A gym shirt should move with you without becoming something you have to think about. The right fit depends on whether you prefer a close performance shape or a more relaxed silhouette, but movement should never feel restricted.
Check the shoulders first
The fabric should sit cleanly across the shoulders without sharp pulling when you reach forward, press overhead or rotate your arms. A fitted shirt can sit close to the body, but it should not limit your normal training range.
Give the chest and torso room to move
A performance fit should follow your shape without making deep breathing uncomfortable. If the fabric strains heavily across the chest or constantly rides upward during rows, presses or pull movements, the fit may be too tight for the way you train.
Get the length right
The hem should give enough coverage to stay comfortable through overhead work, squats and bends without hanging so low that it bunches around the hips. The best length stays stable through movement rather than needing repeated adjustment.
Test it like you train
Raise both arms, rotate your torso, squat and mimic a few of your usual exercises. Check the underarms, sleeves and hem. If the shirt stays in place and your movement feels natural, the fit is doing its job.
A good gym shirt should feel intentional, keep its shape and let the session take priority. Fit should support movement, not compete with it.