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GSM Calculator — Fabric Weight in Grams per Square Metre
Calculate fabric GSM from a cut sample's weight and dimensions. Switch to Verify mode to check if received fabric meets your buyer's GSM spec. Includes a pass/fail result against ±5% industry tolerance and a reference guide for common fabric weights by end use. No account needed — numbers update as you type.
For accurate GSM measurement, cut at least three samples from different positions across the fabric width (avoid selvedge) and average the results. Test under standard atmosphere: 20°C ± 2°C and 65% ± 4% RH, after 24-hour conditioning per ISO 139.
Step By Step
How to Use This Calculator
5 steps
- Use a sharp, straight-edged cutter to cut a fabric sample — a 30×30 cm square is easiest to cut accurately; a standardised 100 cm² circular template (per ISO 3801) gives the most consistent results.
- Weigh the dry, conditioned sample on a scale with at least 0.01 g resolution.
- Enter the sample weight in grams.
- Enter the sample length and width in centimetres.
- For QC verification: switch to Verify mode and enter the buyer's specified GSM. The calculator shows pass/fail against ±5% tolerance.
Worked Example
Worked example — 30×30 cm sample from jersey roll
Use this sample to sanity-check your inputs and understand what the final result represents.
- 1Sample dimensions: 30 cm × 30 cm = 900 cm² = 0.09 m²
- 2Sample weight on scale: 14.4 g
- 3GSM = 14.4 ÷ 0.09 = 160 g/m²
- 4Buyer spec: 160 GSM · Tolerance: ±5% → acceptable range 152–168 GSM
Final Result
Measured GSM: 160 · Result: PASS ✓. Category: medium-weight knit, suitable for standard T-shirts.
Methodology
GSM Formula — ISO 3801:1977 and ASTM D3776
This section explains the calculation logic, assumptions, and source material used to make the result more trustworthy and easier to verify.
GSM (g/m²) = Sample Weight (g) ÷ Sample Area (m²). Sample Area (m²) = Length (cm) × Width (cm) ÷ 10,000. Standard test methods: ISO 3801:1977 'Textiles — Woven fabrics — Determination of mass per unit length and mass per unit area'; ASTM D3776/D3776M 'Standard Test Methods for Mass Per Unit Area (Weight) of Fabric'. Both specify conditioning at 20°C ± 2°C and 65% ± 4% relative humidity per ISO 139. Acceptance tolerance of ±5% is industry convention; some buyers specify ±3% in their technical pack.
Practical Guidance
GSM testing — what QC inspectors do in practice
- 1Cut samples from at least 3 positions — near each selvedge and the centre — and average; GSM can vary 5–10% across a roll width due to tension differences during knitting or weaving
- 2Never test wet or damp fabric — moisture adds to apparent weight; cotton at 65% RH has an 8.5% moisture regain (ISO 6741-1); drying the sample first then retesting in conditioned atmosphere is the most reliable approach
- 3A 100 cm² circular template cutter (available from testing equipment suppliers) gives more consistent sample areas than hand-cut squares — blade sharpness and cutting angle introduce variability
- 4Test early in the roll and again at the middle and end — some mills have tighter tensions at roll ends that alter GSM
- 5Keep a GSM test register by PO number and roll number — this creates a traceability chain if a buyer raises a quality dispute
- 6If GSM fails, test at least 10 more samples before rejecting the roll — one outlier may not represent the full roll
Frequently Asked Questions
What is GSM in fabric and why does it matter?+
GSM stands for Grams per Square Metre — the standard measure of fabric weight defined by ISO 3801 and ASTM D3776. It determines fabric cost (more weight = more raw material), hand feel, opacity, warmth, durability and end-use suitability. In the Bangladesh RMG industry, buyers specify GSM in the Technical Data Sheet (TDS) for every fabric. Delivering fabric significantly outside the specified GSM is a quality non-conformance that can result in rejection, chargeback, or requirement to recut and remake.
What GSM should a T-shirt fabric be?+
T-shirt GSM varies by market and product position. Economy/promotional T-shirts use 140–160 GSM single jersey — lightweight and low cost. Standard retail T-shirts use 160–185 GSM — the most common range for high-street brands. Premium and fashion T-shirts use 185–220 GSM for a heavier hand feel and better drape. Heavyweight T-shirts (streetwear, workwear) use 220–260 GSM. The Bangladesh RMG industry produces the largest volume in the 160–180 GSM range. The right GSM also depends on fibre blend: a 100% cotton 160 GSM and a 60/40 cotton-polyester 160 GSM feel and cost differently.
What is the standard GSM tolerance in garment QC?+
Industry-standard tolerance is ±5% of the specified GSM value. A 160 GSM specification therefore accepts any result between 152 and 168 GSM. Some premium buyers and technical garment programs specify tighter tolerances of ±3%. The Verify mode in this calculator defaults to ±5% per industry convention. If your buyer's TDS specifies a different tolerance, override the pass/fail assessment accordingly. Reference: AATCC Test Methods Manual, ASTM D3776 and individual buyer quality manuals (H&M, Inditex, PVH all publish their own QC requirements).
Does fabric GSM change after washing?+
Yes — washing typically increases measured GSM because the fabric shrinks in both length and width. A fabric that is 160 GSM before washing may be 170–175 GSM after a standard wash cycle, depending on shrinkage rate. This is why buyers increasingly specify 'after wash GSM' in their TDS, alongside pre-wash measurements. If the TDS specifies both, test samples in each condition. For jersey fabrics with 5% length shrinkage and 3% width shrinkage, post-wash area is approximately 8% smaller, which inflates measured GSM by roughly the same percentage.
How does GSM affect fabric cost per garment?+
Higher GSM means more fibre weight per square metre, which directly increases fabric cost. At a fabric price of $3.50/kg: a 160 GSM T-shirt consuming 1.4 m of 1.5 m-wide fabric weighs 160 × 1.5 × 1.4 ÷ 1000 = 0.336 kg → fabric cost $1.18. Upweight to 200 GSM: 0.420 kg → $1.47/pc — a 25% cost increase per piece for a 25% GSM increase. Use the Fabric Weight Calculator on this site to compute total lot weight from GSM, width and length.