USPS Large Envelope Size


March 17, 2024

Envelopes serve a dual purpose in mailing. The first, quite obvious purpose, is safeguarding the contents they encase.

Fortunately, many envelopes sport safety features to accomplish this first objective, from standard adhesives to prevent the contents from falling out in transit to security tints that make it challenging to identify what’s inside.

However, envelopes have a second, equally crucial function: ensuring your mail items reach their intended destinations on time. Yet, to fulfill this purpose effectively, envelopes require your assistance.

When sending these usually rectangular paper containers through the United States Postal Service (USPS), selecting the appropriate size is essential. Failure to do so may lead to your mailpiece being rejected or incurring postal rates reserved for packages.

This article focuses on large envelopes or “flats” in USPS terminology, shedding light on their definition, requirements, rates, and other critical details you must remember.

Whether you’re an individual mailer or a small business owner, navigating the complexities of large envelope shipping guarantees your mail’s safe and timely journey. Visit to locate your nearest post office for additional information on USPS packaging guidelines. 

USPS Large Envelopes – Flats | Overview

If you want to send a letter and prefer that mailpiece to arrive within five business days at an affordable price, you should ship it through the U.S. Postal Service’s (USPS) First-Class Mail option.

However, this mail service has strict size guidelines. For instance, First-Class Mail letters must be at least 3½” tall x 5” long x 0.007” thick. However, they also cannot exceed the maximum dimensions of 6⅛” and 11½” in height and length and ¼” in thickness.

While USPS may still allow you to send a letter surpassing these size specifications in exchange for a surcharge, USPS may also categorize and charge it as a large envelope or “flat.”

What Are Large Envelopes?

At its core, a large envelope is a mail item that doesn’t fit the size standards of a First-Class Mail letter. In other words, it has at least one dimension past 6⅛” tall, 11½” long, or ¼” thick.

What Is Considered a Large Envelope for Mailing?

As mentioned, a U.S. Postal Service flat exceeds the letter size requirements of First-Class Mail. However, like letters, large envelopes must also meet specific guidelines to be considered one.

The maximum size for First-Class large envelopes is 12” tall, 15” long, and ¾” thick. Flats must also be 13oz or less and be rectangular, uniformly thick, and not rigid.

Under the First-Class Mail service, large envelopes that exceed these restrictions will be charged package postage rates and shipped through other USPS mailing options, such as USPS Ground Advantage.

What Are First-Class Mail Flats Retail Rates?

First-Class Mail is the U.S. Postal Service’s most cost-effective mailing solution for items weighing up to 13oz. In other words, it’s the most economical option for large envelopes.

To demonstrate this claim, here’s USPS postage pricing for First-Class Mail envelopes, based on October 2023 information:


Domestic Priority Mail Flat Rates

Although USPS First-Class Mail is an economical choice for mailing envelopes, it isn’t your only option. Enter Priority Mail, a one- to three-business-day shipping service.

The most significant advantage of sending an envelope via Priority Mail over First-Class Mail is that it can weigh up to 70lbs. It also offers flat-rate pricing, allowing you to ship to any state at the same price.

Here are the available flat-rate envelopes you can ship through Priority Mail, including their dimensions and October 2023 shipping prices:

ProductDimensionsRetail Price
Flat Rate Envelope12½” x 9½”$9.65
Window Flat Rate Envelope10” x 5”12½” x 9½”$9.65
Small Flat Rate Envelope10” x 6”$9.65
Padded Flat Rate Envelope12½” x 9½”$10.40
Legal Flat Rate Envelope15” x 9½”$9.95

What Is First-Class Mail Shape-Based Pricing?

As cited, your letters and flats must meet size and weight requirements to qualify for the U.S. Postal Service’s First-Class Mail option. However, your mailpieces must also follow shape guidelines.

As a rule of thumb, First-Class Mail items, including large envelopes, must be uniformly thick, rectangular, and not rigid. If your letters fail these prerequisites, they’ll incur a $0.40 nonmachinable surcharge.

Nonmachinable surcharges are tariffs tacked onto mailpieces that cannot go through the Postal Service’s automated mail process and require manual sorting.

Unfortunately, USPS will charge odd-shaped large envelopes as parcels and ship them through USPS Ground Advantage. When this happens, the postage calculation will include the origin and destination distance or zone.

Here are your potential expenses if your large envelope is shipped via USPS Ground Advantage based on USPS’ October 2023 pricing:

Weight not overZone 1 (1-50 miles)Zone 2 (51-150 miles)Zone 3 (151-300 miles)
Weight not overZone 4 (301-600 miles)Zone 5 (601-1,000 miles)Zone 6 (1,001-1,400 miles)
Weight not overZone 7 (1,401-1,800 miles)Zone 8 (over 1,801 miles)Zone 9 (U.S. territories and some military addresses)

Note that the abovementioned prices are based on USPS’ October 2023 shipping costs and are subject to change. For up-to-date information, visiting the USPS website at or your local post office is best.

Use’s Postal Service facility finder tool to locate your area’s nearest and most convenient post office locations.

Mail in 1–5 Business Days; Small Packages in 1–5 Business Days

Aside from large envelopes, First-Class Mail is ideal for mailing postcards and letters. This USPS service delivers within five business days, making it an excellent option for mailpieces that must arrive promptly.

Features and Pricing

If you’re sending greeting cards, invoices, or lightweight merchandise, First-Class Mail is your go-to for reliable and cost-effective shipping. Here are some features and shipping costs for the eligible mail items.


As mentioned, First-Class Mail letters have dimensions under 6⅛” high x 11½” long x ¼” thick. The U.S. Postal Service has two pricing structures for these letters: stamped and metered.

Stamped letters utilize postage stamps. On the other hand, metered letters have postage printed directly on them. Here are the rates for both types of First-Class Mail letters, based on October 2023 data:

Stamped First-Class Mail Letters
Metered First-Class Mail Letters


A standard-sized postcard is at least 3½” by 5” in height and length and 0.007” thick and doesn’t exceed the dimensions of 4¼” high x 6” long x 0.016” thick. As of October 2023, the mailing cost of postcards is $0.51.

Large Envelopes (Flats) or Small Packages

If your mailpieces don’t fall under USPS’ standard letter size, they’re treated as large envelopes or small packages. These mail items can weigh as much as 13oz, with postal rates ranging from $1.35 to $4.23.

Flats Must:

Note that for the U.S. Postal Service to consider an item as a “flat,” it must have one dimension surpassing 6⅛” tall, 11½” long, or ¼” thick. However, it must also not go over 12” in height, 15” in length, and ¾” in thickness.

To Decide if First-Class Large Envelope (= First Class Flat) Rate Will Work for Your Book:

You can use large envelopes to mail books via the First-Class Mail service. However, some considerations must be remembered, including the following:

  • The book must overhang the envelope’s long and short sides. Otherwise, your flat may not be uniformly thick.
  • Send thinner than ¼” or thicker than ¾” books through other USPS services, such as Media Mail, which specializes in shipping educational materials.
  • Books must flex at least an inch in all directions to qualify for First-Class Mail.

Packages (Parcels)

If your First-Class Mail item exceeds large envelopes’ size, shape, and weight limits, you must send it through other USPS mailing services. One of these alternative shipping options includes USPS Ground Advantage, offering two- to five-business-day delivery for parcels over 15.999oz and up to 70lbs.

There’s also the Priority Mail Express service, the U.S. Postal Service’s fastest package solution. It allows you to send mailpieces up to 70lbs while guaranteeing next- to two-business-day delivery or your money back.

Note that these services have higher postage rates than First-Class Mail. Contact USPS customer service at 1-800-ASK-USPS (1-800-275-8777) to help you decide the best service for your unique package-mailing needs.

Conversely, you can bring your parcel to a local post office and talk to a Postal Service representative to receive in-person advice. Visit to find your area’s most convenient USPS facility.

First-Class Mail Quick Tips

Here are some quick tips to help you understand the nuances of shipping via First-Class Mail and simplify the mailing process:

  • USPS charges First-Class Mail letter rates for square greeting cards that meet the standard letter size. In other words, they’re also subject to the $0.40 nonmachinable surcharge.
  • The Postal Service will classify non-flexible letters as nonmachinable mailpieces, regardless of whether they meet the size and weight requirements.
  • Nonmachinable surcharges only apply to First-Class Mail letters. This additional tariff doesn’t extend to eligible postcards and large envelopes.
  • First-Class Mail items that exceed their maximum dimensions are charged the cost of the next larger shape. For instance, a postcard surpassing 4¼” high x 6” long x 0.016” thick will be priced as a letter.
  • You can mail padded bags through First-Class Mail if they meet flats’ maximum weight, size, and shape limits.

Weight, Size, and Shape Requirements

Due to its fast and affordable nature, the U.S. Postal Service’s First-Class Mail service has strict mailpiece size, shape, and weight guidelines.

Although we’ve already discussed them repeatedly throughout this article, here’s a quick overview of First-Class Mail size and weight standards:

Minimum SizeMaximum SizeMinimum ThicknessMaximum ThicknessMaximum Weight
First-Class Mail Postcards3½” x 5”4¼” x 6”0.007”0.016”3.5oz
First-Class Mail Stamped Letters3½” x 5”6⅛” x 11½”0.009”¼”3.5oz
First-Class Mail Metered Letters3½” x 56⅛” x 11½”0.009”¼”3.5oz
First-Class Mail Large envelopes6⅛” x 11½”12” x 15”¼”¾”13oz

Nonmachinable Envelopes

Square-shaped, vertical, rigid, lumpy, or letter-sized envelopes with clasps, buttons, or strings must be hand-canceled by postal workers instead of processed by automated machines.

Therefore, these mail items will incur a $0.40 nonmachinable surcharge, regardless of whether they weigh under 1oz.

Large Mailpieces, Envelopes, and Small Packages

As mentioned, the U.S. Postal Service doesn’t charge nonmachinable surcharges on large envelopes deviating from the size, shape, and weight criteria. Instead, it treats these mailpieces as packages.

While you can send international mail that doesn’t fit First-Class large envelope requirements via the First-Class Package International service, you must send domestic pieces via services like USPS Ground Advantage or Priority Mail.

For context, First-Class Package International allows you to ship packages measuring up to 24” long, at least 4” tall, and a combined length, height, and thickness of 36”.

Additional Information

Apart from the critical size, shape, and weight guidelines, there are additional details you must consider when sending large envelopes via USPS’ First-Class Mail service. These considerations include the following:

Mailing Restrictions and Prohibitions

USPS prohibits and limits you from mailing specific items through First-Class Mail and other services, even if they fit inside a First-Class Mail-eligible large envelope.

These prohibited items include ammunition, explosives, and marijuana. At the same time, domestically restricted items include alcoholic beverages, firearms, live animals, and perishable items.

For an exhaustive list of mailable items, refer to the Postal Service’s publication on hazardous, restricted, and perishable mail on the USPS website.

Alternatively, you can consult a Postal Service representative via the USPS customer care center or your local post office, which you can find using’s USPS facility finder tool.

For Businesses

Due to its cost-efficiency in shipping lightweight mail items, First-Class Mail can benefit businesses such as eCommerce retailers. Here’s how USPS can streamline your mailing process and reduce shipping expenses:

Data-driven Shipping

First-Class postcards, letters, and large envelopes are targetable, trackable, and measurable. As a result, this service can be ideal if you’re a business owner looking to expand your reach, especially considering that USPS offers address correction and forwarding services.

Commercial Prices and Automation Discounts

First-Class Mail retail prices are already affordable: postcards start at $0.51, letters at $0.66, and large envelope prices begin at $1.35. However, you can lower these rates further with commercial pricing and automation discounts.

You need a minimum mail volume of 500 pieces to qualify for commercial base rates. On the other hand, you must presort or bundle mailpieces by ZIP codes and use automation-compatible mailpieces to be eligible for automation discounts.

For more information on commercial pricing and automation discounts, contact USPS at 1-800-ASK-USPS (1-800-275-8777) or your local post office. Find your area’s nearest U.S. Postal Service facilities with’s post office locator.


  1. How much does it cost to mail a 9×12 envelope?

Whether your mailpiece is 9” high x 12” long or 9” long x 12” high, it’s still within the maximum dimensions of a large envelope, which are 12” tall, 15” long, and ¾” thick. 

So, if your flat is under 13oz and ¾” thick, you can ship it via First-Class Mail. However, the postage cost will vary depending on its weight. For instance, if it weighs 9oz, expect to pay $3.27. At the same time, you’ll pay $1.59 if it only weighs 2oz.

  1. How many stamps do I need for a 9×12 envelope?

The number of stamps you must affix to your 9” x 12” envelope depends on the shipping cost, which is influenced by weight. Fortunately, the U.S. Postal Service simplifies this stamp-calculation process with its specialty and denomination stamps.

If you’re sending a 6-oz large envelope, the postage cost is $2.55. In this scenario, you can use the following stamps: Forever stamps, postcard stamps, Additional Ounce stamps, and definitive stamps.

Forever stamps are postage stamps that are always equal in value to the cost of a 1-oz First-Class Mail letter, which is $0.66 as of October 2023. On the other hand, Postcard stamps are equivalent to $0.51 or the cost of mailing a postcard.

The value of an Additional Ounce stamp is tied to the cost of subsequent ounces past the 1-oz letter price of First-Class Mail, which is, as of October 2023, $0.24. Lastly, definitive or denomination stamps are valued based on the price printed on their face.

A possible combination to cover the $2.55 shipping fee would be three Forever stamps, one Postcard stamp, and a 6¢ definitive stamp ([$0.66 x 3] + $0.51 + $0.06 = $2.55).Alternatively, you can use three Forever stamps, two Additional Ounce stamps, plus a 9¢ denominated stamp ([$0.66 x 3] + [$0.24 x 2] + $0.09 = $2.55).


  1. Direct Mail and Data Security: Top 5 Ways to Stay Safe | Letter Jacket Envelopes
  2. First-Class Mail
  3. Sizes for Letters
  4. USPS Price List
  5. Priority Mail
  6. What is the Non-Machinable Surcharge for First-Class Mail®?
  7. What are Zones, and how do they affect your shipping costs?
  8. What is a postage meter?
  9. Sizes for Postcards
  10. Can I use First-Class ‘Large Envelope’ Rate to send my book?
  12. USPS Ground Advantage
  13. Priority Mail Express
  14. Contact USPS
  15. First-Class Mail Fact Sheet
  16. Types of First-Class Mail®
  17. First-Class Package International Service
  18. Mailing Standards of the United States Postal Service
  19. Postage Stamps – The Basics