How Big Is a Priority Mail Box? USPS Flat Rate Box Sizes, Shipping Options & Money-Saving Tips

If you run an online store, you’ve probably wondered: how big is a Priority Mail box, and which one should I use to ship my orders? USPS Priority Mail Flat Rate is one of the most popular and cost-effective shipping services for sending packages across the United States — especially for growing eCommerce businesses trying to keep costs low and operations simple.

This guide breaks down everything you need to know about Priority Mail Flat Rate box sizes, USPS shipping options, and how to save money on shipping.

What Is Priority Mail?

Priority Mail is a fast and reliable USPS shipping service that delivers packages within 1 to 3 business days anywhere in the U.S. You don’t need to weigh your package (as long as it’s under 70 lbs), and USPS tracking is included at no extra charge.

With USPS Flat Rate shipping, the same price applies no matter how far the package travels, as long as it fits within the box and doesn’t exceed the maximum weight. You get predictable flat rate pricing, which is perfect for store owners who want to simplify logistics.

What Types of Boxes Does USPS Have?

The postal service offers three main types of flat rate packaging:

1. Flat Rate Boxes and Envelopes – Fixed-price boxes and envelopes in various sizes.
2. Regional Rate Boxes – Prices depend on the delivery zone and box type.
3. Forever Prepaid Boxes – Flat Rate boxes with prepaid postage that never expires.

Most USPS flat rate boxes and flat rate envelopes are free shipping supplies that can be ordered online or picked up from your local Post Office. You can also use your own box, but Priority Mail Flat Rate requires USPS-branded packaging.

Flat Rate Shipping Box Sizes

USPS offers a variety of Flat Rate boxes and envelopes, each designed for specific item types. The most common sizes include:

Flat rate envelopes are also available, starting at around $8.95, and work well for clothing, documents, or slim boxed items. Using Easyship’s shipping API, you can create a shipment with rules that will automatically allot a box size based on the weight of your shipment.

Medium Flat Rate Box Measurements

Medium Flat Rate Boxes come in two shapes:

• Top Loading (11 1/4 x 8 3/4 x 6)
• Side Loading (14 x 12 x 3 1/2)

Both versions handle up to 70 lbs. The side loading box is best for wide or flatter products, while the top loading version works well for bulkier items with height.

It’s also important to know when to avoid using medium and large flat rate boxes, since it might not always be ideal.

USPS Flat Rate Envelope and Box Dimensions

Here are the sizes for popular USPS envelopes:

• Flat Rate Envelope: 12 1/2 x 9 1/2
• Legal Flat Rate Envelope: 15 x 9 1/2
• Padded Flat Rate Envelope: 12 1/2 x 9 1/2

Padded envelopes are best for fragile or boxed items. Be sure the package weighs under 70 lbs and fits without altering the container flaps or changing the normal folds of the packaging. 

Prepaid Post Boxes (Forever Prepaid Flat Rate)

Forever Prepaid Flat Rate boxes come with postage included and never expire. You can mail a flat rate box anytime, even if USPS shipping rates increase.

These boxes aren’t free like standard Flat Rate packaging, but they’re a good investment if you want commercial pricing protection long-term. Keep in mind that they’re not ideal if your return address changes — each box is printed with your location.

USPS Regional Rate Box Options

Regional Rate Boxes offer affordable options for short-distance shipping. These boxes are priced by zone and come in the following types:

• Box A: Up to 15 lbs
• Box B: Up to 20 lbs
• Box C: Up to 25 lbs

You can also use your own packaging with this method, but it may cost more than using USPS-provided boxes.

How to Ship a USPS Priority Mail Box

Here’s how to send your package step-by-step:

1. Choose your box or envelope
   Make sure the item fits properly and the box or envelope closes without bulging.
2. Pack your item securely
   Avoid shipping hazardous materials or mailing restricted items like smokeless tobacco.
3. Print a shipping label
   Use USPS.com or a provider like Easyship.
4. Schedule a pickup
   The postal service offers free pickups or you can drop off your item at your local Post Office.
5. Track your shipment
   Every Flat Rate package includes USPS tracking, so you can easily track delivery online.

How to Save on Shipping

Here’s how to get the cheapest option when using Priority Mail:

• Use Easyship or similar tools to get discounted rates with Commercial Plus Pricing.
• Choose the smallest Flat Rate box or envelope that fits your order.
• Compare USPS shipping rates with other shipping services using a rate calculator.
• Print labels online and schedule pickups to save time and gas money.

These steps can help you reduce overhead, increase profit per shipment, and keep your customers happy with fast, trackable deliveries.

Where to Get USPS Shipping Boxes

You can get Flat Rate boxes and envelopes in several ways:

• Order online from USPS – free delivery to your business or home.
• Visit your local Post Office – some packaging may be limited in supply.
• Use USPS shipping software tools – some services include supplies or packaging kits.

Remember: if you’re using Priority Mail Flat Rate, you must use USPS-branded boxes or envelopes to qualify for flat rate pricing.

Final Takeaway

So, how big is a Priority Mail box? It depends on the size you choose — but more importantly, it depends on what you’re shipping and how you want to ship it. USPS gives you plenty of free, reliable options. Whether you're sending out 10 orders a week or 2,000, using the right Priority Mail box — and the right shipping software — can save you time, money, and stress.

Want to see how much you can save? Try Easyship’s Free Shipping Rate Calculator and compare rates from USPS and other major carriers.

Frequently Asked Questions (FAQs)

How big is a USPS Priority Mail box?

USPS Priority Mail Flat Rate boxes range from 8 11/16" x 5 7/16" x 1 3/4" (Small) to 12 1/4" x 12 1/4" x 6" (Large). Each holds up to 70 lbs.

Can I use my own box for USPS Priority Mail?

Yes, but only if you're using regular Priority Mail. If you're using Flat Rate, you must use USPS-branded Flat Rate boxes or envelopes.

Where can I get USPS Flat Rate boxes?

Order free Flat Rate boxes online at USPS.com or pick them up at your local Post Office. Delivery is free for online orders.

How much does it cost to ship with Flat Rate?

Prices start at $8.95 for envelopes and go up to $21.50 for large boxes. Discounts are available through platforms like Easyship.

Is tracking included with USPS Flat Rate?

Yes. Every USPS Priority Mail shipment includes free USPS Tracking, so you can easily monitor delivery status from origin to destination.