What Does GSM Mean for Paper?

Quick answer

GSM means grams per square metre. For paper and board, it reports grammage: the mass of a one-square-metre area under the specified test method. A higher number means more mass per area, not automatically greater thickness, stiffness, durability, or print quality.

What the number actually measures

ISO 536 specifies a method for determining the grammage of paper and board. The unit normalizes mass to area, so sheets of different sizes can be compared. You do not need a physical one-square-metre sheet; a measured sample can be weighed and its result converted to grams per square metre.

For two papers cut to the same dimensions, the higher-GSM sheet has more mass. For an entire order, multiply the area of one sheet by its grammage and sheet count to estimate paper mass, allowing for manufacturing tolerances.

Formula

Sheet mass (g) = grammage (gsm) × sheet area (m²). Rearranged: grammage (gsm) = sheet mass (g) ÷ sheet area (m²). A4 has a fixed area of about 0.06237 m² (0.210 m × 0.297 m), so grammage and single-sheet mass convert directly for that size.

Worked example: A4 sheet and ream mass

An A4 sheet's mass follows directly from its grammage using the formula above. The table below applies it to grammages commonly seen in stationery and print, plus the mass of a standard 500-sheet ream at that grammage.

A4 sheet and 500-sheet ream mass by common grammage
Grammage	One sheet (approx.)	500-sheet ream (approx.)
80 gsm	5.0 g	2.5 kg
90 gsm	5.6 g	2.8 kg
100 gsm	6.2 g	3.1 kg
120 gsm	7.5 g	3.7 kg
160 gsm	10.0 g	5.0 kg
200 gsm	12.5 g	6.2 kg
250 gsm	15.6 g	7.8 kg

For example, ordering 500 sheets of 80 gsm A4 paper (one standard ream) comes to roughly 2.5 kg — matching the weight most retail reams of everyday 80 gsm copy paper are labeled with. Doubling the grammage to 160 gsm roughly doubles both the per-sheet and per-ream mass, because grammage and mass scale together at a fixed sheet size; it does not, by itself, double thickness or stiffness (see the next section).

What GSM does not tell you

Two 120 GSM papers can feel different. Fiber choice, density, bulk, coating, calendering, moisture, and internal structure can change thickness and bending stiffness without changing the nominal grammage. A bulky uncoated sheet may feel thicker than a denser coated sheet with the same GSM.

GSM is also not a universal quality score. A lower grammage may be desirable for mailing weight or book portability, while a higher grammage may suit cards or cover stock. The right value depends on the printing process, opacity needs, folding, binding, and handling.

Same grammage, different feel: what can vary at equal GSM
Property	Bulky uncoated sheet	Dense coated sheet
Relative thickness at equal GSM	Can be noticeably thicker	Can be noticeably thinner
Relative bending stiffness	Often stiffer for its weight	Often more flexible for its weight
Typical reason	Looser fiber structure, more trapped air	Calendering and coating compress and fill the sheet

How to use GSM well

Treat it as one comparable specification. Pair it with thickness, finish, opacity, stiffness, and printer limits. When substituting stock, test the actual paper rather than assuming equal GSM means equal behavior.