If you have ever bought paper for a printer or a business card, you have met gsm, lb, caliper, and maybe even micron. These are all units of paper weight or thickness — and they measure different things.
GSM: Grams per Square Metre
GSM (or g/m²) is the metric standard. It measures the mass of one square metre of paper. A standard sheet of office paper is 80 gsm. A business card is around 300 gsm. A heavy poster might be 250 gsm. A thin magazine cover could be 120 gsm.
GSM is easy to compare because it is a direct physical measurement — 100 gsm paper is always twice as heavy as 50 gsm paper of the same area.
LB: Pounds per Ream
The US and UK use lb (pounds per ream). A ream is 500 sheets. But here is the catch: the "basis size" matters. 20 lb paper in the US is measured against a basis size of 17 × 22 inches; 20 lb paper in the UK used to be measured against a different basis. This is why "80 gsm" and "20 lb" describe roughly the same paper but are not mathematically identical.
Common GSM Values
- 35–50 gsm — newspaper, telephone directories
- 60–75 gsm — cheaper books, letterhead
- 80–90 gsm — office paper, photocopying
- 100–130 gsm — brochures, flyers, magazines
- 150–200 gsm — posters, invitations
- 250–350 gsm — business cards, book covers
- 350–450 gsm — premium cards, packaging
Caliper: Thickness in Microns
Caliper (or thickness) is measured in microns (µm) or "thou" (thousandths of an inch). The caliper of a sheet depends on the density of the fibre — 80 gsm paper can have a range of thicknesses depending on how compressed it is. Caliper matters when you need to fold without cracking, or when mail is priced by thickness (as in the UK).
Choosing the Right Weight
- Everyday printing — 80 gsm
- Newsletters and brochures — 100–130 gsm
- Posters — 150–200 gsm
- Business cards — 300–400 gsm
- Covers and boxes — 250–350 gsm
Why Weight Matters for Folding
Light paper (under 100 gsm) folds easily with no scoring. Mid-weight paper (100–200 gsm) needs a light score line to fold cleanly. Heavy paper (over 250 gsm) needs a proper score to prevent cracking along the fold. This is why the same printer might produce beautiful brochures on 130 gsm but crack on 300 gsm without scoring.