GSM (grams per square metre)
The single most important number on any paper spec sheet. What it measures, what it isn't, how to read it.
GSM — grams per square metre — is the universal unit for paper weight. It tells you the mass of one square metre of that paper. Nothing more, nothing less. If you can only look at one number on a datasheet, make it this one.
What it actually measures
A sheet 1 metre × 1 metre of 80 gsm copy paper weighs 80 grams. A sheet of 170 gsm art card weighs 170 grams. The measurement is absolute: no need to know the sheet dimensions, basis size, or ream count. That universality is why gsm dominates globally.
The standard governing the measurement is ISO 536:2019, which specifies sample conditioning (23 °C / 50% RH, 24-hour equilibration) and the weighing procedure. In practice, mills run inline basis-weight sensors continuously on the paper machine — the gsm figure on a datasheet reflects a reel average, not a single point.
What gsm does not tell you
GSM is a mass-per-area figure. It says nothing about:
- Thickness (caliper) — two papers at identical gsm can have dramatically different caliper depending on furnish, refining intensity, and calendering. See caliper.
- Stiffness — bulk × caliper³ drives stiffness; gsm is only one input.
- Surface quality — a 90 gsm coated sheet and a 90 gsm uncoated offset sheet are very different products.
- Basis weight in US lb — the US system uses a different reference sheet size per grade family. See basis weight vs grammage.
GSM ranges by grade family
These are practical market ranges, not absolute limits. Actual products exist outside these bands.
| Grade family | Typical GSM range | Notes | |---|---|---| | Tissue / hygiene | 12–40 g/m² | Single-ply base tissue; converted products often quoted per-ply | | Newsprint | 40–52 g/m² | Standard newspaper runs at 45–48.8 g/m² | | Uncoated printing / offset | 60–115 g/m² | 80 g/m² is the ubiquitous A4 office standard | | Coated art paper | 90–350 g/m² | Lightweight = 90–115; heavy art card = 200–350 | | Kraft / sack | 40–120 g/m² | Sack kraft for cement bags typically 70–90 g/m² per ply | | Liner / board | 100–400 g/m² | Testliner typically 125–175; kraftliner 125–225 g/m² |
Standard office copy paper sits at exactly 80 g/m² — confirmed in the canonical graph at WPI-g-000136. Classic newsprint at 45 g/m² sits in the family anchored by WPI-g-000869.
GSM vs US basis weight
US grades quote weight in pounds per 500 sheets (a "ream") cut to a grade-specific "basic size." For bond/writing, basic size is 17 × 22 inches; for text/book, it's 25 × 38 inches. This means a 20 lb bond equals roughly 75 g/m², while a 20 lb text equals roughly 30 g/m². The same number means different things depending on which grade family you're quoting.
The conversion factor for bond is: g/m² = lb × 3.760. For text/book: g/m² = lb × 1.480.
WPI editorial note: When a supplier quotes paper weight in pounds without specifying the basic size and grade family, ask. The ambiguity is real and can result in significant under- or over-specification of substrate.
For more on the conversion, see basis weight vs grammage.
Measuring gsm yourself
Cut a 100 cm² sample (10 cm × 10 cm), weigh it on a balance to 0.001 g precision, multiply by 100. That's your gsm. No calibration standard required — just accurate scissors and a decent scale.