The standard postcard size in the US is 5.8 x 4.2 inches, which is the equivalent of A6 and is broadly based on the traditional 6 inch x 4 inch standard postcard format.

Is a UK postcard A6?

The two standard postcard sizes in the UK are A5 (210mm x 148mm) and A6 (148mm x 105mm).

What size paper is postcard?

The standard Postcard size is 4” x 6”.