Explanation

As per IS : 10811 - 1983, the size of a trimmed A4 sheet is 210mm x 297mm. This standard defines the specific size and the tolerance of the sheet size of paper which is used in Offset printing, and it's referenced as ISO paper size. A4 is a standard paper size that is commonly used for letterhead, forms, catalogs, brochures, and many other printed materials. This size is based on the German DIN 476 standard, and it is part of a larger set of paper sizes known as ISO A or ISO 216 paper sizes.

The width of an A4 sheet is always 210mm and the length of an A4 sheet is 297mm, no matter what the orientation of the page is.