Why do cotton undershirts get shorter after washing?

Short answer: Cotton jersey is knitted and finished under tension along its length, so the loops are stretched lengthwise. When the fabric is washed and especially tumble dried, the loops relax and the fabric shortens, often more in length than in width. Factors that increase length shrinkage include fabric finished under high tension, no or insufficient compacting at the mill, loose knits, heavy tumble drying and hot washing. For undershirts, which rely on length to stay tucked, even moderate shrinkage matters. Reduce it by specifying a maximum length shrinkage for the fabric, asking the mill for compacted or pre-shrunk finishing, testing the bulk fabric with ISO 5077 after washing and drying as on the care label, and adding a pattern allowance for remaining shrinkage. Tell customers how to wash and dry, and avoid hot tumble drying on the care label if tests show high shrinkage.

Why length shrinks more

• Fabric is pulled lengthwise during knitting, dyeing and finishing.
• Relaxation in washing and drying lets loops return to their natural shape.
• Width often changes less, and can even grow as length shrinks.

Controlling shrinkage

• Specify maximum length and width shrinkage in the fabric order.
• Ask for compacting or sanforizing equivalents for knits.
• Test each bulk fabric lot before cutting.
• Add a length allowance to the pattern for the remaining shrinkage.

Testing

Mark and measure specimens following ISO 3759, wash and dry with ISO 6330 procedures at the care label settings, and calculate dimensional change with ISO 5077. Test finished garments as well, since sewing and pressing can change results.

Care label and advice

If tumble drying causes high shrinkage, choose a care label that says line dry or tumble dry low, and explain this on the product page.

A worked example

As an illustration, if garment tests show length shrinkage well above width shrinkage after tumble drying, the brand could ask the mill for improved compacting, retest, and add the remaining length shrinkage to the pattern before the next order.

Explaining it to customers

State the care instructions clearly and give the body length per size on the product page. Customers who know the expected length can check it after washing, and complaints become easier to judge.

Limits

Some shrinkage is normal for cotton. Hot tumble drying at home can exceed lab conditions.

Facts, recommendations and uncertainty

Established: length shrinks more because of lengthwise tension in finishing. Recommendation: specify shrinkage limits, ask for compacting, test and allow in the pattern. Uncertain: customer drying habits.

Sources and further reading

• ISO 5077:2007 — Determination of dimensional change in washing and drying
• ISO 6330:2021 — Domestic washing and drying procedures for textile testing
• ISO 3759:2011 — Preparation, marking and measuring of fabric specimens and garments for dimensional change

Standards and regulations are referenced by their published scope. Test parameters, cycles and pass levels should be agreed with your laboratory for the specific product; legal requirements should be confirmed for each destination market.