Mirror glass thickness explained: 3mm, 4mm or 5mm?

Almost every domestic mirror starts as float glass, the same flat sheet used in windows, made by floating molten glass on a bath of molten tin so both faces come out flat and parallel. A float glass mirror is that sheet with a reflective coating applied to the back. The coating is a thin layer of silver on most quality mirrors, or aluminium on the budget ones; silver reflects more light and gives a truer colour, which is why a good mirror looks bright and an aluminium one looks slightly grey.

Over the silver go two or three layers of protective paint. On older mirrors that included a copper layer, which is where the familiar black spots and dark edges on an old bathroom mirror come from as the copper corrodes. Modern mirrors made to the British and European standard BS EN 1036, which covers silver-coated float glass mirrors for internal use, are copper-free and resist that edge spotting far better. If a product page says copper-free backing, that is what it means.

The last layer on a decorative mirror is usually a safety backing film, covered below. Then the sheet is cut to size, the edges are polished or bevelled, and it goes into a frame or onto a backboard.

Glass bends. Not much, but a sheet held only at its edges sags under its own weight, and a sheet glued to a backboard follows every ripple in that board. The eye does not notice a bend of half a millimetre across a 60cm bathroom mirror. Across 170cm it is a different story, because a reflection magnifies small curvature into a visible stretch or squash. That is the answer to why does my mirror make me look wider, or taller, or oddly narrow at the hips: the glass is not flat, and it is not flat because it is thin.

The physics is unforgiving. The stiffness of a sheet rises with the cube of its thickness. Go from 3mm to 4mm and the glass resists bending about 2.4 times as well. Go to 5mm and it is about 4.6 times stiffer than 3mm. At 6mm it is 8 times stiffer. So a modest step in thickness buys a large step in flatness, which is why mirror distortion is almost always a 3mm problem and rarely a 5mm one.

The frame matters too. A welded steel frame or a solid timber frame holds a 4mm sheet flat over 180cm. A thin extruded aluminium frame with a plastic corner joint does not add much, and a frameless mirror adds nothing, which is why those two cases push you to 5mm.

Weight is the hidden cost of thicker glass. It decides whether one person can carry the mirror, what fixings the wall needs, and how much the courier charges. The figures below are for the glass alone at the three sizes that cover most of what we sell. Add the frame: a slim aluminium frame is 2 to 4kg, a welded steel frame 5 to 12kg, a solid oak frame 10 to 20kg, and an MDF backboard on a frameless shape 4 to 8kg.

The practical thresholds are 15kg, above which we recommend a two-person lift and keyhole or batten fixings rather than a wire, and 25kg, above which the mirror should go into masonry or timber studs, never into plasterboard anchors alone. The how to hang a mirror guide works through the fixings for each weight.

Three technologies make mirror glass safer when it breaks, and only one of them is common on decorative mirrors. A safety backed mirror has a thin polymer film bonded to the painted back of the glass. If the mirror cracks, the film holds the fragments in place instead of letting them fall as shards. It adds almost no weight, costs little, and is what every mirror on this site carries. It does not make the glass harder to crack; it changes what happens afterwards.

Toughened glass is heat-treated so that it breaks into small blunt granules rather than shards. It is used for shower screens and glass doors, and sometimes for gym mirrors. It is rare on decorative mirrors because toughening slightly warps the sheet, which shows in a reflection, and because the glass cannot be cut or drilled after treatment, so every size has to be made to order. Laminated glass sandwiches a plastic interlayer between two sheets, the way a car windscreen does. It is the safest of the three and also the heaviest and dearest, and you will only find it on bespoke panels.

For a full length mirror in a bedroom or hallway, a 4mm or 5mm safety backed float glass mirror is the right answer. Ask for it by name, and look for the film mentioned on the product page. Where a listing says nothing about backing, assume there is none.

A growing number of cheap full length mirrors are not glass at all. They are polymer sheets, usually acrylic or PET, with a reflective coating, often sold with words like shatter-resistant or child-safe. They weigh a third of glass, they will not cut you if they break, and they cost very little. For a nursery, a play room, a gym where a ball might hit them, or a rented room where you cannot fix anything to the wall, they have a place.

They are not a substitute for a glass mirror anywhere you care about the reflection. Polymer sheets are far less rigid than glass of the same thickness, so a 150cm acrylic mirror bows and gives a funfair reflection unless it is glued flat to a board across its whole area. The coating scratches with a cloth and a bit of grit. The surface clouds over a few years in sunlight. If you see a 160cm mirror for £20, it is almost certainly acrylic, and the reviews that mention a wobbly or warped reflection are describing exactly this.

The edge of a mirror tells you how it was finished. A polished edge is ground smooth and square; it is the standard finish inside a frame. A bevelled edge is ground to a slope, usually 10 to 25mm wide, so the edge catches the light in a band around the mirror. Bevelling only works properly on 4mm glass and thicker, because there has to be enough depth to grind the slope, and it is one of the reasons a bevelled frameless mirror is nearly always 5mm.

Black spots and dark edges, sometimes called mirror rot or desilvering, happen when moisture reaches the silver through the edge or through a scratch in the backing paint. Old copper-backed mirrors in bathrooms are the classic case. A modern copper-free, safety backed mirror with sealed edges will go decades without spotting as long as the edges are not left sitting in water and the back is never cleaned with ammonia or vinegar, which can creep around the edge and attack the coating. The how to clean a mirror guide covers the safe method.

You do not need to take a mirror out of its frame to find out what it is made of. Hold the edge of a coin against the glass. You will see the coin and its reflection, with a gap between them. The reflection comes from the silver on the back of the glass, so the gap is the thickness of the glass, viewed at an angle. Compare the gap to the coin: on a 3mm mirror the gap is thinner than a pound coin; on a 5mm mirror it is noticeably wider than one. For a precise reading, hold a ruler against the coin and read the gap, then adjust for the angle you are looking from, which is why the straight-on comparison with a coin is quicker.

Two other quick checks. Tap the glass with a knuckle: thin glass rings, thick glass gives a duller note. And look at a straight line, a door frame or a skirting board, in the reflection while you walk slowly past the mirror. If the line stays straight, the glass is flat. If it bows or wobbles as you move, the glass is bending, which on a tall mirror almost always means 3mm or acrylic.

Every mirror on this site is float glass with a silver coating, copper-free backing paint and a safety film. We list 4mm glass on framed mirrors up to 180cm tall, because in a rigid aluminium alloy or welded steel frame 4mm stays flat at that height and keeps the weight where one person can still manage the smaller sizes. That covers the arch mirror range, the free standing mirror range and most of the wall mirrors.

We specify 5mm on three kinds of mirror: frameless and pond shapes, where there is no frame to add stiffness, so the whole flatness of the wavy mirror range comes from the glass and its backboard; mirrors taller than 180cm, where the extra rigidity shows in the reflection and the large full length mirror sizes carry it; and the bevelled designs, which need the depth to grind the edge.

We do not list 3mm glass on anything over 120cm, and we do not list acrylic full length mirrors. Both are cheaper to buy and dearer to live with. If a mirror you want is not here in the thickness you want, email us; the same suppliers usually offer a 5mm version of a 4mm design to order. The size guide helps you settle the height first, which decides the thickness, and the delivery page explains how the heavier sizes travel.