than 600 miles (Zone 0-4) $7.02 – $7.56 $0.19+
Weighs 1-20 lbs. and traveling less than 50 miles (Zone 0,1,2) $7.46 $0.29

When to Use USPS Flat Rate Boxes 

  • For 1 – 20 lb. packages, choose Flat Rate for long distances and your own box for short distances.
  • For <1 lb. packages, use your own box and ship via USPS First Class Package Service. 
  • For >20 lb. packages, use a Flat Rate box. 

Use your own box (NOT a Flat Rate box) for the following scenarios:

Sm. Flat Rate Box (8 5/8″ x 5 3/8″ x 1 5/8″) – $7.65

Qualifications for Using Own Packaging Cost to Ship Savings
Weighs 1-20 lbs. and traveling less than 600 miles (Zone 0-4) $7.02 – $7.56 $0.09+

Med. Flat Rate Box (11 1/4″ x 8 3/4″ x 6″) – $13.20

Qualifications for Using Own Packaging Cost to Ship Savings
Weighs 1-20 lbs. and traveling less than 1400 miles (Zone 0-6) $8.21 – $12.78 $0.42+

Lg. Flat Rate Box (12 1/4″ x 12 1/4″ x 6″) – $18.30

Qualifications for Using Own Packaging Cost to Ship Savings
Weighs 1-2 lbs. $7.02 – $17.45 $0.85+
Weighs 3-4 lbs. and traveling anywhere except U.S. Territories (Zone 0-8) $7.84 – $18.14 $0.16+
Weighs 5 lbs. and traveling less than 1400 miles (Zone 0-6) $8.04 – $16.14 $2.16+
Weighs 6-8 lbs. and traveling less than 1000 miles (Zone 0-5) $8.15 – $17.35 $0.95+
Weighs 9-13 lbs. and traveling less than 600 miles (Zone 0-4) $9.28 – $17.68 $0.62+
Weighs 14-16 lbs. and traveling less than 300 miles (Zone 0-3) $13.27 – $17.97 $0.33+
Weighs 17-20 lbs. and traveling less than 50 miles (Zone 0,1,2) $14.85 – $16.10 $0.44+

Keep in mind that the maximum weight is 70 pounds. And, when sealing a Flat Rate envelope or box, the container flaps must be able to close within the normal folds.

Is Priority Mail (Weight-based) or Flat Rate Faster?

Technically speaking, they are all part of the Priority Mail mailstream, so they all ship within 1-3 business days. 

Are There Any Other Restrictions?

There are specific regulations and standards for mailing restricted, prohibited, and hazardous materials, including cigarettes and smokeless tobacco. You also can’t ship airbags, ammunition, explosives, or gasoline. For more details, visit the USPS Shipping Restrictions page.

How Do I Get a USPS Priority Mail Flat Rate Label?

It takes just a few minutes to set up a free Shippo account, and then you can print the label from your office or warehouse. (To get a clearer picture upfront of how you can save on USPS shipping rates by using Shippo, feel free to check out our USPS shipping calculator.) Affix the label to the USPS Priority Mail Flat Rate box. Then, put it in your mailbox or schedule a pick-up with your local mail carrier in the Shippo dashboard.

Make sure the label you’re using matches the packaging. For example, if you are using a Priority Mail Express Flat Rate box, you need to use a Priority Mail Express Flat Rate label.

If you liked this post, you may also be interested in a video that our co-founder Laura Behrens Wu did with the co-founder of packaging solution company Lumi on shipping with the USPS.


Shippo is a multi-carrier API and web app that helps retailers, marketplaces and platforms connect to a global network of carriers. Businesses use Shippo to get real-time rates, print labels, automate international paperwork, track packages and facilitate returns. Shippo provides the tools to help businesses succeed through e-commerce shipping.

Michelle McNamara is a Senior Marketing Manager at Shippo

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