The USPS has minimum and maximum sizes for the various classes of mail.The following information will enable you to stay within the allowable size ranges for the mail type you desire.
1st Class Letter Standards:
- Minimum Allowable Letter Size: 3.5 x 5 Inches
If the width is less than 5 inches or the height is less than 3.5 inches the USPS will NOT mail your letter. - Maximum Allowable Letter Size: 6.12 x 11.5 Inches
If the width is greater than 11.5 inches or the height is greater than 6.12 (6 1/8) inches, the letter must be mailed as a flat, higher rate and all. - Minimum Thickness: .007 Inches
Any mail less than the minimum is considered Non-Mailable and the USPS will NOT deliver it. - Maximum Thickness: .25 inches
Exceeding the maximum puts the mail piece into a different mail class, called a Parcel. - Weight cannot exceed 1 oz.
- Intermediate Sizes are Allowable
A 4x5.5 inch letter is mailable at the 1st Class rate. As long as a mail piece meets the size, thickness, weight and aspect ratio requirements above, it can be mailed as a 1st Class Letter. - Minimum Thickness: .007 Inches
- Maximum Thickness: .25 inches
Postcard Standards:
- Minimum Allowable Postcard Size: 3.5 x 5 Inches
If the width is less than 5 inches or the height is less than 3.5 inches the USPS will NOT mail your postcard. - Maximum Allowable Postcard Size: 4.25 x 6 Inches
If the width is greater than 6 inches or the height is greater than 4.25 inches, the postcard must be mailed as a letter, higher rate and all. - Minimum Thickness: .007 Inches
Any mail less than the minimum is considered Non-Mailable and the USPS will NOT deliver it. - Maximum Thickness: .25 inches
Exceeding the maximum puts the mail piece into a different mail class, called a Parcel. - Intermediate Sizes are Allowable
A 4 x 5.5 inch postcard is mailable at the postcard rate. As long as a mail piece meets the size, thickness, weight and aspect ratio requirements above, it can be mailed as a postcard. - Minimum Thickness: .007 Inches
- Maximum Thickness: .25 inches
- Space reserved for USPS Barcode and Endorsements
Aspect Ratios
- Aspect Ratio Rule: a 1st Class mail piece (letter or postcard) must have an aspect ratio that falls between 1 to 1.3 and 1 to 2.5 inclusive.
- Definition of Aspect Ratio: The length (width) of a piece divided by the height of the piece.
- A letter with the dimensions of 4x6 has an aspect ratio of 1 to 1.5 which falls into the allowable aspect ratio range.
Height = 1 Width = 1.5 ( 6 / 4 )
