ISO Paper Size Identifier
There are many charts available that show ISO paper designations and the corresponding physical measurements. For example, you can look up what A4 is and you'll find that it has dimensions 210mm x 297mm.
However there doesn't seem to be any way to obtain the International ISO 216 paper size designation from the dimensions of a sheet of paper. The Wikipedia charts all assume you want to see what sizes paper the ISO standard defines. Not how to get back to the ISO letter/number from having a known size.
Here is a tool to determine the ISO series A, B, C from the measurements of a piece of paper.
Measurement can be made in mm or inches.
Just enter the paper measurement into the box. The program will identify the ISO Series and Size number.
Proposed Ruler Design
(Click for full-sized view. Image is scaled at 114 DPI)
The illustration is an original design proposed for a ruler 25 cm or just under 10 inches. It is designed to find the ISO Series and Size number of paper.
A piece of paper can be measured directly against this ruler for sizes 4 through 10. If the paper is larger then it is folded a number of times until it fits within the green area - the section on the right. You can then read off the ISO designation subtracting the number of folds from the number shown. The green section is "4". Exceptions are the rarely used "overformats" sizes such as 2A0, 4A0. The subtraction would evaluate to -1 and -2 respectively.
Using the ISO Paper Size Identifier program the ruler can be rendered on your screen in the proper size with either metric or imperial scale.
Revised 3 Aug 2020