How should an underwear size chart be graded between sizes?

Short answer: Grading turns an approved base size into a size range by adding or subtracting set amounts, called grade increments, at key points such as waist, hip, thigh, rise and leg. Underwear is graded from body measurements, typically waist and hip, then converted into garment measurements that allow for fabric stretch. Increments are often larger at the waist and hip than at length points. The range must then be fit-tested at the extremes, not only in the base size.

Body chart versus garment specification

The customer-facing size chart usually lists body measurements, such as waist and hip in centimetres or inches, so customers can choose a size. The factory works from a garment specification, which lists finished garment measurements per size with tolerances. Because underwear stretches, the relaxed garment waist is usually smaller than the body waist; the amount depends on fabric and elastic. ISO 8559-1 defines how body measurements such as waist girth and hip girth are taken, which helps keep the body chart consistent.

Setting grade increments

• Waist and hip: the largest increments, reflecting body size steps.
• Thigh and leg opening: graded proportionally to the hip.
• Rise (front and back): smaller increments, as rise changes less between sizes.
• Inseam: often small or no grade.
• Pouch depth and width: graded modestly.

Grade rules should follow body data for your market, not a copied chart from another brand.

Allowing for stretch

Fabric and elastic stretch change how much negative ease is needed. A high-stretch modal may use more negative ease than a firmer cotton. When the fabric changes, the conversion from body to garment measurements may need to change too.

Checking the graded range

• Make samples in the smallest, base and largest sizes.
• Fit-test on people matching each body size.
• Check waist comfort, pouch, rise and leg opening.
• Wash and remeasure, since stretch and shrinkage affect large sizes noticeably.

A worked example

If the medium waist relaxed measurement is 72 cm and the grade is 6 cm per size, the large is 78 cm and the extra large 84 cm. Check that these relaxed measurements, stretched by the fabric and elastic, cover the body waist ranges in the size chart with comfortable tension, not at maximum stretch. Leg openings and rise usually need smaller increments than the waist, so grade them separately rather than scaling everything by the same ratio.

Measuring the graded samples

Measure each size flat and stretched, try them on testers near the middle and edges of each range, and adjust the grade before bulk.

Limits

Body proportions vary between markets and age groups. Return reasons and fit feedback should be used to refine grading after launch.

Facts, recommendations and uncertainty

Established: grading applies increments from a base size, with body charts for customers and garment specs for production. Recommendation: base increments on market body data and fit-test the extremes. Uncertain: whether your increments suit your customers until fit and returns data confirm them.

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.