Fabric GSM, Density and Shrinkage: Testing Workwear Fabric Before Bulk
The short answer
Before a bulk order, test five fabric properties: mass per unit area (GSM, tolerance ±5%), construction density (warp and weft threads), shrinkage after washing (3% maximum for poly-cotton, 5% for cotton), colour fastness (grade 4 or better) and surface durability (pilling and abrasion). Testing costs US$80–400 per fabric and takes 5–10 working days, far less than finding a problem across 5,000 garments.
GSM is the number buyers argue about most, and it is also the easiest to verify with a fabric cutter and a laboratory scale in under five minutes.
Last updated: September 2026.
Key takeaways
- GSM tolerance is normally ±5%, and it belongs in the specification.
- Shrinkage must be measured after washing, not on the greige fabric.
- Colour fastness of grade 4 or better is the practical minimum for workwear.
- Test the fabric, not the garment, before cutting: after cutting, the cost is the whole order.
- Ask for the mill report and spot-check it yourself.
1. The five tests that matter
| Test | What it measures | Typical acceptance |
|---|---|---|
| Mass per unit area | GSM of the fabric | Within ±5% of the specification |
| Construction density | Warp and weft threads | Matches the approved sample |
| Dimensional stability | Shrinkage after washing | 3% maximum poly-cotton, 5% cotton |
| Colour fastness | Fading and bleeding | Grade 4 or better |
| Surface durability | Pilling and abrasion | Grade 3–4 or better |
2. How to check GSM yourself in five minutes
| Step | Action | Notes |
|---|---|---|
| 1 | Cut a circular sample with a GSM cutter | 100 cm² is standard |
| 2 | Weigh it on a laboratory scale | 0.01 g resolution |
| 3 | Multiply the weight in grams by 100 | The result is g/m² |
| 4 | Repeat three times and average | Avoids an unrepresentative reading |
| 5 | Compare with the specification | A 240 g/m² fabric weighing 205 g/m² is a substitution |
This single check catches the most common quiet downgrade, because a lighter fabric is almost invisible in a photograph.
3. Shrinkage: what to specify
| Fabric | Typical shrinkage after three washes | Recommended maximum |
|---|---|---|
| 100% cotton | 3–5% | 5% |
| Poly-cotton 65/35 | 1–3% | 3% |
| 100% polyester | under 1% | 2% |
| Untreated cotton canvas | 5–8% | 5%, using pre-shrunk fabric |
| Knitted hi-vis fabric | 2–4% | 3% |
A garment that shrinks 6% loses a full size in the body and sleeve. On a 5,000-piece order that is a returns problem, not a quality conversation.
4. What the specification sheet should state
| Line | Example |
|---|---|
| Composition | 65% polyester, 35% cotton |
| Mass | 240 g/m², ±5% |
| Construction | Twill, 20 × 16 threads per cm |
| Shrinkage | Maximum 3% after three washes at 40 degrees C |
| Colour fastness | Grade 4 minimum, wet and dry rub |
| Finish | Pre-shrunk, sanforised, or FR treated as applicable |
| Certification | Report number and issuing laboratory |
If the specification omits these lines, every later quality discussion becomes an opinion instead of a measurement.
5. What testing costs and when to do it
| Test | Typical cost | Timing |
|---|---|---|
| GSM and construction check | $0–30 in house | On fabric arrival |
| Shrinkage after washing | $80–200 | Before cutting |
| Colour fastness | $60–150 | Before cutting |
| Pilling and abrasion | $100–250 | On new fabrics |
| FR or hi-vis certification testing | $400–4,000 per style | Before bulk, once per construction |
Testing is a one-time cost per fabric. A US$200 shrinkage test against an US$8,000 order is the cheapest insurance in the production process.
6. Frequently asked questions
What does GSM mean for workwear fabric?
It is the mass of one square metre of fabric in grams, and it drives weight, warmth and much of the durability. Standard workwear runs from about 190 g/m² for summer garments to 300 g/m² and above for heavy canvas.
What is an acceptable GSM tolerance?
Plus or minus 5% is standard in apparel. Some buyers allow only 3% for technical fabrics, or for hi-vis garments where coverage and performance relate directly to fabric mass.
How do I test fabric GSM without a laboratory?
Use a 100 cm² circular cutter and a scale accurate to 0.01 g. Weigh the sample, multiply by 100, repeat three times, and compare the average with the agreed specification.
How much shrinkage is acceptable in cotton workwear?
Up to 5% after three washes for untreated cotton, and up to 3% for poly-cotton or pre-shrunk cotton. Anything more affects fit and should be treated as a specification failure.
What colour fastness rating should workwear have?
Grade 4 or better on wet and dry rub for general workwear. Hi-vis garments have stricter requirements, because colour performance is part of the certification and fading reduces the compliant area.
Should I test the fabric or the finished garment?
Test the fabric before cutting, when a problem is cheapest to solve. Certified protective garments also require testing on the finished garment, which is a separate mandatory step.
How long does fabric testing take?
In-house GSM and construction checks take minutes. Laboratory shrinkage, fastness and durability tests take 5–10 working days, so schedule them before the cutting date rather than after.
Can I rely on the mill's test report?
Treat it as evidence, not proof. A mill report describes the fabric they shipped, which is not always the fabric in your garments. Spot-check GSM and shrinkage on the production lot.
Ask for the test data with your quotation
Tell us the fabric and the performance you need, and we will include GSM, shrinkage and fastness data in the quotation, plus the mill report where one exists, so the specification is measurable before production begins.
