Us postal service priority mail box sizes

As we enter the busiest time of the year for many in the e-commerce industry, it’s important for carriers to maximize their output to meet customer demand. One way the United States Postal Service is doing this is by adjusting the size of their free USPS Priority Mail boxes.

The new sizes were originally rolled out in the last week of September. However, e-commerce merchants were allowed to use the Priority Mail boxes they already had until they ran out.

With more and more holiday shopping orders rolling in this month, stocking up on packaging supplies will be critical. We’ll explain what these new sizes mean for your holiday shipping processes and beyond.

Why Are Free USPS Priority Mail Packages Getting Smaller

There are likely two primary reasons for USPS Priority Mail packages getting smaller.

The size decrease for Priority Mail poly bags allows them to be more compatible with sorting equipment. This helps automate the process for USPS so that they can sort faster and stay organized, especially during a time of year like now.

The other reason is to maximize the space available within their transportation vehicles. Every inch counts when it comes to logistics, and with holiday shipping volumes estimated to reach all time highs, getting the most out of every delivery will be vital to getting gifts delivered on time.

It is worthy to note that these package size differences will be put in place indefinitely, so the effects on your e-commerce business will last even after the holiday rush slows down.

New Sizes For USPS Priority Mail Boxes

The size differences are now being seen across two different envelopes and six different boxes. The sizes themselves range from as little to ⅛” to 2 1/2” as seen below.

Item/Service (SKU)Previous Outer DimensionsNew/Current outer DimensionsNet Change In SizePriority Mail Tyvek Envelope (EP14)15 and ⅛” Length times 11 and ⅝” Width15” Length times 11 and ⅝” Width⅛” in LengthPriority Mail APO/FPO Tyvek Envelope (MILIPAC)15 and ⅛” Length times 11 and ⅝” Width15” Length times 11 and ⅝” Width⅛” in LengthPriority Mail Large Flat Rate Box (LFRB)12-¼” Length X 12-¼” Width X 6” Height12-¼” Length X 12” Width X 6” Height¼” in WidthPriority Mail APO/FPO Large Flat Rate Box (MILIFRB)12-¼” Length X 12-¼” Width X 6” Height12-¼” Length X 12” Width X 6” Height¼” in WidthPriority Mail (0-1092 Box)13-11/6” Length X 12 ¼” Width X 2-⅞” Height13-11/6” length X 12” Width X 2-⅞” Height¼” in WidthPriority Mail (0-1095 Box)15-⅝” Length X 12-½ Width X 3-⅛” Height15” Length X 12” Width X Height 3-⅛”½” in Width

⅝” in Length

Priority Mail Regional Rate Box B (RRB-B2)16-¼” Length X 14-½” Width X 3” Height15” Length X 12” Width X 3” Height1-¼” Length

2-½” Width

Priority Mail (Box 7)12-¼” Length X 12-¼” Width X 8-½” Height12-¼” Length X 12” Width X 8-½” Height¼” in Width

How The New USPS Priority Mail Box Sizes Affect E-commerce Merchants

These size changes in all likelihood will not and have not affected most e-commerce merchants.

With sizes shrinking by just a fraction of an inch for most Priority Mail packages, your products should still fit in the same.

One difference may lie in how much you stuff these boxes in order to protect your products. With the smaller dimensions, you are able to use less stuffing per package, which could save you money if you are shipping a lot of your products through USPS Priority Mail.

How To Get USPS Priority Mail Boxes For Free

There are two ways to get USPS Priority Mail boxes for free for your business. The first way is to head into your local post office and purchase/pick up boxes from there. However, e-commerce merchants don’t always have the time to commute and wait in line to get these supplies.

By going to the USPS Store online, you can order boxes and envelopes, and have them delivered to your place of business for free. Some packages come in packs of 10, while others come in packs of 25. The maximum permitted quantity of packs you can order at one is 20.

Why Free USPS Priority Mail Boxes Are Still Effective

Using free USPS Priority Mail boxes is still an effective way to ship your products because, even though the size of the box has changed, the trusted USPS service levels and competitive rates are still in place.

By shipping with USPS, you’ll have the ability to ship out your products to customers at every residential address within the U.S. With Flat Rate boxes you’ll even be able to ship heavier products without having to worry about paying for any additional weight as long as it fits.

On top of that, Shippo customers will still be able to purchase discounted USPS shipping labels  for USPS  – up to 90% off retail rates – no matter which service they choose to use. Shippo customers can also compare USPS Priority Mail rates as well as rates from other top carriers.

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 2022 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. On top of this, Shippo merchants get access to special pricing on Priority Mail Flate Rate services during peak season. Sign up for a free Shippo account, to see live discounted rates and print USPS labels for Flat Rate Boxes.

2022 USPS Priority Mail Flat Rate
Shippo vs. Retail Rates
ProductsDimensionsShippo RateRetail RateSavings Using Shippo vs. Retail

% Savings with Shippo

Flat Rate Envelope12 ½” x 9 ½”$7.75$8.95$1.2013.41%Legal Flat Rate Envelope15” x 9 ½”$8.05$9.25$1.2012.97%Padded Flat Rate Envelope12 ½” x 9 ½”$8.45$9.65$1.2012.44%Small Flat Rate BoxOutside:

8 11/16” x 5 7/16” x 1 ¾”

Inside:
8 ⅝” x 5 ⅜” x 1 ⅝”

$8.25$9.45$1.2012.70%Medium Flat Rate Box – 1 (Top-Loading)Outside:

11 ¼” x 8 ¾” x 6”

Inside:
11” x 8 ½” x 5 ½”

$14.25$16.10$1.8511.49%Medium Flat Rate Box – 2 (Side-Loading)Outside:

14” x 12” x 3 ½”

Inside:
13 ⅝” x 11 ⅞” x 3 ⅜”

$14.25$16.10$1.8511.49%Large Flat Rate BoxesOutside:

12 ¼” x 12 ¼” x 6”

Inside:
12” x 12” x 5 ½”

$19.20$21.50$2.3010.70%

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 discussed 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 must 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″) – $7.40
Qualifications for Using Own PackagingCost to ShipSavingsWeighs 0.5-1 lb. and traveling less than 50 miles (Zone 0,1,2)$7.16$0.24
Legal Flat Rate Envelope (15″ x 9 1/2″) – $7.70
Qualifications for Using Own PackagingCost to ShipSavingsWeighs 0.5-1 lb. and traveling less than 600 miles (Zone 0-4)$7.16 – $7.67Up To $0.54
Padded Flat Rate Envelope (12 1/2″ x 9 1/2″) – $8.00
Qualifications for Using Own PackagingCost to ShipSavingsWeighs 0.5-1 lb. and traveling less than 600 miles (Zone 0-5)$7.16 – $7.88Up to $0.84Weighs 2 lbs. and traveling less than 50 miles (Zone 0,1,2)$7.79$0.21

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″) – $7.90
Qualifications for Using Own PackagingCost to ShipSavingsWeighs 0.5-20 lbs. and traveling less than 600 miles (Zone 0-4)$7.16 – $7.81Up to $0.74
Med. Flat Rate Box (11 1/4″ x 8 3/4″ x 6″) – $13.75
Qualifications for Using Own PackagingCost to ShipSavingsWeighs 0.5-2 lbs. and traveling anywhere except U.S. Territories (Zone 0-8)$7.16 – $11.69Up to $6.59Weighs 3-20 lbs. and traveling less than 1400 miles (Zone 0-6)$8.00 – $13.58Up to $5.75
Lg. Flat Rate Box (12 1/4″ x 12 1/4″ x 6″) – $19.30
Qualifications for Using Own PackagingCost to ShipSavingsWeighs 0.5-1 lb. and traveling to any U.S. State including Hawaii and Alaska$7.16 – $11.69Up to $12.14Weighs 2-4 lbs. and traveling anywhere except U.S. Territories (Zone 0-8)$7.79 – $18.96Up to $11.51Weighs 5-20 lbs. and traveling less than 1800 miles (Zone 0-7)$8.20 – $18.08Up to $11.10

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 mail stream, 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?

Setting up a free Shippo account takes just a few minutes, and then you can print the label from your office or warehouse. (Feel free to check out our USPS shipping calculator to get a clearer picture of how you can save on USPS shipping rates by using Shippo.) 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 use 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.

Is it cheaper to use a Flat Rate box or your own box USPS?

As we mentioned above, combining your own custom packaging with the right USPS service is often more cost-effective than using flat-rate boxes. One such option is taking advantage of cubic pricing. Priority Mail cubic pricing is a more efficient USPS service for shipping small packages that weigh up to 20 pounds.

What are the sizes and prices of priority mail boxes?

USPS Priority Mail Flat Rate Options.

What size is a large priority mail box?

The Priority Mail Large Flat Rate Box comes in one size: Large Flat Rate Box: 12 x 12 x 5.75 inches.

What boxes can I use for USPS Priority Mail?

If you use your own envelope or box for Priority Mail, identify it with the marking, "Priority Mail." Priority Mail packaging provided by the USPS must be used only for Priority Mail.