Inseam on underwear is the length of the inner leg. For a boxer brief it is measured from the crotch seam to the bottom of the leg opening along the inside of the leg. Different brands offer short, mid and long leg lengths, and customers choose by style and by how the leg sits on the thigh. The numbers matter less than the method: if the measuring method is not written down, the factory and the buyer will measure differently.

Write the measuring method

State where the measurement starts and ends, and that the garment is laid flat and relaxed, not stretched. Measure the leg opening too, laid flat and at half width, because a leg opening that is too tight pushes the hem upward and one that is too loose can bag. Include a sketch in the tech pack with measuring points marked.

Should it grade by size

Larger sizes are for larger bodies, and a thigh is longer and fuller. Many brands increase inseam slightly across size groups so the leg sits in the same place on the thigh, while others keep one inseam for all sizes to keep the look the same. Decide by fitting each size group on real wearers, and write the graded inseam for each size into the size chart.

Allow for shrinkage

Knit underwear can shrink in length after washing, so the finished inseam can be shorter than the sample. Wash samples with a named procedure such as ISO 6330 and measure again. Ask the mill about stretch and recovery of the fabric with a method such as ASTM D2594 for low-stretch knits, since a leg that does not recover will ride up.

Tolerances and checks

Set a tolerance for inseam and leg opening in the spec, based on what the factory can hold. At inspection, measure inseam and leg opening on several pieces from each size, and compare with the approved sample.

What to write

Give inseam, leg opening, tolerance, shrinkage limit, and the measuring sketch. Approve fit samples in the smallest, middle and largest sizes, and record the finished inseam of each after one wash as the reference for bulk. If you later add a longer or shorter leg option, give it its own name and size chart rather than changing the existing style.