Interlock or single jersey for undershirts?

Main answer

Single jersey is a lighter, stretchier knit made with one layer of loops that tends to curl at the edges, while interlock is a thicker double-layer knit that is smooth on both sides and more stable. For undershirts, choose single jersey for light, breathable value, and interlock for opacity, stability and a premium feel, comparing both at similar gsm.

What the knits are

Single jersey is the common T-shirt knit, with a smooth face and a looped back. Interlock is made by knitting two layers of rib interlocked together, which gives a smooth surface on both sides and a denser structure. Interlock needs more yarn and a special machine, which raises cost.

Feel and appearance

Single jersey is light, drapey and breathable, but may be translucent in pale colours and curl at hems. Interlock is smoother, thicker and more opaque, so it suits white and pale colours, and it lies flat when cut. It can feel warmer and drapes less, and it is heavier at the same look.

Stability and wear

Interlock tends to be more stable, with less curling, less twisting and a cleaner edge, which makes sewing and finishing easier. Single jersey can twist and shrink unevenly if not well finished. Both depend on yarn quality and finishing.

Checking before you commit

Ask for both fabrics as finished undershirts in white, wash them five times and compare opacity, hem curl, twist and shrinkage. Feel is easy to judge, but stability shows only after washing, and interlock’s extra cost should show up there.

Cost and comparison

Interlock usually costs more because it uses more yarn per square metre. Compare at similar gsm, as ASTM D3776 describes how mass per unit area is measured, and check opacity on a white sample. Ask for shrinkage and pilling results from the same method. See undershirt fabric guide and undershirts.