First Class Mail
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Letters, postcards, matter wholly or partially handwritten or typewritten, matter sealed or closed against postal inspection. Bills or statements of account are considered First Class mail.
LETTER SIZE
Letter size envelopes must be rectangular in shape for automated processing by United States Postal Service.
The following dimensions must be met for automated processing by USPS:
| Minimum | Maximum | |
Height: | 3 1/2 inches | 6 1/8 inches |
Length: | 5 inches | 11 1/2 inches |
Thickness: | .007 inch | 1/4 inch |