Underwear waistbands are designed to be smaller than the body when relaxed, and to stretch onto the body. If a size chart shows the relaxed garment waist, customers compare it with their own waist, see a smaller number and size up. The garment then sits loose, which is what your customers describe. For shoppers, a size chart should normally be based on body measurements, such as waist and sometimes hip, for each size.

Two sets of measurements

Keep two separate documents. The customer chart lists body waist and hip ranges per size. The factory specification lists garment measurements: relaxed waist, extended waist, leg opening, rise and inseam with tolerances. Do not copy garment measurements into the customer chart without explaining them.

Linking body and garment

The link between the two is stretch. Measure the relaxed and extended waist of approved samples and check how much the band stretches to fit the body range for each size. Elastic fabrics and bands can be tested with methods such as ASTM D4964 for elastic fabrics or ASTM D2594 for knits with low power, so you know how much force the band applies across the range.

Check after washing

Cotton-rich boxer briefs may shrink, and elastic may change after washing and drying. Wash samples with a named procedure such as ISO 6330 at your care label settings and measure again. Base the chart on washed garments if they change noticeably.

Wear trials

Ask people whose waist sizes fall at the low, middle and high end of each size range to try the samples. If someone at the top of a range finds the band tight, adjust the ranges, not just the garment.

Presenting the chart

Show body ranges in both centimetres and inches, explain how to measure the waist, and say what to do between sizes. If you also publish garment measurements, label them clearly as flat or relaxed garment measurements.

Keep the chart and the factory specification under the same version control, so a pattern change triggers a chart review.

Finally, read your returns data by size. If many customers return a size as too loose or too tight, compare their stated waist with the chart range before changing the pattern, since the chart, not the garment, may be the problem.