Why film prices use different units
A supplier may sell film by mass, surface area or running length. Each basis can describe the same reel, but a bare price does not tell you enough to compare them. GSM connects mass to area, and web width connects area to running length.
GSM means grams per square metre. A film with a measured basis weight of 60 GSM contains 0.060 kg per square metre. A one-metre-long section of a 0.8 m wide web contains 0.8 m², so its mass is 0.048 kg. These are net film quantities; a reel core is not part of the material basis weight.
Convert from a kilogram quotation
Price/m² = Price/kg × GSM ÷ 1,000. Price/running metre = Price/m² × Width in metres. At €3.20/kg, 60 GSM and 800 mm width, first convert 800 mm to 0.8 m. The area price is €0.192/m², and the running-metre price is €0.1536/m.
For 5,000 running metres, the area is 4,000 m² and the net film mass is 240 kg. All three pricing bases give the same material subtotal: €768. Checking the subtotal through two different units is a useful way to catch a width or factor-of-1,000 error.
Reverse the calculation
Price/kg = Price/m² × 1,000 ÷ GSM. To convert a running-metre price, divide it by the web width first. A €0.16/m offer for an 800 mm web is €0.20/m². At 60 GSM, that is approximately €3.3333/kg.
Keep unrounded values through intermediate calculations, then round the final quotation to the precision appropriate to the purchasing decision. Small rounding differences can become visible when multiplied over a large order.
Use the correct material weight
Measured GSM from a representative sample is preferable to a dimensional estimate when available. For a uniform material, GSM can be estimated as thickness in micrometres multiplied by density in g/cm³. For example, an assumed density of 0.92 g/cm³ and a thickness of 50 µm imply 46 GSM. That is an illustrative assumption, not a specification for every film.
For laminates, use total measured GSM or sum the contribution of each layer, including coatings where known. A single resin density applied to the whole structure may distort the conversion. Film width and GSM also do not account for startup scrap, unusable edges or defects; include those when costing useful production output.
Common questions
Is a linear metre the same as a square metre?
Only when the web is exactly one metre wide. Area equals running length multiplied by width in metres.
Can thickness alone convert price per kg to price per m²?
No. You also need density to estimate GSM, or a measured GSM value that already connects mass and area.