10/21/2026
A quarter inch can change your postage. That's the part most people don't expect when they pick a postcard size.
Make a card a little taller or a little longer, and the Postal Service stops treating it as a postcard at all. It becomes a letter, or a flat, and the price goes with it.
Which is why picking a size is a money decision as much as a design one. Postage per card, room for your offer, whether anyone notices it on the kitchen counter. Size touches all three. Let's go through the usual options.
Common postcard sizes and how USPS treats them
Direct Mail Postcard Guide
Size (inches)USPS categoryWhat it means4 x 6CardLowest First-Class price tier5 x 7LetterLetter pricing, more room6 x 9LetterLetter pricing, popular for promos6 x 11LetterBiggest card that stays letter-size6.5 x 9FlatWorks for Every Door Direct Mail8.5 x 11FlatLots of space, flat pricing6 x 6 (square)Nonmachinable letterShape can push the price up
What size qualifies for postcard rates?
Postcard sizes compared: which get card rates, when a card becomes a letter or flat, why square cards cost more, EDDM sizes and usable design space.