Caliper (thickness)
Micrometers, microns, and points. Why two papers at the same GSM can have completely different thickness — and why it matters.
Caliper is paper thickness, measured in micrometres (µm). One micrometre = 0.001 mm. In the US, caliper is also quoted in points (1 point = 25.4 µm = 0.001 inch) or mils (same as points, imperial usage). The international standard for the measurement is ISO 534.
How it's measured
A flat-faced anvil and platen press on a sample at a defined pressure (100 kPa per ISO 534) for two seconds. The gap between faces is the caliper. Sample conditioning: 23 °C / 50% RH, minimum four hours.
Caliper varies across a sheet and across a reel — formation unevenness, filler distribution, and moisture profiling all introduce variation. A datasheet figure is a reel average; individual sheet caliper typically varies ±2–5%.
Caliper vs GSM: the bulk relationship
The ratio of caliper to gsm is bulk (also called specific volume):
Bulk (cm³/g) = caliper (µm) / (gsm × 1000)
A paper with 100 µm caliper at 80 g/m² has a bulk of 1.25 cm³/g. Bulk is what determines stiffness at a given weight and what determines how thick a book will be per page.
Two papers at identical gsm can have very different caliper if their bulk differs. This matters enormously for:
- Book and magazine design — spine width, feel, page count estimation
- Envelope machinery — caliper variation causes feed jams
- Board packaging — stiffness, which scales with caliper³, determines box rigidity at a given basis weight
- Roll geometry — given a target roll diameter, caliper determines how many metres fit per reel
What drives caliper up or down
| Factor | Effect on caliper | |---|---| | More mechanical pulp (TMP, CTMP) | Increases — bulky coarse fibers | | Heavier calendering | Decreases — physical compression | | More filler (CaCO₃, clay) | Decreases — fills voids between fibers | | Higher freeness (less refining) | Increases — less inter-fiber bonding, more void space | | Wet pressing intensity | Decreases — removes water and compacts sheet | | Coating | Slight decrease — consolidates surface |
Dense vs bulky: a canonical contrast
Bible paper represents the densest end of the market: 20–50 g/m² with maximum opacity per unit thickness. A 40 g/m² bible paper might run at 35–45 µm caliper, giving a bulk of ~0.9–1.1 cm³/g. Every gram of mass does maximum optical work — PCC fillers and tight formation at the expense of bulk.
Standard office copy paper (WPI-g-000136) runs at 80 g/m², typically 95–115 µm caliper — bulk around 1.2–1.4 cm³/g. Nothing exotic: a tight virgin kraft furnish, moderate calendering for surface smoothness.
At the bulky extreme: uncoated book papers for large-print editions or lightweight editorial design can reach 1.6–2.0 cm³/g bulk at 70–80 g/m², using high-yield mechanical pulp and minimal calendering.
Points and the US system
US book and commercial printers quote caliper in points:
- 4pt = ~100 µm — lightweight text paper
- 6pt = ~152 µm — standard uncoated text
- 8pt = ~203 µm — light card / reply card
- 12pt = ~305 µm — standard card stock
- 24pt = ~610 µm — heavy board / signage
The conversion: µm ÷ 25.4 = points. When a printer quotes "10pt coated" they mean roughly 254 µm — check this against the actual datasheet if you're designing to a specific spine width.
WPI editorial note: Caliper is often the forgotten specification. Engineers calculate by gsm and forget that two papers at the same gsm can differ by 30–40% in caliper. If you're designing packaging where crease quality matters, specify caliper alongside gsm — don't assume the relationship.
For the interaction between caliper and opacity, see opacity. For how caliper drives stacking strength in containerboard, see burst strength.