What size is first class mail

In the United States, the size limits for First-Class Mail are as follows:

Letters:

• Maximum length: 11.5 inches (29.2 cm)
• Maximum height: 6.125 inches (15.6 cm)
• Maximum thickness: 0.75 inches (1.9 cm)

Large Envelopes (Flats):

• Maximum length: 12 inches (30.5 cm)
• Maximum height: 15 inches (38.1 cm)
• Maximum thickness: 0.75 inches (1.9 cm)

Packages:

• Maximum length: 13 inches (33 cm)
• Maximum height: 17 inches (43.2 cm)
• Maximum thickness: 12 inches (30.5 cm)

Oversized Envelopes (Flats):

• Maximum length: 15 inches (38.1 cm)
• Maximum height: 20 inches (50.8 cm)
• Maximum thickness: 0.75 inches (1.9 cm)

Note that these size limits apply to the exterior dimensions of the mailpiece, including any packaging or wrapping. Additionally, the weight limit for First-Class Mail is 13 ounces (369 grams).

It's worth noting that if your mailpiece exceeds these size limits, you may need to use a different class of mail, such as Priority Mail or Parcel Post, which have different size and weight limits.