What is the standard postcard size? Dimensions and postal limits

Size is set by the post office, not the printer

Print shops will make a postcard almost any size, but only cards inside the postal rules are delivered at the postcard or small letter price. The US Postal Service defines a standard postcard as 5 to 6 inches long and 3.5 to 4.25 inches high, between 0.007 and 0.016 inches thick. The Universal Postal Union sets wider limits for international post, 140 by 90 millimetres at the smallest and 235 by 120 at the largest, and asks that the length be at least the height times the square root of two. Australia Post treats a postcard as a small letter, up to 240 by 130 millimetres and 5 millimetres thick.

Why 6 by 4 and A6 are the defaults

The 6 by 4 inch card matches the most common photo print size, so postcards, photo prints and frames share the same stock and the same proportions, a 3 to 2 ratio. A6 follows ISO 216 and is a quarter of an A4 sheet, so it prints four up with no offcuts. Both sit inside the US postcard limit, the UPU minimum and the Australia Post small letter size. Larger cards, such as 5 by 7 or 6 by 9 inches, are popular for direct mail because they stand out, but in the United States they pay the letter rate rather than the postcard rate.

Laying out the address side

The back of a mailed postcard has a fixed structure. The right half carries the delivery address and the stamp or postage indicia, and the left half carries the message. Keep that address area plain, with no images or dark tints under it, so sorting machines can read it, and keep the strip under the address plain too, since machines may print routing marks there. A card that will go in an envelope, such as a thank you note, has no such rules and can be designed edge to edge on both sides.

Common mistakes

The first is a card just over the postcard limit, for example 6.25 inches long, which silently costs the letter rate on every piece. The second is thin stock: a card under the minimum thickness can be rejected or damaged in machines. The third is a design with the important message on the address side, where the label or stamp covers it. The fourth is gloss coating on the writing side, which smudges ballpoint pens and makes handwritten notes impossible.

Postcards and the flyer editor

The flyer editor does not have a postcard canvas. Its sizes are A4 portrait and landscape plus six social formats, and none has a 3 to 2 or A6 proportion that matches a postcard exactly. An A4 landscape design can be scaled down to A6 landscape without cropping, since A sizes share one proportion, and the flyer size resizer outputs exact pixel sizes for A6 or 6 by 4 inches from the 2480 by 3508 export. Exports carry no bleed, so add it with the bleed and trim calculator. The blank PNG above is 1800 by 1200 pixels, a 6 by 4 inch card at 300 DPI, ready to open in another editor.

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