In America we are used to being given the ppi (pages per inch) as part of the description of a book paper, and we all know how to figure the bulk of the book block — page count divided by ppi.  It’s dead easy.

In Britain (and Hong Kong) not only don’t they use inches, they never (at least within my working memory) gave paper bulk in this way.  What you will be told there is the thickness of one sheet, in microns.  A micron is one thousandth of a millimeter.  You can of course use this information to come up with a bulk.  The formula would be:

Half the page count, multiplied by the micron value, divided by 1000 multiplied by 0.03937.

An example will make it clearer.  The book has 840 pages.  Divide by 2 — because each leaf has two sides, and the micron measurement, which in this example is 86, refers to the thickness of a leaf not a page.  420 x 86 = 36120.  Divide by 1000, which just means move the decimal point 3 spaces to the left, and you come up with a book bulk of 36.12 millimeters.  Multiply millimeters by 0.03937 to come up with inches.  In this example the bulk is 1.422 inches.

If you must have it as ppi you can get there by dividing half the micron value (to get back to pages rather than leaves) into 25400 (which is the number of microns in an inch.  In this example your answer will be 590.697, so you could say the paper is 600ppi, or 591 if you are a stickler for accuracy, as you must be, or you would never have embarked on this calculation in the first place.