How wide a size range can seamless underwear cover?

Main answer

Seamless knits stretch, so fewer sizes can cover more bodies than cut-and-sew garments, but each size still has a limit, so define the body range for every size, test wearers at both extremes and avoid claims such as one size fits all. A wide range works only if the knit recovers well and the garment does not become too tight or too loose.

Why seamless covers more

A seamless garment is knitted as a tube with elastane and zones of different density, so it can stretch to fit a range of body measurements. This allows fewer sizes, lower stock and simpler ranges. But stretch has limits: at the extremes the knit thins, becomes see-through and loses recovery.

Define the range per size

For each size, state the body measurements it should fit, such as waist and hip range, and the garment’s measurements relaxed and at stated stretch. Choose ranges so that neighbouring sizes overlap slightly. ASTM D2594 covers tension testing of low-power stretch knit fabrics, which is relevant to how the fabric stretches.

Test the extremes

Test each size on people at the low and high end of its range, and record comfort, marks, transparency and rolling. Check how the waistband and leg openings behave. Repeat after washing, because recovery changes. Adjust the number of sizes if the extremes do not work.

Stock and returns

Fewer sizes lower stock but can raise returns if the range is too wide. Compare the cost of an extra size with the cost of returns and discounts, using your own data after the first sales period, and adjust the range for the next order.

Labelling and claims

Show the body range for each size on the product page and label, and use clear words. Avoid claims of fitting everyone. Compare with your returns, which tell you if the range is working. For very wide ranges, add a size, since returns cost more than stock. See seamless knitting construction and seamless apparel sourcing.