Glass · Furniture
Glass shelf thickness and deflection calculator, for shelves and table tops
An 800 × 300 mm glass shelf on two supports, with 150 kg/m² on it and a 50 kg object resting on 300 mm in the middle, needs 10 mm toughened glass, and it sags 4.3 mm. 8 mm would be strong enough, at 60 % of its strength, but it would sag 8.5 mm, more than the span / 100 a shelf can show before it looks bent. The heavy object does most of the damage: 19.6 N/mm² against 7.1 from the whole spread load, and most thickness guides check only the spread load. Type your own span, loads and glass below for the thinnest stock thickness and its deflection; the tables underneath give what each thickness carries.
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Glass shelf load tables
What a toughened glass shelf on two supports carries, for a load that stays on it (kmod = 0.29) and a sag of no more than span / 100. These are the calculator's own figures: every cell is worked out by the same code as the result above. Laminated or annealed glass, a table top in a frame, a stricter sag limit: type it in above.
Spread load, kg/m²
| Span, mm | 5 mm | 6 mm | 8 mm | 10 mm | 12 mm |
|---|---|---|---|---|---|
| 500 | 455 | > 500 | > 500 | > 500 | > 500 |
| 600 | 260 | 455 | > 500 | > 500 | > 500 |
| 700 | 165 | 285 | > 500 | > 500 | > 500 |
| 800 | 110 | 190 | 455 | > 500 | > 500 |
| 900 | 75 | 135 | 320 | > 500 | > 500 |
| 1000 | 55 | 95 | 230 | 455 | > 500 |
| 1200 | 30 | 55 | 135 | 260 | 455 |
| 1400 | 20 | 35 | 85 | 165 | 285 |
One object in the middle, kg, on a foot 100 mm wide
| Span, mm | 5 mm | 6 mm | 8 mm | 10 mm | 12 mm |
|---|---|---|---|---|---|
| 500 | 14 | 24 | 58 | 94σ | > 100 |
| 600 | 9 | 17 | 40 | 79σ | > 100 |
| 700 | 7 | 12 | 29 | 58 | 97σ |
| 800 | 5 | 9 | 22 | 44 | 77 |
| 900 | 4 | 7 | 18 | 35 | 60 |
| 1000 | 3 | 6 | 14 | 28 | 49 |
| 1200 | 2 | 4 | 10 | 19 | 34 |
| 1400 | 1 | 3 | 7 | 14 | 25 |
σ marks a figure set by the strength; every other one is set by the sag. “> 500” is more than 500 kg/m², over three times a shelf full of books, and “> 100” more than 100 kg on one foot: there the brackets, pins or rails decide, so check what they are rated for. With both loads at once the two shares add up: half of each figure, or a third of one and two thirds of the other, still passes.
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 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 kmod = 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, 0.41 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.
Strength: the method of EN 16612
The design strength is worked out in the form of EN 16612, with the edge and surface factors at 1.0, and the loads are taken as they are, with no load factor:
EN 16612 is written for glazing in buildings and does not cover furniture: this page uses its method and its coefficients, which is not a claim that a shelf or a table complies with it or with any other standard.
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.
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.
Questions people ask
- How thick should a glass shelf be?
- Above all it depends on the span between the supports, then on the load and on the glass. Toughened glass on two supports, with 100 kg/m² of books on it for good and no more sag than span / 100: 5 mm up to 800 mm of span, 6 mm up to 950 mm, 8 mm up to 1300 mm and 10 mm up to 1650 mm. One heavy object changes that a lot: add 20 kg on a 100 mm foot in the middle and 8 mm only reaches 750 mm, 10 mm 1000 mm. The calculator gives the thinnest stock thickness for your own span and loads.
- How much weight can a glass shelf hold?
- Less than its spread-load figure suggests, because of the heavy objects. An 8 mm toughened shelf 800 mm between supports carries 455 kg/m² spread evenly, about 109 kg on an 800 × 300 mm shelf, but only 22 kg as one object in the middle on a 100 mm foot (a load that stays, a sag of up to span / 100). With a spread load the glass is rarely what gives out first: the brackets, pins or rails are, so check what they are rated for.
- What glass do display cabinet shelves need?
- Toughened glass with ground or polished edges. A display cabinet shelf 600 mm between its supports, in toughened glass, carries 260 kg/m² spread or a 9 kg object in the middle on a 100 mm foot in 5 mm, 455 kg/m² or 17 kg in 6 mm, and 40 kg in 8 mm. A shelf on pins near its four corners is close to the two-edge case of this page only if it is clearly longer than it is deep; a deep or square shelf on corner pins bends more than that, and this page does not calculate it.
- How thick should a glass table top be?
- For a top sitting in a frame that holds all four edges, 1200 × 800 mm, with 150 kg/m² and a 50 kg object over 300 mm left on it: 8 mm toughened. With 150 kg/m² and someone leaning 100 kg on it, both counted as ten-minute loads: 10 mm. A top laid on four legs at its corners, with no frame, is a different calculation that this page does not do, and over people's legs laminated glass is the safer answer.
- Why does a glass shelf sag, and how much is too much?
- The sag grows with the fourth power of the span and falls with the cube of the thickness: with 100 kg/m² of books an 8 mm toughened shelf sags 1.75 mm over 800 mm and 4.28 mm over 1000 mm. No standard sets a limit for furniture; this calculator uses span / 100 (8 mm on an 800 mm shelf) and offers span / 175 and span / 250 for a shelf that must look flat. In the spread-load table on this page the sag decides every figure, not the strength.
- Toughened or laminated glass for a shelf or a table top?
- Toughened for nearly every shelf and top: it breaks into small blunt pieces, and under a load that stays its design strength here is 69.75 N/mm², against 7.25 for annealed glass. Laminated glass holds together when it breaks, so it is the better answer over people's legs or above head height; this calculator takes it as two plies with no shear transfer through the interlayer, on the safe side. Annealed glass breaks into blades: not for anything people touch, whatever the numbers say.