Glass · Furniture

Glass thickness for a table top or shelf

The weight spread over a table top is almost never what breaks it — a single concentrated load is. A 800 mm shelf under a normal spread load sees around 7 N/mm²; put one 50 kg object in the middle of the same shelf and it sees nearly three times that. This page checks both and tells you which one governs, because most thickness guides quietly only do the easy one.

The piece

How it is supported

Four legs under the corners of an unframed top is a different case again; see the limits below.

Clear span (mm)

The other side (mm)

Span is the distance between supports. For a four-edge top, the short side is what does the work.

The glass

Thickness (mm)

Make-up

Glass type

Furniture glass that people touch should be safety glass. See the note below — this is not a detail to save money on.

What goes on it

Spread load (kg/m²)

Books on a shelf run 60–100 kg/m². A dining top rarely exceeds 100. Use 150 if you want margin.

Point load (kg)

Spread over (mm)

The single heavy thing: a TV foot, a stack of plates, someone sitting on the edge. A narrower contact means a higher stress — a spike heel is worse than a suitcase.

Load duration for k_mod

Deflection limit

No standard sets this for furniture. It is about how it looks and how it feels underhand: glass that visibly sags reads as unsafe even when it is not.

—

Stress · point load

— N/mm²

Stress · spread load

— N/mm²

Deflection

— mm

Thinnest that passes

— mm

same type and make-up

How this is calculated

Everything is worked out per millimetre of width, as a strip spanning between the supports. The spread load on a four-edge top uses plate theory; the point load is always treated as a strip on the short span, which is the conservative reading — a real plate shares that load sideways and comes out lower.

two edges, spread load q: σ = 0.75 · q · L² / t² w = 5·q·L⁴ / (32·E·t³) two edges, point load P: σ = 1.5 · (P/c) · L / t² w = (P/c)·L³ / (4·E·t³) four edges, spread load q: σ = β · q · b² / t² w = α · q · b⁴ / D c = the width the point load is spread over, D = E·t³/(12(1−ν²)), E = 70 000, ν = 0.23

Two things fall out of that, and they are why this page exists:

Stress goes with the square of the span but the inverse square of the thickness. Doubling the span quadruples the stress; going from 8 to 10 mm cuts it by a third. Shortening the span is always the cheapest fix.
Deflection goes with the fourth power of the span. A shelf twice as long sags sixteen times as much. This is why long glass shelves look bent even when they are perfectly safe.

Load duration matters more here than anywhere else

Glass gets weaker the longer a load sits on it. EN 16612 handles that with k_mod = 0.663 · t^−1/16 for a load lasting t hours, and the range is brutal: 1.00 for a five-second shove, 0.74 at ten minutes, 0.44 over three months and 0.29 for something permanent. A shelf carrying books for a decade has less than a third of the strength it would have resisting a gust. Most furniture guides quietly use the short-term figure.

Worked example you can check by hand

800 mm shelf, 300 mm deep, 10 mm toughened, two edges, 150 kg/m² spread and a 50 kg object over 300 mm.

spread: q = 150 · 9.81 / 1e6 = 0.001472 N/mm² σ = 0.75 · 0.001472 · 800² / 10² = 7.07 N/mm² w = 5 · 0.001472 · 800⁴ / (32 · 70000 · 10³) = 1.35 mm point: P = 50 · 9.81 = 490.5 N over c = 300 mm → 1.635 N/mm σ = 1.5 · 1.635 · 800 / 10² = 19.6 N/mm² w = 1.635 · 800³ / (4 · 70000 · 10³) = 2.99 mm

The point load produces 2.8 times the stress of the spread load, on the same shelf. That is the whole argument of this page.

The part that is not about arithmetic

A table top or a shelf is furniture people touch, lean on and let children near. Passing a stress check is not the same as being safe to break:

Annealed glass breaks into blades. It is the cheapest and it is the wrong answer for anything at hand height, whatever the numbers say.
Toughened glass dices into blunt fragments and is the normal choice for tops and shelves. It is also the one that occasionally fails spontaneously from a nickel sulphide inclusion, years later, for no reason you will ever see.
Laminated glass holds together when it breaks. For a top over someone's legs, or a shelf above head height, it is the grown-up answer.
Edges do the breaking. Ground and polished edges are stronger than a raw cut one, and a chipped edge is where the crack will start. Never use a piece with a nibbled corner because it is only furniture.

What this page does not do

No corner supports. An unframed top on four legs concentrates everything at the corners and needs a plate analysis with those boundary conditions. This page does not do it, and the answer is not close.
No impact. Someone dropping a pan, a child pulling on the edge, a chair knocking it: none of that is a static load and none of it is here.
No holes, notches or cut-outs. Every one of those is a stress raiser and changes the answer completely.
No cantilever. Glass overhanging its support is a different calculation and a much less forgiving one.
Nominal thicknesses, and for laminated pieces the conservative bound with no shear transfer through the interlayer.

Read this. A pre-dimensioning aid. For a piece of furniture that will be sold, or one that sits above people, get the specification from someone who takes responsibility for it. Glass is unforgiving of optimism.