Adding a small share of elastane (spandex) to cotton jersey lets a fitted tee stretch with the body and come back afterwards. Without it, cotton jersey stretches and tends to stay stretched. The fibre percentage on the quote tells you what is in the fabric, but not how it behaves: stretch, recovery and growth depend on the yarn, the knit structure, the fabric weight, how the elastane is fed and how the fabric is finished and relaxed.

Cotton spandex jersey: define the three behaviours

Stretch is how far the fabric extends under a set load. Recovery is how much of that extension it gives back. Growth, sometimes called set, is the permanent extra length or width left after repeated stretching. A fitted tee needs enough stretch to put on and move in, and recovery good enough that the chest, waist and neckline do not grow. Ask the mill for stretch, recovery and growth in both directions, not only across the width.

Which test method fits

ASTM D2594 covers stretch properties of knitted fabrics with low stretch power, which includes many cotton knits with a little elastane. ASTM D4964 covers tension and elongation of elastic fabrics using a constant-rate-of-extension tester. Ask your lab which method suits your fabric, and use the same method and load for every lot, so results can be compared. Do not compare numbers from different methods.

Test after washing, not only on the roll

Fabric that recovers well on the roll can behave differently after laundering. Wash the fabric or a finished tee with a named procedure such as ISO 6330, then repeat the stretch and recovery test. Measure the garment too: chest width and length before and after washing and after being stretched over a body form or a standard stretch cycle.

What to write in the spec

Put the fibre content, yarn count, knit type, weight with tolerance, finished width and the test method into the fabric spec. Add your own pass limits for stretch, recovery and growth, based on a sample you have approved and worn. Limits are your decision, not a universal figure, so tie them to the approved sample and keep that sample sealed as the reference.

Label and quality notes

Elastane must appear on the fibre content label by its generic name if it is present in the required amount; the FTC rules at 16 CFR Part 303 set how fibres are named and ordered. Also ask the mill to confirm the elastane percentage is controlled from lot to lot, since lower elastane in one lot is a common cause of tees that do not recover like the sample.

Finally, check the first bulk lot before cutting by measuring a few rolls against the approved fabric.