In the U.S. book business when we buy paper we talk about basis weight in referring to text stock but when we move to cover stock we talk about caliper. Caliper is the thickness of a single leaf of paper. Thus a cover measured by a micrometer at 0.010 inches is referred to as 10 point cover stock (or 10pt) — a common caliper for paperback covers. We indicate basis weight by the pound sign: #. Basis weight is always the weight in pounds of a ream of paper of a standard, basis, size. Superficially pretty straightforward.

Cover and text stocks each have both caliper and basis weight, yet we never think of the basis weight of cover stock, and I found myself last week asserting that they were in fact the same. They are not.

There are two variables in that formula for figuring basis weight: the size of a ream, and the size of the standard sheet. The ream is (almost) always 500 sheets, but there are different standard sheet sizes for different types of paper.

PAPER BASIS SIZES

What is the reason for these different basic sheet sizes for different print businesses? Tradition/inertia is the simple answer. When paper was made by hand, the sheet size was determined by the dimensions of the mould used by the papermaker. For example, because sheets measuring 17″ x 22″ leant themselves to cutting into four 8 ½” x 11″ sheets, this became the standard for business stationery as the papermakers serving that business made their moulds that size. Strangely we in America have never changed our units from this quaint basis.

Why do we need to know the different basis weight calculation of cover and text stock? It comes to the fore when you venture overseas for your book production. The rest of the world tends to deal in gsm (grams per square meter, sometimes written g/m2 or g/m²) a much simpler and logical, if less picturesque, system of measurement. A book manufacturer in China may well quote you cover stock referred to in basis weight terms which they’ve converted from their gsm data. (To convert from text basis weight to gsm divide the basis weight by 0.67565. For cover stock the magic number is 0.36989. You’ll no doubt need to do a little rounding to make the answer look right.) So be aware: 100# cover stock is not the same as 100# text stock. Because the area of the basis sheet of cover stock is 1806sq.ft. while the area of the basis sheet size of text stock is 3299sq.ft., your 100# cover stock would be more like a 180# text sheet — if such a thing were available. Verso has quite a useful conversion tool.

For caliper calculation, see PPI. See also Basis weight calculation.