To determine approximate GSM (grams per square meters) of a given fabric, you can use the following formula. One of my colleagues used to use this formula in the testing lab. In case different counts of warp thread or weft thread are used then this formula will not give you correct GSM. It is also known as conversion formula from fabric construction to fabric weight.
For example, construction of a fabric is 40 X40 /120 X 60 and warp and weft crimp is 6%. Then GSM of this fabric will be,Fabric GSM = (EPI/warp count + PPI/Weft count) x (100+crimp %) X 0.2327
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| Woven Fabric |
GSM = (120/40 + 60/40) X (100+6%) X 0.2327
GSM =104.77
Also See: What is Crimp% in Fabric and How to Measure Warp and Weft Crimp%
I found others to use this formula with different multiplying factors.
Fabric GSM= ((EPI/warp count X 1.1) + (PPI/weft count x 1.04)) x 23.5 By Dharmi
In the above formula, 1.1 and 1.04 are warp and weft crimp factors respectively.