GSM of one film
GSM means grams per square metre. A film's GSM is its thickness in microns times its density in g/cm³. Typical densities: PET 1.40, BOPP 0.91, LDPE 0.92, CPP 0.90, BOPA 1.14, and aluminium foil 2.71.
Example: 12 micron PET is 12 × 1.40 = 16.8 GSM. 50 micron LDPE is 50 × 0.92 = 46 GSM.
GSM of a laminate
Add up the films, then add the ink and adhesive between them, which together usually come to a few GSM.
Example: PET 12 / Met PET 12 / LDPE 50 is 16.8 + 16.8 + 46 = 79.6 GSM of film. With 5 GSM of ink and adhesive the laminate is 84.6 GSM.
Roll weight
Roll weight in kilograms = GSM × roll width in metres × roll length in metres ÷ 1000.
Example: an 84.6 GSM roll, 1 metre wide and 1,000 metres long, weighs 84.6 × 1 × 1000 ÷ 1000 = 84.6 kg, plus the core.
Working backwards
Laminates are usually sold by weight, so turn it round to get running length: length in metres = roll weight in kg × 1000 ÷ (GSM × width in metres). Divide that by your pack's cut-off length to see how many packs one roll makes. The calculator below runs the numbers for you.