How the box weight is calculated
For a regular slotted carton the sheet length is twice the length plus twice the width, plus four times the board thickness, a 35 mm joint flap and 10 mm trim. The sheet width, called the deckle, is the width plus the height plus twice the board thickness and 10 mm trim.
Each liner weighs the sheet area times its GSM. Each fluting paper weighs more than its GSM because it is corrugated: it is multiplied by the take-up factor, about 1.52 for A flute, 1.36 for B flute, 1.42 for C flute and 1.25 for E flute. Starch adds about 10 grams per square metre for each glue line. The total is the box weight.
How bursting strength is estimated
The bursting strength of a paper in kg per square centimetre is roughly its GSM times its BF divided by 1000. For the board, add the strength of every liner and about half the strength of every flute. This is a quick estimate for quoting; a lab test on the bursting strength tester gives the real value.
How the price is built up
Paper cost is the weight of each layer times its price per kg, plus wastage. Starch, conversion (corrugation, slotting and handling, often taken as a rate per kg of box), printing per colour and stitching or die-cutting are added to get the cost per box. Your margin on top gives the price per box, and dividing by the box weight gives the price per kg that many buyers compare.
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Questions people ask
How do I calculate the weight of a corrugated box?
Find the sheet area from the box size, then add the weight of each paper layer (area x GSM, with flutes multiplied by the take-up factor) and the starch. For example, an 18 x 12 x 12 inch 5-ply box with 150 GSM liners and 120 GSM flutes weighs about 830 grams.
What do GSM and BF mean for kraft paper?
GSM is the weight of the paper in grams per square metre. BF, the burst factor, measures how strong the paper is for its weight. Higher BF paper gives a stronger box at the same weight.
What is deckle in a corrugated box?
Deckle is the width of the sheet across the corrugator, which for a regular slotted carton is the box width plus height plus allowances. It decides which reel width the job runs on.
What is the difference between 3-ply, 5-ply and 7-ply?
A 3-ply board has two liners and one flute, 5-ply has three liners and two flutes, and 7-ply has four liners and three flutes. More plies make a heavier, stronger box for heavier or export loads.