Bag weight from geometry and fabric weight

Woven sacks are priced by the weight of polypropylene in them, not by the piece. If you know the bag's dimensions and the fabric weight, you know roughly what it must cost — and you can tell a real quote from a padded one.

fabric area (m²) = 2 × (width + gusset) × (length + bottom allowance) ÷ 10,000
bag weight (g) = fabric area (m²) × GSM

Worked example. A 55 × 95 cm open-mouth sack at 60 g/m²: area ≈ 2 × 55 × 100 ÷ 10,000 = 1.10 m²; weight ≈ 1.10 × 60 = 66 g.

Why it matters: fabric weight is the easiest place for a bag to get quietly cheaper. Two quotes at the same price can differ by 15% of GSM, and only the weight shows it.

Next: check a quoted GSM against a real sample, or read what gsm a job actually needs.