How many grams should this bag be?
Woven sacks are priced by the weight of polypropylene in them, not by the piece. So the fabric weight — grams per square metre, written g/m² or gsm — is the number your price is really made of. It is also the easiest number in the industry to shave without anyone noticing.
Start with the arithmetic, not with a price
A sack is a flattened tube, so its fabric area is close to 2 × width × length, plus a
few percent for the hem and bottom seam. Multiply by the gsm and you have the grams of PP in the
bag. A 55 × 95 cm open-mouth sack at 60 g/m² is about 1.10 m², so about 66 g.
That is the whole basis of the quote. The calculators do this, the reverse calculation from tape denier and mesh, and the weigh-it-yourself check on a sample.
What gsm a job actually needs
There is no single right answer, but the ranges are well established, and a spec far outside them is usually telling you something:
| Typical use | Common range | What drives it |
|---|---|---|
| Grain, rice, feed sacks — 25–50 kg | 50–100 g/m² | Fill weight and stacking height. Most of the market sits here. |
| Cement and construction | 70–100 g/m² | Abrasion and drop resistance, not just static load. |
| Retail, promotional and reusable bags | 100–200 g/m² | Service life and shape retention, not strength. |
| FIBC body fabric — 1,000 kg bulk bags | 170–200 g/m² | Safety factor on a suspended load. |
The last row is there for comparison, because it is the range people most often misquote. We supply fabric, yarn and components into that trade — we do not make finished bulk bags.
Read a specified gsm for intent, not only for load. 200 g/m² on a 10 kg bag is far above what the weight requires — that buyer is paying for reuse, abrasion resistance and shape, not for strength. Offering to value-engineer it down reads as having misread the brief. Equally, 50 g/m² under a 50 kg fill is at the bottom edge of what will survive handling.
The tolerance band matters more than the number
Woven fabric has real process variation. If you specify 200 g/m² and the factory targets exactly 200, then with ±5% variation half the output falls below 200 and, against a ±10 tolerance, some of it is out of spec.
The fix costs very little resin: target slightly above the centre of the band, not at its edge. On a 200 ±10 spec, aiming at 203–205 removes most of the rejection risk. Ask what the factory is targeting, not just what tolerance it accepts — those are different questions and only one of them predicts what arrives.
Why gsm alone can be gamed
Calcium carbonate filler is cheaper than polypropylene and it adds weight. So a factory can hit your gsm exactly while producing fabric that is substantially weaker than virgin PP. Filler also makes fabric stiffer and more prone to cracking at folds and seams — which is precisely the failure mode for a bag that ships folded flat and gets refolded in service.
Typical levels run from near zero on premium fabric to around a fifth of the fabric weight on economy grades. Specify the filler content and ask for tensile and tear figures, not weight alone. A quote that answers on gsm but goes quiet on tensile has told you something.
Three things to put in your enquiry
- Fabric area per bag, in m². It removes the ambiguity between flat-laid and finished dimensions, which can move consumption by several percent, and it exposes anyone quoting from a different reading of your drawing.
- Filler content, and tensile or tear figures. Weight without strength is not a specification.
- The gsm target, not just the tolerance. See above — it is the number that decides what actually turns up.
Everything on this page is arithmetic or a published range; none of it is specific to us. What we cannot evidence is listed separately, by product and by process.