USPS gives postcards their own cheaper rate, but only inside a specific size window. Get the dimensions right and one postcard stamp does the job; get them wrong and the card either costs letter postage or comes back.
The postcard size window
- Minimum: 3.5 inches high x 5 inches long.
- Maximum: 4.25 inches high x 6 inches long.
- Shape: rectangular, at least slightly wider than tall.
- Stock: card stock sturdy enough to survive automated sorting — copy paper is too thin.
Why 4x6 is the standard
A 4x6 card uses the full allowed canvas while staying machinable, matches photo-print sizes, and is exactly what home printers and blank postcard stock are made for. Every pack on our printable postcards page is built as an 1800x1200 JPG for clean 6x4 printing.
Outside the window
- Bigger than 4.25 x 6: mails as a letter, not a postcard.
- Square cards: non-machinable; extra fee applies.
- Smaller than 3.5 x 5: too small to mail at all.
- Odd shapes or attachments: hand-cancelled or surcharged.
Verify before a big batch
Rules and rates are maintained by USPS and can change. Before printing a large batch, check the current standards on the official USPS Postal Explorer. For which stamp the finished card needs, see the postcard stamp guide.