Paper Specifications Explained (Cobb, Bursting Strength & More)

Grammage (GSM)

Grammage, expressed in grams per square metre, is the weight of the paper and usually the first specification agreed. It strongly influences thickness, stiffness, and strength: heavier paper is generally more durable and rigid. Grammage is quoted with a tolerance — a small acceptable variation — so always agree both the target and the tolerance. A paper consistently below its stated grammage is effectively underweight and lower in value, so confirming grammage per shipment protects what you pay for.

Cobb value (water absorption)

The Cobb value measures how much water a paper surface absorbs in a set time, expressed in grams per square metre. It matters because many packaging papers must resist moisture — a high Cobb means the paper soaks up water readily, while a low Cobb indicates better water resistance, often from sizing or coating. Buyers in humid climates or with moist products should pay close attention to Cobb. If your application involves damp conditions, specify a maximum Cobb value and confirm it is met, as moisture resistance directly affects how the paper and any box made from it will hold up.

Bursting and tensile strength

Two specifications describe how the paper resists force, and both are important for packaging:

• Bursting strength — the pressure the paper withstands before it ruptures, key for liners and sacks.
• Tensile strength — the pulling force the paper withstands before tearing, important for bags and wrapping.
• Higher values mean a tougher paper, but also usually a heavier and costlier one.

Moisture content

Paper naturally contains some moisture, and its level affects both performance and handling. Too much moisture weakens the paper and can cause problems in printing and converting; too little can make it brittle. Moisture content is also why paper must be packed against humidity and stored properly — paper that arrives damp may be out of specification even if it left the mill correct. When sourcing for humid markets, discuss moisture-barrier packing with your supplier and confirm the moisture content on the data sheet, as it underpins many of the other strength figures.

Brightness, whiteness, and surface

For papers that will be printed, appearance specifications matter alongside strength. Brightness and whiteness describe how light and clean the surface looks, affecting how vivid printed colours appear. Smoothness and the presence of a coating influence print sharpness. These are most important for graphic and retail packaging, where shelf appeal counts, and less critical for plain industrial liners. Match the appearance specification to the visibility of the surface: pay for high brightness and coating only where the paper will be seen and printed, and economize where it will not.

Why agreeing specifications in writing matters

Specifications only protect you if they are agreed and documented. Before ordering, set out the target values and tolerances for the parameters that matter to your application, and ask the supplier to certify them per shipment on a mill certificate or data sheet. This turns vague quality expectations into measurable commitments and gives you a basis to inspect and, if necessary, reject non-conforming paper. When comparing quotes, make sure all suppliers are bidding on the same specifications, because a lower price on a lower specification is not a genuine saving.