Large Flat Rate USPS Box: Dimensions, Weight, Cost (2025)

Shipping can sometimes feel complicated, with all the weigh ins, measurements, and zone calculations. The United States Postal Service created its Flat Rate program to simplify things, and the large flat rate USPS box is a popular choice for sending bigger or heavier items. With its straightforward pricing and generous size, it’s a go to option for everyone from small business owners to families sending care packages.

This guide breaks down everything you need to know about the large flat rate USPS box, from its dimensions and weight limits to when it’s the absolute cheapest way to ship.

What Are the Dimensions of a Large Flat Rate Box?

Understanding the size of the box is the first step. You have to make sure your items will actually fit inside before you start packing.

Exterior and Interior Dimensions

The standard large flat rate USPS box has exterior dimensions that are easy to remember.

Outside Dimensions: 12¼ inches x 12¼ inches x 6 inches

This gives the box a roughly square shape that’s about half a foot deep. Because of the cardboard thickness, the inside space is slightly smaller.

Inside Dimensions: 11⅞ inches x 11⅞ inches x 5½ inches

That small difference matters. An object that is exactly 12 inches wide might not lie perfectly flat inside. It’s always smart to leave a little wiggle room for your items and any packing materials you plan to use; if you’re considering other sizes, see our USPS Flat Rate Boxes guide.

Weight Limits and Pricing

The magic of flat rate shipping lies in its simple pricing structure. As long as your item fits, the price stays the same up to a very generous weight limit.

How Much Can It Weigh?

For shipments inside the United States, the rule is simple.

Domestic Weight Limit: 70 pounds

This is the maximum weight for any domestic Priority Mail package, and it’s a huge advantage of the large flat rate USPS box. Whether your package weighs five pounds or 65 pounds, the price doesn’t change. This makes it incredibly economical for shipping dense, heavy goods like books, tools, or auto parts.

For international shipments, the rules are different.

International Weight Limit: 20 pounds

If you’re sending a package overseas using a large flat rate USPS box, you must keep the weight at or below 20 pounds.

How Much Does a Large Flat Rate Box Cost?

The cost to ship a large flat rate USPS box is a single, fixed price for delivery anywhere in the U.S. As of late 2023, the retail price you would pay at the Post Office was about $22.80. Keep in mind that USPS adjusts its rates, so it’s always a good idea to check the current price.

You can often find slightly lower prices by purchasing your postage online through a commercial provider instead of at the counter—see our shipping discounts guide. For military addresses, there’s even a built in discount, which we’ll cover later. International shipping costs are much higher and vary by destination country; compare options in our cheapest international shipping guide.

How Does Shipping with a Large Flat Rate Box Work?

Using the large flat rate USPS box is a straightforward process that makes shipping easy. You are using the USPS Priority Mail service, which comes with several key benefits.

Speed: Delivery is typically within one to three business days anywhere in the U.S. (see our guide on how fast is Priority Mail).
Tracking: Free USPS tracking is included with every shipment, so you and your recipient can follow the package’s journey.
Insurance: Your package automatically comes with $50 of insurance coverage for domestic shipments.

One of the biggest advantages is that you don’t have to worry about shipping zones. Normally, sending a package across the country costs more than sending one to a neighboring state. With flat rate, the price is the same whether you’re shipping locally or from coast to coast. There are also no surprise fuel surcharges or residential delivery fees.

When Should You Use a Large Flat Rate Box?

While flat rate shipping is convenient, it isn’t always the cheapest option. The key is knowing when it provides the best value.

The Golden Rules: When Flat Rate Wins

Consider using a large flat rate USPS box in these situations:

For Heavy Items: If your package weighs around 18 pounds or more, the large flat rate box is almost always the cheaper choice compared to standard Priority Mail. For a 20 lb shipment, see our 20 lbs: UPS or USPS? comparison.
For Long Distances: For packages traveling to farther zones (like from New York to California), flat rate can be more economical even for moderately heavy items weighing five to ten pounds.
For Predictable Pricing: If you’re a small business owner who wants to offer a consistent shipping price to all customers, flat rate removes the guesswork. You also get the boxes for free from USPS, saving on supply costs.
For Military or Remote Addresses: The flat rate price applies to all U.S. states and territories, including Alaska, Hawaii, and military APO/FPO addresses, often providing significant savings over other services.

Large Flat Rate Box vs. Priority Mail: A Cost Comparison

So, when is it better to use your own box and pay by weight? For lighter packages traveling short distances, standard Priority Mail is usually cheaper. For example, a seven pound package going to a mid range zone would cost significantly less if shipped by weight instead of in a large flat rate box.

The breakeven point depends entirely on the package’s weight and destination. Instead of guessing, you can get a clear answer in seconds. Using an online shipping calculator lets you compare rates for the large flat rate USPS box against standard Priority Mail and other carriers to see which option saves you the most money. For a deeper dive, read our guide on which is cheaper: UPS or USPS.

Special Versions of the Large Flat Rate Box

Over the years, USPS has offered a couple of variations on the large flat rate box.

The Discontinued Large Flat Rate Board Game Box

There used to be a special large flat rate USPS box designed specifically for board games. Its dimensions were approximately 24 inches x 12 inches x 3 inches, making it perfect for long, flat items. However, USPS discontinued the Large Flat Rate Board Game Box in early 2022. The box’s unusual size required manual sorting, and with low customer usage, it was retired. You can no longer order this specific box from USPS.

The APO/FPO/DPO Military Mailbox

USPS offers a special large flat rate USPS box for sending packages to military and diplomatic personnel stationed overseas. This box is the exact same size as the standard version and has the same 70 pound weight limit.

The key difference is the price. Shipments to APO, FPO, and DPO addresses receive an automatic discount of around $1.50 to $2.00 off the standard retail rate. While USPS does produce boxes specifically branded for military mail, you don’t need to use one. Any large flat rate USPS box sent to a valid military or diplomatic address will automatically get the discounted price.

Getting and Packing Your Box Like a Pro

A successful shipment starts with getting the right supplies and packing your items securely.

Where to Get a Large Flat Rate Box for Free

You never have to pay for the boxes themselves. USPS provides them for free as part of the Priority Mail service.

At Your Local Post Office: Most post offices keep a stock of flat rate boxes in the lobby for you to take.
Online at USPS.com: You can order packs of 10 or 25 boxes directly from the USPS website. They will ship them to your home or business for free, though delivery usually takes 7 to 10 days.

Once your box is ready, you can schedule a free USPS pickup so you don’t have to visit the Post Office.

Pro Packing Tips for a Secure Shipment

Packing a large flat rate USPS box correctly is crucial, especially when shipping heavy items.

Don’t Overstuff: Make sure the box can close easily without bulging. If the box is deformed, USPS may charge you a different rate.
Fill Empty Space: Use packing materials like bubble wrap or crumpled paper to prevent items from shifting around during transit. A well packed box should feel solid.
Reinforce the Seams: Use strong packing tape to seal the top and bottom of the box. The “H tape” method (taping the center seam and all edge seams) provides excellent strength.
Label Heavy Boxes: While not required by USPS, it’s a good practice to write “HEAVY” on boxes over 50 pounds to give postal workers a heads up.

Need help formatting or printing the label? Check out our shipping label guide.

By following these tips, you can ensure your package arrives safely and securely.

Frequently Asked Questions (FAQ)

What are the exact dimensions of a large flat rate USPS box?

The exterior dimensions are 12¼″ x 12¼″ x 6″, while the interior dimensions are slightly smaller at 11⅞″ x 11⅞″ x 5½″.

How much does it cost to ship a large flat rate box?

The price is fixed for domestic shipments up to 70 pounds. While rates change, the retail price in 2023 was around $22.80. You can often get a lower price by paying for postage online.

Is it cheaper to use a large flat rate box or my own box?

It depends. For heavy items or items traveling long distances, the large flat rate USPS box is often cheaper. For lightweight items going a short distance, using your own box and paying for weight based Priority Mail is usually a better deal.

What’s the heaviest I can ship in a large flat rate box?

For domestic shipments within the United States, the weight limit is 70 pounds. For international shipments, the limit is 20 pounds.

Can I still use the large flat rate board game box?

No, USPS discontinued the board game box in early 2022. You can no longer order it, though if you have an old one, USPS should still honor the flat rate postage.

Do I get a discount for shipping to military addresses?

Yes. Any large flat rate USPS box sent to an APO, FPO, or DPO address automatically qualifies for a discounted postage rate, typically saving you around $1.50 to $2.00.