Board Basis Weight (Grammage)

Mass over area. Paper has been sold this way since paper has been sold, and the number connects a blank size to a fibre bill, a freight cost and, increasingly, a packaging levy charged by mass.

The whole difficulty is units, and there are three conventions in play. Grammage in grams per square metre is unambiguous everywhere and is what most of the world quotes. Pounds per thousand square feet is the North American combined-board unit, also unambiguous, and it is what this site returns in imperial — 520 g/m² is 106.5 lb/1000 ft², the same board. The third convention is the trap: the North American "basis weight" of a PAPER grade is quoted per a basis ream whose size depends on the grade. A 42 lb linerboard and a 42 lb bond are nothing like the same mass per unit area, and a "26 lb medium" is a ream weight, not an areal density. When a specification says a number in pounds, find out per what before doing arithmetic with it.

Measure the area flat. A blank is substantially larger than the box it becomes — every flap, every glue lap, every allowance. Computing a basis weight from the assembled box's outside dimensions overstates the grammage badly, and it is an easy mistake to make when the only thing on the bench is a folded box.

Weigh it conditioned, or say what you did not condition it to. Paper at 90% relative humidity carries several percent more mass than the same paper at 50%, and all of that difference is water. As an error in a grammage figure it is modest. As a signal it is not modest at all: it is the same moisture that is quietly halving the compression strength on the derating page. A board that weighs heavy is telling you something about the warehouse, not just about the paper.

Run backwards, the equation is the standard way to audit a delivery without counting anything: weigh the pallet, divide by the certified grammage, and compare the implied area against what the count says should be there. The discrepancy is far more often moisture than short delivery, which is why the comparison is only worth making on conditioned material, or on two lots weighed in the same room on the same day.