How should a work jacket be sized to layer over a hoodie?

Size a work jacket for its working condition — worn over layers — not for the bare body. As a rule of thumb, add about one size of chest ease relative to a standard fit: roughly 15–20 cm of total chest allowance over a hoodie-and-base-layer combination, so arms can reach forward without the jacket pulling at the shoulders.

Three specification tips: (1) check sleeve length and shoulder drop with the layers on — sleeves ride up noticeably over a bulky midlayer; (2) if the jacket must fit over a hood, verify the hood or collar clears it; (3) most importantly, request the fit sample evaluated over the actual midlayer, not on a bare model. Tell the factory what your workers or customers will wear underneath and put it in the tech pack — that single line prevents most fit complaints.