A paper specification sheet is a list of numbers, and each number answers a practical question: how heavy is the sheet, how well does it resist pressure and moisture, and how will the finished box behave when stacked on a pallet. This glossary explains the most common tests in plain language, names the ISO method behind each one, and shows which results matter for testliner, fluting and kraftliner buyers.
1. Grammage (GSM), Caliper and Moisture: The Basics
Grammage, written GSM or g/m², is the mass of one square metre of paper (method ISO 536). It is the basis on which paper is priced and sold, and a small deviation across a whole reel changes the real weight of the shipment. Caliper is the thickness of one sheet; a thicker sheet is not always stronger, so read it together with grammage.
Moisture is the water content of the paper, usually 6 to 9 percent for finished paper, measured by drying a sample in an oven (ISO 287). Paper that is too wet is heavier, weaker and prone to mould; paper that is too dry is brittle. Strength values only mean something if the sample was conditioned at a standard temperature and humidity before testing (ISO 187: 23 °C and 50 percent relative humidity).
2. Strength Tests: Burst, SCT, CMT, RCT, ECT and BCT
Burst strength is the pressure a sheet withstands before it ruptures when pushed by a rubber diaphragm. Dividing it by the grammage gives the burst index, which lets you compare papers of different weights. SCT (short-span compression test) measures how well a strip of paper resists being crushed edge-wise over a very short span; it is a key value for liners and for fluting. CMT (Concora medium test) measures the crush resistance of a fluted sample of the medium and predicts how well the flute will hold up in the box.
The results combine as you move from paper to board to box: RCT (ring crush) and ECT (edge crush) measure the compression strength of the paper and the corrugated board, and BCT is the compression strength of the finished box. For a buyer, SCT for liners and CMT or SCT for fluting are the values that most directly predict how strong your boxes will be.
Common Paper and Board Tests at a Glance
| Test | What it measures | Applies to | ISO method |
|---|---|---|---|
| Grammage (GSM) | Mass per square metre | All paper and board | ISO 536 |
| Moisture | Water content of the paper | All paper and board | ISO 287 |
| Burst index | Resistance to pressure before rupture, per unit of grammage | Liners, testliner, kraftliner | ISO 2758 (paper), ISO 2759 (board) |
| Cobb (for example Cobb60) | Water absorbed by the surface in a set time | Liners and sized papers | ISO 535 |
| SCT | Short-span compressive strength | Liners and fluting | ISO 9895 |
| CMT | Crush resistance of fluted medium | Fluting (medium) | ISO 7263 |
| RCT | Ring crush strength | Paper and board | ISO 12192 |
| ECT | Edge crush resistance of corrugated board | Corrugated board | ISO 3037 |
3. Water Resistance: What the Cobb Value Tells You
The Cobb test puts water on the surface of the paper for a fixed time, for example 60 seconds, and weighs how much was absorbed, in g/m². A lower number means the surface resists water better because the paper has been sized. It matters for boxes that will meet humidity, cold storage, condensation or wet handling, and for papers that are printed or coated.
Always state the test time and the side tested when you compare values: a Cobb60 result and a Cobb30 result are not comparable, and the two faces of a sheet can differ.
4. How to Read a Supplier's Specification Sheet
Check first whether each value is a typical average or a guaranteed minimum or maximum: a "typical" burst index is not a promise. Then look at the tolerance on grammage and width, the test method and the conditioning used, and whether values refer to machine direction (MD) or cross direction (CD); compression values are usually quoted in the cross direction for liners.
Finally, ask for a certificate with your order that lists the results for the lot you will receive, and compare it with your own test on arrival. A supplier who is happy to document the lot is usually confident in the product.
5. Which Tests to Request for Each Product
For testliner and kraftliner ask for grammage, moisture, burst index, Cobb (at the time you use, for example Cobb60) and SCT. For fluting ask for grammage, moisture, CMT or SCT and, for semi-chemical fluting, the Cobb value if moisture resistance matters. For finished corrugated board, ECT and BCT show whether the boxes will meet your stacking requirement.
If you are not sure which values to specify for your boxes, tell us the box size, load and storage conditions and we will suggest a specification.
6. Worked Example: Reading a Testliner Specification Sheet
Imagine a supplier's sheet for a testliner with these values: grammage 125 g/m², moisture 8 %, burst index 2.2 kPa·m²/g, Cobb60 30 g/m², SCT cross direction 1.8 kN/m. Here is how to read each line.
Grammage 125 g/m²: each square metre weighs 125 grams. A tolerance, such as plus or minus 4 %, should also be stated; it tells you how much the real weight can differ from the label. Moisture 8 %: normal for finished paper; much higher values suggest it was stored badly. Burst index 2.2: the burst strength divided by the grammage. Multiply by 125 to get the burst strength of about 275 kPa. It suggests medium-high strength for a recycled liner. Cobb60 30 g/m²: after 60 seconds, one square metre of surface absorbs 30 grams of water, a moderate value that suits general packaging but is not ideal for very humid or cold-chain use. SCT cross direction 1.8 kN/m: the compressive strength across the paper; it feeds into the box compression calculation.
Now check the paper against your needs. If you make boxes for humid export, look for a lower Cobb. If your boxes must be stacked high, look at the SCT of the liner and the CMT of the medium. If you print in high quality, ask about the surface. A sheet only helps when you know what you need from it.
7. A Simple In-House Routine for Testing Reels on Arrival
You do not need a large laboratory to protect yourself. A short routine on every delivery catches most problems.
1. Check the reel labels and count against the packing list. 2. Inspect wrapping and edges for damage, water marks or crushing. 3. Weigh a sample of reels and compare with the packing list. 4. Take a sample from the outer layers (discard the first few metres, which may be damaged) and condition it under standard conditions before testing. 5. Measure grammage, moisture, burst and Cobb, and, if you can, compression values (SCT or CMT). 6. Record the results with the reel number and date, and compare them with the certificate and the specification. 7. Report any difference to the supplier in writing, with photographs and test reports.
Keeping these records over time also builds a picture of how consistent a supplier is, which is very useful when you negotiate the next order.
8. Common Misunderstandings About Paper Tests
A higher grammage always means a stronger paper. Not necessarily: fibre quality and processing matter as much as weight. Two papers of the same grammage can differ greatly in strength.
Burst strength predicts box stacking strength. Only partly: burst relates to puncture and rupture, while stacking depends mainly on compression values such as SCT and ECT.
Lower moisture is always better. Not always: very dry paper can be brittle, and paper conditioned at standard humidity is what the tests assume. The goal is a stable, correct moisture level.
A single result describes the whole reel. A test measures a small sample. Paper varies along and across the reel, so look at averages, tolerances and the number of samples tested.
Different labs give the same number. Only if the method and conditioning match. When comparing results, confirm the standard used.
