When underwear feels tighter after washing, two parts can be changing. The cotton body fabric can shrink, which shortens the rise and narrows the leg and hip. The waistband elastic can also change: some elastics shrink, and heat can affect the stretch fibre inside them. A hot wash makes both more likely. Customers who wash above the label temperature are outside the care instructions, but if many customers report the same thing, it is worth checking how close to the edge the product is even at the labelled temperature.

Measure the body fabric

Wash and dry garments with a named procedure such as ISO 6330, or measure garments using AATCC TM150, which AATCC revised in its 2026 manual. Run the test at the labelled temperature and, separately, at the higher temperature customers seem to use. Measure waist relaxed, hip and rise before and after. This shows how much of the change comes from the fabric.

Measure the elastic

Ask the elastic supplier for stretch and recovery data, and test washed and unwashed lengths of the elastic using a named method such as ASTM D4964 for elastic fabrics. Measure the relaxed length and the force at a set extension. If the elastic shrinks or gets stronger after washing, the waist will feel tighter even if the body fabric is stable.

Check how the waistband is sewn

If the elastic is stretched while sewing, the waistband is set at a ratio of elastic to fabric. A small change in that ratio between production lines can change the finished waist. Compare washed garments from different lines and different elastic lots.

Fix and label

Depending on the results, use a more stable or pre-shrunk fabric, a different elastic, a changed sewing ratio or a pattern allowance. Under the FTC Care Labeling Rule, 16 CFR Part 423, the label must give instructions for regular care and warn about procedures that would harm the product. If a hot wash would shrink the briefs out of size, the label should clearly state the safe maximum temperature.

Keep washed reference garments at both temperatures from approval, so future lots can be compared.