USPS Flat Rate Boxes: Complete Size & Price Guide 2026

Introduction

USPS Flat Rate boxes are one of the most popular shipping options in the United States, and for good reason. The concept is simple: if it fits, it ships for one flat price regardless of weight or destination zone. Whether you are sending a package from New York to California or from Maine to Hawaii, the price stays the same. This makes Flat Rate shipping predictable, affordable, and easy to calculate.

In 2026, USPS continues to offer a comprehensive lineup of Flat Rate packaging options through Priority Mail and Priority Mail Express services. This guide covers every Flat Rate box and envelope available, their exact dimensions, current pricing, weight limits, and strategies to get the most value from flat rate shipping.

How USPS Flat Rate Shipping Works

Flat Rate shipping eliminates the two biggest variables in shipping cost calculations: weight and distance. With standard Priority Mail, your cost depends on the package weight and the distance between origin and destination (measured in postal zones 1 through 9). Flat Rate removes both variables entirely.

Key Principles

• Fixed price regardless of weight: A 1-pound package costs the same as a 70-pound package in the same box
• Fixed price regardless of distance: Shipping across town costs the same as shipping across the country
• Free packaging: USPS provides all Flat Rate boxes and envelopes at no cost
• Priority Mail service included: All Flat Rate shipments include Priority Mail speed (1-3 business days)
• Insurance included: Up to $100 of insurance is included at no extra charge
• Tracking included: Full USPS Tracking is provided with every shipment

When Flat Rate Makes Sense

Flat Rate is most cost-effective when:

• Your package is heavy relative to its size
• You are shipping to distant zones (zones 5-9)
• The items fit snugly into a Flat Rate container
• You want predictable shipping costs for pricing your products

Flat Rate is less cost-effective when:

• Your package is lightweight (under 2-3 pounds)
• You are shipping to nearby zones (zones 1-2)
• Your items are bulky but light
• The package dimensions exceed Flat Rate box sizes

Complete Flat Rate Box Lineup 2026

Priority Mail Flat Rate Envelopes

Priority Mail Flat Rate Boxes

Priority Mail Express Flat Rate

Detailed Box Dimensions and Best Uses

Small Flat Rate Box (8.69" x 5.44" x 1.75")

The Small Flat Rate Box is the most compact option and is ideal for small, dense items. At just 1.75 inches tall, it works best for flat items that have significant weight.

Best for:

• Jewelry and watches
• Small electronics (phone cases, adapters, cables)
• Coins and collectibles
• Small auto parts
• Gift cards and small books

Volume: Approximately 83 cubic inches

Pro tip: This box is perfect for items that weigh 3 pounds or more. At that weight, you are almost certainly saving money compared to weight-based Priority Mail for zones 4 and above.

Medium Flat Rate Box — Top-Loading (11.25" x 8.75" x 6")

The top-loading medium box is the most versatile Flat Rate container. With a 6-inch depth, it can accommodate a wide range of products and is the go-to choice for most e-commerce sellers.

Best for:

• Clothing and apparel (folded)
• Books and media
• Coffee and food items
• Beauty and skincare products
• Small kitchen appliances

Volume: Approximately 591 cubic inches

Pro tip: This is the sweet spot for most shipments. Items weighing 5 pounds or more shipping to zone 5+ will almost always be cheaper in this box than with standard Priority Mail.

Medium Flat Rate Box — Side-Loading (14" x 12" x 3.5")

The side-loading variant offers a wider, flatter profile. While it has the same price as the top-loading medium box, its dimensions make it better suited for different types of items.

Best for:

• Framed photos and artwork (small)
• Tablets and laptop accessories
• Board games and puzzles
• Clothing laid flat
• Documents and large envelopes

Volume: Approximately 588 cubic inches

Large Flat Rate Box (12.25" x 12.25" x 6")

The Large Flat Rate Box provides the most volume for your money and is the best choice for heavy, bulky items.

Best for:

• Multiple clothing items
• Shoes and boots
• Large electronics
• Tools and hardware
• Pantry items and food gifts

Volume: Approximately 901 cubic inches

Pro tip: If your item weighs 8 pounds or more and ships to zone 4+, the Large Flat Rate Box is almost always the cheapest Priority Mail option.

Large Flat Rate Board Game Box (24.06" x 11.63" x 3.13")

This specialty box was introduced for board games and similar flat, wide items. Despite its name, it is useful for many product types beyond board games.

Best for:

• Board games and card games
• Wall calendars and posters (rolled)
• Flat automotive parts
• Sewing patterns and fabric cuts
• Long, thin items

Volume: Approximately 875 cubic inches

How to Get Free Flat Rate Boxes

USPS provides all Flat Rate packaging completely free of charge. Here are the ways to obtain them:

Order Online at usps.com/shop

Visit the USPS Postal Store

Navigate to "Shipping Supplies"

Filter by "Priority Mail" or "Priority Mail Express"

Select the Flat Rate containers you need

Place your order — shipping is free

Quantities: You can order boxes in quantities of 10 or 25, depending on the container type. There are no limits on how many you can order.

Pick Up at the Post Office

Walk into any USPS Post Office and pick up Flat Rate boxes from the self-service area. Most locations stock all standard sizes, though specialty boxes like the Board Game Box may need to be ordered online.

Carrier Pickup

If you use USPS Carrier Pickup, your mail carrier can bring Flat Rate supplies when they come to pick up your outgoing packages.

Retail vs. Commercial Pricing

One of the biggest ways to save on Flat Rate shipping is to access Commercial Pricing rather than paying retail rates at the Post Office counter.

How to Get Commercial Pricing

Savings Comparison

Flat Rate vs. Standard Priority Mail: Cost Comparison

Understanding when Flat Rate beats standard Priority Mail (weight-based) is crucial for optimizing your shipping spend.

Break-Even Weight by Zone (Medium Flat Rate Box at Commercial Pricing)

Key insight: The farther your package travels, the lower the break-even weight. For cross-country shipments (zones 7-9), Flat Rate is cheaper for almost any package over 2 pounds.

Flat Rate Shipping Rules and Restrictions

What You Can Ship

• General merchandise and retail products
• Food items (non-perishable or properly packaged perishable)
• Clothing and textiles
• Books, media, and documents
• Electronics and gadgets
• Cosmetics and toiletries (non-hazardous)

What You Cannot Ship

• Hazardous materials (HAZMAT) unless specifically permitted
• Live animals (except bees, scorpions, and certain insects under specific conditions)
• Explosives and firearms (with limited exceptions)
• Perishable items without proper packaging
• Items exceeding 70 pounds

Packaging Rules

• Items must fit inside the Flat Rate container with the lid or flap fully closed
• You cannot alter the box dimensions, add extensions, or tape extra material to increase volume
• The box must close naturally without bulging or tearing
• Flat Rate envelopes can be sealed with the clasp, tape, or both
• Padding and cushioning material is allowed and recommended inside the box

Tips for Maximizing Flat Rate Value

1. Use Commercial Pricing

Never pay retail rates if you can avoid it. Sign up for a shipping platform like atoship to access Commercial Plus pricing on every Flat Rate shipment. The savings of 17-25% add up quickly.

2. Match the Right Box to Your Product

Do not default to the Large Flat Rate Box for everything. If your item fits in a Small or Medium box, the savings are significant. Conversely, do not try to squeeze items into a smaller box if they fit better in the next size up — damaged items cost more than the price difference.

3. Combine Orders When Possible

If you have multiple small orders going to the same address, combining them into a single Flat Rate box can save significantly on per-unit shipping costs.

4. Consider Weight Distribution

Since weight does not affect Flat Rate pricing, use these boxes for your heaviest items. Ship lightweight items using standard Priority Mail or Ground Advantage where weight-based pricing works in your favor.

5. Stock Up on Supplies

Order Flat Rate boxes in bulk from USPS.com so you always have them on hand. Running out and having to use your own packaging with weight-based rates can erode your margins.

6. Use the Padded Envelope for Small Heavy Items

The Padded Flat Rate Envelope at $8.70 (Commercial) is one of the best deals in shipping. Small, heavy items like phone cases, jewelry, small tools, and cosmetics can ship for less than the Small Flat Rate Box if they fit in the envelope.

Flat Rate for Business Shippers

Volume Discounts and Negotiated Rates

High-volume shippers (typically 50,000+ packages per year) may qualify for Negotiated Service Agreements (NSAs) with USPS that provide additional discounts beyond Commercial Plus pricing. Contact your local USPS Business Mail Entry Unit for details.

Integrating Flat Rate into Your Shipping Strategy

Smart e-commerce businesses use a hybrid approach:

Flat Rate for heavy items shipping to distant zones

Standard Priority Mail for lightweight items or short-distance shipments

Ground Advantage for non-urgent, lightweight packages

Regional Rate for items that fall between Flat Rate and standard Priority Mail

Accounting and Tax Considerations

Flat Rate pricing simplifies shipping cost accounting because each shipment has a fixed, predictable cost. This makes it easier to:

• Calculate accurate shipping charges for customers
• Forecast shipping expenses
• Maintain consistent margins across different destinations
• Simplify bookkeeping and reconciliation

Frequently Asked Questions

Can I use my own box instead of the USPS Flat Rate box?

No. Flat Rate pricing is only available when you use official USPS Flat Rate containers. If you use your own box, you will be charged standard Priority Mail rates based on weight and distance.

Can I reuse a Flat Rate box?

You can reuse a Flat Rate box only if you turn it inside out so the Flat Rate branding is not visible. If the Flat Rate markings are showing, you will be charged the Flat Rate price even if you intended to ship at standard rates.

Is there a weight limit for Flat Rate boxes?

Yes, the maximum weight for any Flat Rate container is 70 pounds. In practice, most items shipped in Flat Rate boxes weigh far less, but the 70-pound limit is important for dense items like metals, tools, or books.

Do Flat Rate boxes include insurance?

Yes, all Priority Mail Flat Rate shipments include up to $100 of insurance at no additional cost. You can purchase additional insurance up to $5,000 for an extra fee.

Can I ship internationally with Flat Rate boxes?

Yes, USPS offers international Flat Rate options through Priority Mail International. Prices vary by destination country and are generally higher than domestic Flat Rate prices. International Flat Rate is available for Small, Medium, and Large boxes.

How fast is Flat Rate shipping?

Priority Mail Flat Rate typically delivers in 1-3 business days domestically. Priority Mail Express Flat Rate provides overnight to 2-day delivery with a money-back guarantee.

Conclusion

USPS Flat Rate boxes remain one of the best values in domestic shipping for 2026, especially for heavy packages traveling long distances. The combination of free packaging, included insurance, full tracking, and zone-free pricing makes Flat Rate an essential tool for both individual shippers and e-commerce businesses. By understanding which box to use, when Flat Rate beats standard pricing, and how to access Commercial Plus rates, you can significantly reduce your shipping costs while providing fast, reliable delivery to your customers.