Third-Class mail must weigh less than 16 ounces

Congressional third-class mass mailings are letter-size or flat size. The processing category is
determined by the size and the weight of the mailpiece.


Letter-Size Mail Dimensions: Min Max
Height:3 1/2"6 1/8"
Length: 5"11 1/2"
Thickness: .007"1/4"

Flats are mail that exceeds one or more of the letter-size dimensions.
Max Height - 12"Max Length - 15"