Dimensions
Letters
| Minimum | Maximum | |
| Length | 5 inches | 11-1/2 inches |
| Height | 3-1/2 inches | 6-1/8 inches |
| Thickness | 0.007 inch | 1/4 inch |
Aspect Ratio: length divided by height must be more than 1.3“ and less than 2.5“.
Non-machinable letter-size pieces and pieces over 3.3 ounces are subject to flat-size rates.
Aspect Ratio: length divided by height must be more than 1.3″ and less than 2.5″.
Flats
| Minimum | Maximum | |
| Length | 11-1/2 inches | 15 inches |
| Height | 6-1/8 inches | 12 inches |
| Thickness | 1/4 inch | 3/4 inch |
Flats exceeding these dimensions, rigid flats, and flats with certain characteristics are subject to parcel rates.
Postcards
| Minimum | Maximum | |
| Length | 5 inches | 6 inches |
| Height | 3-1/2 inches | 4-1/4 inches |
| Thickness | .007 inch | .016 inch |
Aspect Ratio: length divided by height must be more than 1.3″ and less than 2.5″.