Glass, for any mirror you will genuinely train in front of. Acrylic flexes against the wall and hands back a wavy reflection, scratches under ordinary cleaning, and hazes with age. Glass stays flat and reads true. Acrylic earns its place only where weight or breakage rules glass out, such as a rented room or a wall behind heavy equipment.
A gym mirror has one job, which is telling you the truth about a hip hinge, a bar path or an overhead position. That depends completely on the reflection being undistorted. Acrylic sheet is thin and light, so it takes the shape of whatever it is fastened to, and a wall that is slightly out of plane telegraphs straight through as a slow bend in your own reflection.
Glass is rigid enough to hold its own plane. The wall still matters, and a bad one still gets shimmed and corrected, but the panel is not simply obeying it. That difference is the reason commercial studios have used glass for as long as they have existed.
It is the right answer in a few real situations. A wall or partition that cannot carry the weight. A garage setup where a bar might come down near the wall. A kids room. A rented space where nothing can be anchored permanently. A basement with no path wide enough to bring a large panel down.
Just know what comes with it: visible distortion, scratches from ordinary wiping, static that pulls dust onto the face, and yellowing where sunlight reaches it. Mirrored acrylic marketed as a lightweight upgrade is the same material behaving the same way, whatever the listing calls it.
Two things separate a gym installation from a bathroom mirror. The first is safety backing, a film bonded to the back of the glass that holds the pieces together if the panel is ever struck. The second is where the glass stops. Keeping the bottom edge up off the floor, on a rail or above the height a dropped plate can reach, takes the mirror out of the impact zone entirely, and a bench or rack parked in front of the glass is inexpensive insurance.
Long walls go in as multiple panels, set with mastic plus mechanical support so nothing depends on adhesive alone. The seams get placed away from where you normally stand, since a joint sitting in the middle of your working sightline is the one thing people notice afterward.
It should not. Stopping the glass above the floor keeps the bottom edge clear of dropped plates and out of standing water when the floor gets mopped, and a rail or trim there protects it further. Running full height is a studio look rather than a requirement, and nothing about checking your form needs it.
Warm humid air meeting cool glass condenses on it, exactly as it does in a bathroom. That clears on its own once air moves. What does not clear is the effect of years of that humidity on the back edge, so a gym room benefits from real ventilation and a mirror set to breathe against the wall.
Before a mirror wall is quoted, somebody should put a straightedge across that wall and walk the route the glass has to travel to reach it. On residential work that visit carries no charge, and it is what keeps a long wall from arriving in panels that will not make the turn.
