Why does seamless underwear have fewer sizes?

Short answer: Seamless underwear is knitted as a tube on circular knitting machines, with the shape and different zones created by changing stitches and yarn tension rather than by cutting and sewing panels. The tube diameter is fixed by the machine's cylinder, so one machine covers a band of body sizes, and fit is adjusted by knit structure and stretch rather than by pattern grading. Each extra size can mean a different machine diameter or a separate programme, plus its own sampling and testing, which raises cost and minimums. That is why seamless ranges usually have fewer, broader sizes. A wider range is possible but needs more development, and every size still has to be fit-tested.

How seamless knitting works

• A circular machine knits a tube in the body diameter it was built for.
• Zones such as the waistband, pouch and legs are formed by changing stitch type, density and yarn feed.
• The garment is then finished with minimal seams, usually at the crotch and sometimes the waistband.

Why fewer sizes

• Machine diameter: each cylinder size suits a range of body sizes.
• Programme development: each size needs its own knitting programme and samples.
• Stretch: seamless fabrics stretch a lot, so one size can cover more bodies.
• Minimums: each size adds setup time, so small runs per size are costly.

Trade-offs for customers

Broad sizes fit many people comfortably, but customers at the edges of a size band may find the fit too tight or too loose. Leg length and rise cannot change as much between sizes as in cut-and-sew garments.

How to widen the range

• Use more machine diameters, if the factory has them.
• Develop separate programmes for extra sizes.
• Combine seamless bodies with cut-and-sew waistbands or legs.
• Focus on sizes that cover most of your customers first.

Size charts

Give clear waist and hip ranges for each size, based on consistent body points such as those defined in ISO 8559-1. Be honest that one size covers a range.

Testing

Fit-test customers at both ends of each size band. Wash and measure with ISO 5077, since seamless garments can change dimensions after washing. Compare stretch and recovery between sizes with a method such as ASTM D4964.

Limits

The available sizes depend on the factory's machines and experience. Ask early rather than assume.

Facts, recommendations and uncertainty

Established: seamless underwear is knitted as tubes on fixed-diameter machines, so sizes are broader and fewer. Recommendation: ask which diameters the factory has and fit-test the edges of each size band. Uncertain: whether a wider range is economical for your volumes.

Sources and further reading

• ISO 8559-1:2017 — Size designation of clothes: anthropometric definitions for body measurement
• ISO 5077:2007 — Determination of dimensional change in washing and drying
• ASTM D4964 — Tension and elongation of elastic fabrics

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.