Paper Standard Sizes and Conversion

Paper sizes are categorized into two standards, the ISO standard and the North American size standard. ISO standard has A, B and C series, while NA standard is based on inches.
ISO Size Standards
“A-series”, for general printing
“B-series”, for posters
“C-series”, for envelopes, postcards, and folders
The “A” series is used most commonly with sizes ranging from A0,which is the largest, down to A8. The A-series sizes are represented as a part of the area of one square meter with a length to width ratio of 1.414. Size A0 is equivalent to the area of one square meter while each smaller size being 50% of the size of the preceding one. A1 is 50% of the area of A0; A2 is 50% of A1, and so on.  Another way to look at it is that when an A0 sheet cut in half yields two A1 sheets, and an A1 sheet cut in half yields two A2 sheets. 
North American Size Standards
Although the ISO size standards are common in many parts of the world where the metric system is the established standard for measurement, North American sheet sizes are based on inches. The most common size of cut sheet for copier/printers is 8.5”x11” (Letter Size) that is based on basis size of 17”x22”. (Letter being the quarter of the size of the 17” x 22”). Other popular sizes are 8.5”x14” (Legal Size), and 11”x17” (Ledger Size). The other sizes such as 12”x18” and 9”x12” are becoming popular for color applications. These paper sizes are based on paper sizes used in offset printing.
Engineering paper sizes are also in this category with sizes such as 9”x12” (A size), 12”x18” (B size), 18”x 24” (C size), 24”x36” (D), 36”x48” (E size).
NAS To ISO Conversion:
To convert the sizes and weights from North American Standards to ISO standards we have to use a little math. Let’s take a look at how to convert size and weight from North American standards to ISO and ISO to North American standards. As defined by ISO, paper size is described in metric measure, more specifically millimeters.  The first conversion ratio that we need to know is how many millimeters there are in an inch. There are 25.4 millimeters in one inch. The conversion procedure involves multiplying the inches by the conversion factor (25.4), and rounding off to the nearest millimeter.
For example:
To convert the size of the standard 8 1/2 x 11 letter size from inches to millimeters:
Multiply the width in inches by the conversion factor:
8 1/2” x 25.4 mm = 215.9 mm
Round the product off to the nearest millimeter:
Multiply the length in inches by the conversion factor:
11” x 25.4 mm = 279.4 mm.
Round the product off to the nearest millimeter:
The measurement therefore becomes:
US Letter (8 1/2” x 11”) = 216 mm x 279 mm.
This process can be reversed to determine the inch size of paper (or anything else) initially describe in millimeters.
We saw earlier that ISO A4 paper is 210mm x 297mm. How would we find out what the inch size of A4 is?
We would take the width and divide by the conversion factor:
210/25.4 = 8.2677
Round the quotient off to the nearest 1/4 inch:
8 1/4
Divide the length by the conversion factor:
297/25.4 = 11.6929
Round the quotient off to the nearest 1/4 inch:
11 3/4
The measurement therefore becomes:
ISO A4 (210mmx297mm) = 8 1/4” x 11 3/4”