Elastane makes the fabric pull in, so the finished width is smaller and the GSM higher than for the same fabric without elastane. A cotton blend with 3 to 5% elastane is usually finished narrower, and the GSM should be checked after relaxation. The width can also differ after washing.
For stretch knits, ask for both the open width and the finished GSM. The usable cutting width is the one measured after the fabric has relaxed. Fabric stretches more across than along, so patterns should be cut with that in mind, especially for garments that must fit closely to the body.
elastanewidthgsm
