Knowledge Base
Thickness and caliper testing glossary and fact sheet
Key terms
- Caliper: Another word for thickness; the perpendicular distance between the two principal surfaces of a sheet, measured under load.
- Single sheet thickness: The thickness of one sheet of paper or board, measured under a defined static load.
- Bulking thickness: The thickness of a single sheet, calculated from a pack of superimposed sheets measured under load.
- Hard-platen method: Thickness measured between rigid metal faces at a set pressure (ISO 534, TAPPI T411).
- Soft-platen method: Thickness measured with a compressible surface, reporting effective thickness (TAPPI T551).
- Effective thickness: The value reported by the soft-platen method, which reduces the influence of surface roughness.
- Apparent density: Mass per unit volume, calculated from thickness and grammage.
- Bulk: Volume per unit mass; the inverse relationship to density.
- Grammage: Mass per unit area of paper or board, expressed in grams per square meter.
- Gauge block: A precision reference of known dimension, used to calibrate thickness instruments.
- Cross-machine profile: A set of readings taken across the width of the web to show thickness uniformity.
- Deadweight micrometer principle: A measuring method that applies a known static load for accuracy and repeatability.
Standards reference
| Standard | What it covers |
| ISO 534 | Thickness, density, and specific volume of paper and board by the hard-platen method. Covers single sheet and bulking thickness. Not applicable to corrugated fiberboard. |
| TAPPI T411 | Single-sheet thickness (caliper) of paper, paperboard, and combined board, measured between hard metal platens at 50 kPa (7.3 psi). |
| TAPPI T551 | Thickness of paper and paperboard by the soft-platen method (effective thickness). Generally returns lower values than TAPPI T411. |
| ISO 18522 | An ISO thickness method supported by the PROFILE/Plus Thickness instrument. Detailed scope not independently verified, please confirm before publishing. |
| TAPPI T580 | A TAPPI thickness method supported by the PROFILE/Plus Thickness instrument. Detailed scope not independently verified, please confirm before publishing. |
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FAQs
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In paper and board testing, the terms are used interchangeably. Both describe the perpendicular distance between the two principal surfaces of a sheet, measured under a defined load. “Caliper” is common in North American and TAPPI contexts, while “thickness” is more common in ISO terminology. Whichever term a specification uses, it refers to the same physical measurement.
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Thickness influences strength, stiffness, and bulk, so variation affects how a grade performs. If a sheet runs too thin, it may not meet the specified strength, raising the risk of complaints or rejected batches. If it runs too thick, the mill tends to use more fiber than needed, adding cost to every reel. Consistent caliper helps balance product quality against material efficiency.
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The most common are ISO 534 and TAPPI T411 for the hard-platen method, and TAPPI T551 for the soft-platen method. ISO 534 also covers density and specific volume calculations. Automated instruments such as the PROFILE/Plus Thickness can also work to ISO 18522 and TAPPI T580. Always check which standard and method a specification refers to before comparing results.
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The hard-platen method measures thickness between rigid metal faces at a set pressure and is described by ISO 534 and TAPPI T411. The soft-platen method, in TAPPI T551, uses a compressible surface that reduces the effect of surface roughness and reports effective thickness. The soft-platen method generally returns smaller values, so the two are not directly interchangeable.
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Gauge blocks are the main calibration reference for these instruments. A block of known dimension is measured to confirm the device reports the correct gap, and the parallelism of the measuring faces is checked. Alongside calibration, conditioning samples in a controlled atmosphere and keeping platen surfaces clean help keep readings accurate and repeatable.
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Thickness, or caliper, is one of the first properties many mills measure, and one of the most revealing. It tells you how much material is in a sheet, and it points to how that sheet will behave under stress, through a press, and inside a finished package. Getting it right is a balancing act between…
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Thickness, also called caliper, is the perpendicular distance between the two principal surfaces of a sheet of paper or board, measured under a defined static load between hard metal platens. It's one of the most fundamental physical properties in paper and board manufacturing, and it directly influences strength, stiffness, bulk, and how a material runs…
