Paper weight is measured three different ways: grammage in GSM (grams per square metre), basis weight in pounds per ream, and caliper in thickness. Only GSM compares directly across grades; basis weight depends on paper category, and caliper depends on how the sheet is made. Confusing them is the most common cause of specification errors in print buying.

This guide explains each measure, provides conversion tables, and shows how to choose weight by application.

GSM: the universal measure

Grammage, expressed in grams per square metre (gsm), is the weight of one square metre of paper. It is unambiguous and used in most of the world. When a quotation states 100 gsm, any mill anywhere can produce or verify that number the same way.

GSM is measured by cutting a precise area of paper and weighing it on an analytical balance. Tolerances are tight on machine-finished grades, commonly plus or minus 2 to 3 percent.

Basis weight: the North American system

The United States uses basis weight, expressed in pounds. The catch: the ream size depends on the paper category, so '80 lb' means different things for different papers:

• Text papers: 25 x 38 inch ream (500 sheets)
• Cover papers: 20 x 26 inch ream (500 sheets)
• Index papers: 25.5 x 30.5 inch ream (500 sheets)
• Bristol and tag: 22.5 x 28.5 inch ream (500 sheets)

Common conversions

Rule of thumb: never convert basis weight without knowing the category. A 60 lb cover is nearly twice the grammage of a 60 lb text.

Caliper: thickness matters too

Two sheets of the same GSM can have very different caliper. Bulk is the ratio of caliper to grammage, and it varies with fibre type, fillers and calendering. A bulky sheet feels substantial and is preferred for books; a low-bulk sheet is denser and often preferred where opacity per grammage matters.

Caliper is expressed in microns (thousandths of a millimetre) or points (thousandths of an inch). A typical 100 gsm coated woodfree sheet runs roughly 90 to 110 microns; a 100 gsm cast coated sheet can be thinner because its coating is dense and compact.

Choosing weight by application

Self-adhesive labels

Face stocks for labels commonly run 70 to 90 gsm. Lighter face stocks conform to curved surfaces; heavier stocks resist edge lift and scuffing. The face stock must also balance against the liner so the laminate feeds straight on a converter.

Brochures and leaflets

A tri-fold leaflet typically uses 120 to 170 gsm so it holds a fold without cracking. Multi-page saddled brochures use 100 to 150 gsm text weight, with heavier stocks for covers.

Folding cartons and packaging

Cartons generally start at 200 gsm and rise with size and product weight. A small cosmetics box may use 250 to 300 gsm cast coated board; a large appliance carton may use 350 gsm or more. Stiffness, not just grammage, determines how premium the box feels.

Books and publications

Text pages commonly run 70 to 120 gsm depending on page count and show-through requirements. Higher opacity grades allow lighter grammage without print-through.

Practical buying tips

1. Always state GSM plus tolerance. '100 gsm plus or minus 2 gsm' removes arguments at goods-in.
2. Specify caliper where feel matters. For luxury packaging, a caliper range communicates the hand feel better than grammage alone.
3. Check opacity at low grammage. Lightweight sheets with poor opacity show print-through on double-sided work.
4. Match stiffness, not only weight. A stiffer lighter sheet can outperform a heavier limp one in cartoning.
5. Confirm runnability. Heavy stocks on high-speed web presses need the right stiffness and moisture to run without breaks.

China UV Paper produces coated woodfree, cast coated and specialty grades with tight basis weight control on a 6,600 mm wire, and every reel ships with a certificate stating grammage, caliper, brightness, gloss, smoothness and moisture. If you are converting a specification from basis weight to GSM, our team will verify the numbers before you order.