Sizes are stated in centimetres as half chest measurements — the width of the garment, not the body.
To measure: lay the garment flat, then measure straight across the front, side-seam to side-seam, starting a few centimetres below where the sleeve joins the body.
Because it's a flat measurement, double it to compare against an actual chest size. For example, a 50cm half chest measurement doubles to 100cm — a fit that would sit skin-tight on someone with a 100cm chest. For a looser fit, look for a half chest measurement a few centimetres larger than half your actual chest size.
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For more on sizing across our full range, see our size guides.