Caliper (thickness)

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 classic 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.