Post Office Regulation on Small Post Card Sizes
Postcards are a fun and economical way to keep in touch with friends, loved ones or customers, but the U.S. Postal Service regulates the minimum and maximum size of what can be sent at its postcard rates.
Minimum Length
Postcards must measure at least 5 inches in length.
Minimum Width
The short side of a postcard must be at least 3 1/2 inches.
Minimum Thickness
Although length and width are simple measurements, regulations also require that a postcard must be at least .007 inches thick. That's approximately the thickness of a single index card. For business postcards, thin and/or flimsy cards can be damaged during mailing and sorting, so contact your local post office's mail piece design analyst (see Resources) to confirm that your cards are thick enough.
Shape--No Squares
Postcards must be rectangular--square cards with equal lengths and widths will either not be mailed or require extra postage because they cannot be machine-sorted.
Maximum Dimensions
The maximum dimensions for postcards are 4 1/4 inches high by 6 inches long by 0.016 inches thick. If your postcard exceeds any of those dimensions, it can still be mailed but you will be charged the higher letter rate.
