GSM — grams per square metre — is the first number to settle when specifying a paper product, because it decides stiffness, machine settings and cost at the same time. This page gives the working ranges we see across bags, cups and folding cartons, and explains what changes on the machine when the number moves.
GSM is weight, not thickness
Two papers with the same GSM can differ in caliper by 30%, depending on how much the sheet is calendered. Caliper (thickness in microns) and bulk (caliper divided by GSM) matter as much as weight when a product has to stand up or fold cleanly. For machine setup, ask for both GSM and caliper.
Working ranges by application
| Application | Typical GSM | Notes |
|---|---|---|
| Grocery and takeaway paper bags | 80–120 | Kraft or recycled; strength comes from the bag construction as much as the paper |
| Retail and branded paper bags | 120–200 | Higher GSM for handle strength and print quality |
| Luxury shopping bags | 200–250 | Often laminated or with reinforced top fold |
| Paper cup body stock | 190–300 | Plus a PE or aqueous coating of roughly 15–25 µm per side |
| Paper cup bottom stock | 250–320 | Stiffer than body stock to hold the base |
| Folding cartons | 250–400 | FBB or SBS; above 350 the crease line needs more preparation |
| Corrugated board liner | 120–200 | Test liner or kraft liner, matched to flute |
What changes on the machine when GSM moves
- Forming and folding. Heavier paper needs more crease pressure and a larger folding radius; too little and the fold cracks along the print line.
- Gluing. Higher GSM absorbs more adhesive and takes longer to set. Hot-melt open time and compression length have to be adjusted, not just the glue quantity.
- Die cutting. Cutting pressure rises roughly with caliper; a 350 gsm board may need a different make-ready than 250 gsm on the same tool.
- Running speed. Heavier stock usually means lower stable speed, especially on bag machines with a long folding section.
How to check a delivered lot
- Cut a 100 × 100 mm square from three positions across the reel or sheet.
- Condition the samples for 30 minutes in the production area — paper picks up moisture and gains weight.
- Weigh on a scale accurate to 0.01 g and multiply by 100 to get GSM.
- Compare with the specification and with the previous lot; a 5% swing is enough to change machine behaviour.
Specifying paper in a purchase order
State GSM with a tolerance (typically ±3–5%), the grade (kraft, SBS, FBB, recycled), the coating if any, moisture content, and the reel or sheet dimensions. Add the caliper range if the product has to hold a shape. When a supplier quotes only GSM, ask for caliper and bulk before approving the sample.