Cards (or postcards) are an inexpensive way to get an immediate message to customers. When cards arrive in the mail, there’s the message -- no envelope to open! First-Class Mail cards are a great value, too. With First-Class Mail cards, you pay a low rate and get all of the benefits, like forwarding and return, that come with First-Class Mail. And, if you mail at the single-piece First-Class Mail rate, there is no extra work involved -- simply drop the cards in a collection box.

You may think that your mailpiece is a "card," because it is a single sheet of paper. But to qualify for mailing at First-Class Mail card rates, a card must be:

• Rectangular
• At least 3-1/2 inches high x 5 inches long x 0.007 inch thick
• No more than 4-1/4 inches high x 6 inches long x 0.016 inches thick

If your mailpiece does not meet the dimensions above, then the Postal Service considers it a letter (and charges letter rate postage). With Standard Mail, there is a little more flexibility -- there is no separate (lower) postage rate for cards, so you don't have to worry about your postcard being too big -- because you're paying letter rates anyway. But make sure that your card is no larger than 6-1/8" x 11-1/2" x 1/4" thick. Larger than any of those dimensions and you'll have to pay flat (large envelope) postage rates.

Some mailers want to attach stickers, magnets, or other items to their cards. However, an attachment may disqualify a card for mailing at the First-Class Mail card rate—or even make it nonmailable. The rules about attachments to cards are restrictive, so check with your local mailpiece design analyst (MDA[1]) or business mail entry staff[2], who can tell you if your mailpiece design will be mailable.

There are slightly more restrictive size requirements for mailing cards at automation rates[3].

TIPS
-If you’re planning to mail a card, First-Class Mail gives you the best value for your postage dollars. There is no lower card rate in Standard Mail.
-0.007 inches? How do I measure that? As a guide, an index card is thick enough. If in doubt, contact a mailpiece design analyst (MDA[4]) at the post office near you. MDAs have tools for precisely measuring thickness and can tell you if your mailpiece is thick enough.
-Make sure your mailpiece meets the minimum thickness requirement. Thin, flimsy pieces tend to get caught in mail processing equipment. If your mailpiece gets damaged in the equipment, then your message doesn’t reach your customers.
-What is high? What is long? Length is the side parallel to the address. Height is the side that is perpendicular to the length.

Sizes for letters[5]

Sizes for flats[6]

Sizes for parcels[7]