USPS Large Flat Rate Box Size

Looking to ship a USPS large flat rate box? What is the typical size? Here is everything you need to know. Read more below.

It’s almost impossible to overstate the popularity of the USPS Flat Rate Shipping option. From the moment that the USPS offered Flat Rate Shipping it was a huge success, especially when they rolled out the USPS large flat rate box size. This service provided USPS customers with an opportunity to ship pretty much anything and everything (so long as it fit inside of the USPS large flat rate box) at deeply discounted prices compared to traditional rates. That was, and continues to be, a game changer!


Below we highlight (almost) everything you need to know about taking advantage of the USPS large flat rate box size, too. We go over things like:


  • Just how big the USPS large flat rate box size is
  • The benefits of using the USPS Flat Rate Shipping option
  • How much you can expect to spend on USPS large flat rate boxes


… And that’s just the tip of the iceberg!


Ready to get right into it?


priority mail flat rate box


Just How Big is the USPS Large Flat Rate Box Size?


There are a couple of different flat rate box sizes available from the USPS, with the “large” flat rate box size being the biggest of the bunch (as of 2022, anyway). The Flat Rate Envelope is the smallest, measuring in at 12.5” x 9.5”. This envelope is really designed only for sending small documents, pamphlets, and other things that can slide inside of this envelope.


The Legal Flat Rate Envelope is slightly larger, measuring in at 9.5” x 15”. It’s intended (no surprise here) for use with legal documents that need to be shipped quickly yet cost-effectively.


Padded Flat Rate Envelopes are even larger still, measuring 12.5” x 9.5”. These envelopes have a bit of extra space inside of them and work great for books.


Then you start getting into the box sizes.


USPS Small Flat Rate Boxes measure 8.625 by 5.375 x 1.625”. These are the cheapest of the flat rate boxes and are better suited to smaller electronics, jewelry, and things of that nature.


Jump up to the Medium Flat Rate boxes (top loading and sideloading) and you’re looking at either 11 1⁄4 x 8 3⁄4 x 6″for the top loader or 14” x 12” x 3.5” for the side loader.


Finally, you have the actual USPS Large Flat Rate Box size. The biggest of all the flat rate shipping boxes, you’ll have plenty of room to fill this large box as it measures in at 12.25” x 12.25” x 6”. You can fit entire laptops inside of this box and still have space left over!


knowledge of flat rate box


(Almost) Everything You Need to Know About the USPS Flat Rate Box Program


As we highlighted a moment ago, the USPS Flat Rate shipping program at the post office became a runaway hit from the moment that it was first unveiled. It continues to be one of the most often utilized delivery solutions, especially for e-commerce operations and small businesses that don’t want to spend a small fortune on shipping.


Benefits of USPS Flat Rate Shipping


Below we run through some of the biggest benefits of this program, touching on a few of the reasons that people love to use the USPS Flat Rate service.




To kick things off, a big piece of the puzzle behind the popularity of the USPS Flat Rate service is how economical it is. Probably the cheapest way to send heavy items through the mail (with the USPS or anybody else, for that matter), flat rate shipping eliminates the need to weigh items, to worry about dimensionality, or to measure delivery distance. It’s possible to save a lot of money on shipping, particularly high-volume shipping of heavier items.




Another reason that USPS Flat Rate shipping is so popular is because it is so easy! The whole process is simplified dramatically compared to “traditional” shipping options. There’s no more need for trying to track down the right box. There’s no more measuring box size or slipping it on a scale before calculating postage. You don’t even have to worry about how far this package is traveling to figure out how much it’s going to cost, either. All you have to do is get your hands on a USPS large flat rate shipping box, fill it up, seal it up, address it correctly, pay the flat rate shipping fee and you are good to go. It really doesn’t get much easier than that!




While the USPS Flat Rate shipping service isn’t considered expedited by any stretch of the imagination, delivery times are still really short. As a general rule of thumb USPS Flat Rate customers can expect their packages to arrive at their destination between one and three days after they’ve been dropped off at the post office.


Some packages get there even sooner and some packages take a little bit longer. Flat rate boxes to the West Coast from the East Coast, for example, are generally going to take a longer time to get where they are going. As soon as you start talking about flat rate boxes to Alaska and Hawaii you’re talking about extended delivery times, too. Even still, you won’t have to worry about your flat rate packages taking weeks to get to their ultimate destination. They often move just as quickly as First Class mail handled by the USPS will.


Free Shipping Boxes!


Let’s not overlook the benefit of using 100% free shipping materials to get your packages in the mail to begin with! Flat rate boxes are provided by the USPS at no charge whatsoever. That’s a huge boost for everybody but is particularly advantageous for business customers. Any money spent on shipping materials is going to inevitably cut into profits. As soon as you start talking about volume mailing those numbers can add up in a hurry, too. With the free shipping materials from the USPS, though, this isn’t ever going to be a problem. The boxes are sturdy, solid, and easy to put together. They also guarantee that your USPS flat rate box size is always within the parameters.


No Hidden or Surprise Charges


The last big reason that a lot of people love USPS large flat rate shipping box sizes – and flat rate shipping in general – is because there are no hidden or surprise charges with this service. The price you see from the USPS is the price you are going to pay. There are no tricks, no surprises, and no sudden changes. You’ll never have wonder about what it’s going to cause you to send something in large flat rate boxes with the USPS!


block that says price


How Much Do USPS Large Flat Rate Boxes Cost?


The USPS gets to determine exactly how much large flat rate box size deliveries are going to cost, and as of 2022 this number has been pegged at $21.50. Every year it feels like the USPS changes pricing a little bit (always going out, course). Still, you’d be hard-pressed to find anybody else that offers the ability to send a large sized package with up to 70 pounds of stuff in it clear across the country at under $22 right now! Combine that with the speed and reliability of the USPS and it’s not hard to see why these kinds of flat rate boxes are as popular as they are right now.


usps truck driving down the street


How Do I Ship a USPS Large Flat Rate Box?


The actual process for shipping a USPS large flat rate box size is simple and straightforward, too. Let’s break it down step-by-step for anybody that hasn’t gone through this process before, though.


Get the Box and Fill It


The first piece of the puzzle is obviously getting your hands on a large flat rate box to begin with. There are two real approaches to this:


The first (and probably the easiest) is to simply pop on down to your local post office and ask for a USPS large flat rate box size. They’ll be able to hand you one (or 100, depending on how many you need) completely free of charge and you’ll be good to go.


The second method is to jump online, to navigate to the United States Postal Service website ( and then visit the US Postal Service store. You’ll be able to navigate to the SUPPLIES section pretty quickly, and then you can check out the Flat Rate Box selection. Pick how many boxes you want (and the sizes that you are interested in) and then go through the ordering process.


These boxes are 100% free of charge and will be dropped off at your doorstep just like any other piece of mail you have handled by the USPS, too. It’ll take a little bit longer to get your USPS large flat rate box size options with the online approach. But it’s nice to know that the option is there all the same!


Pack and Seal the Box


The next thing you are going to want to do is carefully and strategically fill that large sized box, making sure that you don’t go over the 70 pound weight limit. As long as the things you are trying to send through the mail fit inside this box and come in under 70 pounds you aren’t going to have any trouble whatsoever.


Pack things up nicely with newspaper, foam, bubblewrap, or Styrofoam peanuts. You need to make sure that these things are well protected, as that’s going to be your responsibility completely. After you feel like everything is good to go, though, simply tape things up with quality packing tape. Now you’re one step closer to getting this box in the mail and on its way to its ultimate destination!


Print the Label


If you want to print the label at home now is the time to do so. This can be done through the USPS website, though they also have partner platforms that allow you to take advantage of USPS flat rate services as well. At the end of the day, though, the idea is to print out the label on your own and attach it to your package. You can skip this step, though, and jump right to the next one if you’d like.


usps driving on the road


Drop Everything Off at the Post Office (Or Arrange for Pickup)


The post office can handle all of the heavy lifting as far as printing out shipping labels, getting the package into the USPS infrastructure, and then moving it along its way if you want them to. All you have to do is drop off the correctly addressed, properly taped up, and less than 70 pound flat rate box. They’ll double confirm the delivery address with you, charge you the current going rate for USPS large flat rate box size postage ($21.90 as of 2021), and then you are good to go! It doesn’t get a whole lot easier than that.


What Do I Do If My Items Won’t Fit in a USPS Large Flat Rate Box?


If you are having a tough time getting all of your items to fit into a single USPS large flat rate box size container you might want to break your delivery up into multiple boxes. For example, using one USPS large flat rate box and one small flat rate box might be the easiest way to handle “overflow” without adding a lot of extra expense. Maybe you can fit everything into two different USPS medium-sized flat rate boxes, too.


At the end of the day, there are plenty of options available to get all of your stuff to its ultimate destination without spending a small fortune on shipping. That’s the beauty of the way that the USPS has set up their Flat Rate Shipping program. If you have any questions about flat rate shipping, or want to get your hands on flat rate shipping supplies (like large boxes, for example), visit your local post office today or jump on the United States Postal Service website at


A Virtual Mailbox Alternative


If you are still getting your mail the old way, there’s a better way! More & more people have started using virtual mailbox alternatives, which allow you to view & manage all your mail online from your computer or smart phone.


Once you sign up, you would be given access to a virtual mailbox, where all your envelopes & packages would be uploaded to with a scanned image of them. From there, you can select which envelopes you would like for us to scan, and we can have the scanned document in your account within a few hours.


If you are located overseas or travel a lot, international shipments come with up to 80% off retail shipping rates (cheaper to ship with us, instead of going through the USPS directly.) To learn more, and to sign up, click here.

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