Postcards have been around nearly as long as postal services and are produced in a wide range of sizes, with some even being special shapes, for instance circular. However there are some recognised sizes for the "standard postcard".

International Standard Postcard Size

Although the general consensus is that the dimensions of an international standard postcard are A6 size, 148 x 105 mm or 5.8 x 4.1 inches, the Universal Postal Union (UPU) has defined postcards as having the following maximum and minimum sizes for their members.

Size	Length x Height (mm)	Length x Height (in)
Maximum	235 x 120 mm	9.25 x 4.72 in
Minimum	140 x 90 mm	5.51 x 3.54 in

The UPU specifies that postcards be sufficiently stiff as to "withstand processing without difficulties" and that the length shall be at least equal to the height multiplied by root 2.

US Postal Service Standard Postcard Size

The US Postal Service defines postcards as having a maximum and minimum size and a maximum and minimum thickness. The following table gives the specifications in both inches and millimetres (cm dimensions can be calculated by dividing mm by 10)

Size	Length x Height (in)	Length x Height (mm)	Thickness (in)	Thickness (mm)
Maximum	6.0 x 4.25 in	152.4 x 107.9 mm	0.016 in	0.406 mm
Minimum	5.0 x 3.5 in	127.0 x 88.9 mm	0.007 in	0.178 mm

The thickness specifications for the US postcard equates to a basis weight of 39lb to 105lb bond stock, 55lb to 145lb cover stock (card stock) or 145gsm to 390gsm.

It should be noted while the US minimum size and the UPU International minimum size are very similar the UPU International maximum size is considerably bigger than the USPS maximum size.

Older Postcard Sizes

Old postcards can be found in many different sizes although there have been some standards. In some countries some non-rectangular postcards also made it through the postal system.

Old British Postcard Sizes

In the UK postcards were introduced in 1870, plain and printed by the post office. From 1894 picture postcards were introduced (known as court cards) at 4.75 x 3.5 inches. This size is common up until 1899.

In 1899 a standard postcard size of 5.5 x 3.5 inches was introduced as the maximum size. The minimum size was 3.25 x 2.25 inches. The minimum size was raised to 4 x 2.75 inches in 1906 and in 1925 the maximum size was raised to 5.875 x 4.125 inches. This standard was kept until at least 1968 when special postcard postage rates were abolished in the UK.