Medium Box USPS Flat Rate Guide: Dimensions, Price (2025)

Shipping can sometimes feel like a puzzle, especially when it comes to balancing cost, speed, and size. The medium box USPS flat rate is a popular solution that simplifies the process, offering a straightforward way to mail items across the country. If you’ve ever wondered how it works, what it costs, or if it’s the right choice for your package, this guide has you covered.

We’ll break down everything you need to know about this reliable shipping option, from its dimensions and weight limits to how it stacks up against other services. For a quick comparison of all options, see our guide to USPS Flat Rate Boxes.

What Are the Two Types of USPS Medium Flat Rate Boxes?

USPS offers two different shapes for the medium flat rate box, giving you flexibility depending on what you’re shipping. The best part? They both cost the exact same price to mail. The choice is all about which one fits your items better.

Top Loading Medium Box Size

The top loading box is a classic cube like shape that’s perfect for bulkier items. It opens from the top, making it easy to pack things like shoes, small electronics, or care packages.

Interior Dimensions: 11 inches x 8 1/2 inches x 5 1/2 inches
Exterior Dimensions: 11 1/4 inches x 8 3/4 inches x 6 inches

This style is ideal when your items can be stacked or need more height than width.

Side Loading Medium Box Size

The side loading box, sometimes called the “board game box,” is flatter and wider. It opens along its longest side, which is perfect for sliding in items that need a larger footprint but not much depth.

Interior Dimensions: 13 5/8 inches x 11 7/8 inches x 3 3/8 inches
Exterior Dimensions: 14 1/8 inches x 12 inches x 3 1/2 inches

Think laptops, binders, folded apparel, or photo albums. This shape is excellent for keeping flat items secure during transit.

How Much Does a Medium Flat Rate Box Cost?

The price of a medium box USPS flat rate depends on where and how you buy the postage. This is a crucial detail that can save you a significant amount of money.

Retail Pricing: If you walk into a Post Office and buy postage at the counter, you’ll pay the standard retail rate.
Commercial Pricing: If you use an online postage service, you get access to discounted Commercial Rates. These rates are almost always cheaper than what you’d pay at the retail counter.

For small businesses and even casual shippers, the savings from Commercial Pricing add up quickly. Using a tool to compare shipping rates can instantly show you the difference and help you find the cheapest way to print a label. To see the most current prices and compare your options, check out a free online shipping calculator.

Key Rules for Using a Medium Flat Rate Box

To make sure your package gets where it’s going without any hiccups, there are a few simple but important rules to follow.

Weight Limit

The weight limit is one of the biggest advantages of the medium box USPS flat rate.

Domestic Shipments: You can ship anything up to 70 pounds. As long as your items fit safely inside and the box closes properly, the price remains the same whether it weighs 1 pound or 70 pounds.
International Shipments: The weight limit for international shipments using the same box is lower, typically 20 pounds. If you’re shipping abroad, here’s a quick guide to shipping internationally.

Packaging Requirements

USPS is specific about how you use their branded boxes.

Use the Official Box: You must use the official, USPS branded Medium Flat Rate Box. You cannot use your own box and get the flat rate price.
Don’t Alter the Box: The box must be assembled as intended. You cannot cut it, reshape it, or turn it inside out. The container’s original shape must be preserved.
The Flaps Must Close: The saying “if it fits, it ships” comes with a condition: the box’s flaps must be able to close securely within their normal folds. Bulging boxes that have to be forced shut may be rejected or charged a higher rate.

Label Requirements

The label you use must match the box. You are required to use a Priority Mail Medium Flat Rate Box label for a medium box USPS flat rate shipment.

Putting a different label on the box, like one for weight based Priority Mail or USPS Ground Advantage, will cause a problem. USPS uses an Automated Package Verification (APV) system that scans packages and will likely catch the mismatch, automatically charging you for the correct flat rate postage. To avoid surprises, always select the right service when creating your shipping label.

When Is a Medium Flat Rate Box the Best Choice?

A medium box USPS flat rate is incredibly versatile, making it a great option for many common shipping needs.

Common Use Cases

eCommerce Sellers: Shipping clothing, books, small electronics, or handmade goods.
Care Packages: Sending snacks, books, and home goods to students or family.
Heavy Items: Mailing dense objects like tools, auto parts, or lots of books can be very cost effective. For a heavier‑weight comparison, see is UPS or USPS cheaper for 50 lb?

Flat Rate vs. Weight Based or Cubic Pricing

Is the medium box USPS flat rate always the cheapest? Not necessarily. Here’s a simple breakdown:

Flat Rate is often cheaper for: Heavy items (generally over 2-3 pounds) traveling a long distance (across multiple shipping zones).
Weight Based or Cubic Pricing is often cheaper for: Lighter items traveling a short distance.

For light packages, services like USPS Ground Advantage or weight based Priority Mail can offer better prices. The only way to know for sure is to compare. An online shipping calculator that shows you rates from multiple services side by side is the best tool for making the right decision. For a step‑by‑step walkthrough, see how to calculate shipping costs.

Included Perks: Tracking and Insurance

Every Priority Mail service, including the medium box USPS flat rate, comes with valuable extras at no additional cost.

USPS Tracking: You can track your package from drop off to delivery.
Insurance: Shipments are insured against loss or damage, typically up to $100.

These included features provide peace of mind and are a great value compared to services where they cost extra.

How to Order Medium Flat Rate Boxes

Getting your hands on these boxes is easy and free. You can visit your local Post Office and pick them up in person, or you can order them in packs directly from the USPS website at no cost. They will deliver the empty boxes right to your door. When you’re ready to ship, you can schedule a free USPS pickup from home.

Frequently Asked Questions

What are the dimensions of a medium box USPS flat rate?

There are two sizes. The top loading box is about 11 x 8.5 x 5.5 inches, and the side loading box is about 13.6 x 11.8 x 3.3 inches.

What is the weight limit for a medium flat rate box?

For domestic shipping within the United States, the weight limit is a generous 70 pounds. For international shipping, the limit is 20 pounds.

Can I use my own box for flat rate shipping?

No. To get the flat rate price, you must use the official USPS branded Flat Rate packaging.

How fast is medium flat rate shipping?

As a Priority Mail service, delivery is typically within 1 to 3 business days for domestic shipments. However, this is an estimate and not a guarantee. For details on factors that affect delivery times, see our Priority Mail speed FAQ.

Is the medium box USPS flat rate always the cheapest option?

Not always. It’s most cost effective for heavy items traveling long distances. For lightweight or local shipments, weight based services like USPS Ground Advantage may be cheaper. It’s always a good idea to compare shipping rates before you buy a label.

Which medium box is bigger, top loading or side loading?

They have a very similar total volume but different shapes. The side loader is wider and flatter, while the top loader is more cube shaped and taller. Choose the one that best fits the shape of your items.