Shelf load
How much weight can a glass shelf hold?
Span, depth, thickness and how the shelf is held, to the load it carries spread evenly, with the formula, the design stress and the sag check printed beside the answer. A guide for planning, and one click to price the shelf.
Spread evenly along the shelf
about 9.2 kg
11.0kg in total, of which the shelf itself is 1.8kg. The span/500 sag guide (1.2mm) sets the limit here, before the glass is anywhere near breaking.
- Strength:
- W = 8 x σ x Z / L = 8 x 20 x 1200 / 600 = 320 N, with Z = b t² / 6
- Sag:
- W = 384 E I δ / (5 L³) = 384 x 70000 x 3600 x 1.2 / (5 x 600³) = 108 N, with I = b t³ / 12
- Design stress:
- σ = 20 N/mm² (toughened), a guide figure in trade use, not a certified value.
A guide for planning, never a structural certification. Carrying something heavy, or fixing into a wall you are not sure of? Ask us for a specification.
The link reopens the calculator with the same shelf.
The maths, stated
The formulas the calculator solves
A shelf carrying a load W spread evenly along a span L, with depth b and thickness t. E is the stiffness of glass, 70000 N/mm² (70 GPa). Two limits are worked out and the smaller one is the answer.
A bracket at each end
- Bending moment
- M = W x L / 8
- Centre sag
- δ = 5 x W x L³ / (384 x E x I)
Held in a channel at each end
- Bending moment
- M = W x L / 12
- Centre sag
- δ = W x L³ / (384 x E x I)
Section properties
- Section modulus
- Z = b x t² / 6, so stress σ = M / Z
- Second moment of area
- I = b x t³ / 12
The two limits
- Strength
- σ reaches the design stress: 20 N/mm² toughened, 7 N/mm² annealed. Guide figures in trade use, printed so you can see them, not a certified value.
- Sag
- δ reaches span/500, the usual visible-sag guide for shelving.
A shelf held along its whole back edge in a wall channel, with nothing at the ends, is a cantilever off the depth and a different sum. The calculator does not cover it; ask us for a specification and we will work it for your shelf.
Worked examples
Three shelves, worked through
Bathroom shelf
500 x 120mm, 6mm toughened, a bracket at each end
about 8.6 kg
Sag governs at 1.0mm; the shelf itself weighs 0.9kg.
Alcove bookshelf
900 x 250mm, 10mm toughened, a bracket at each end
about 22.6 kg
Sag governs at 1.8mm; the shelf itself weighs 5.6kg.
Display shelf in channels
700 x 200mm, 8mm toughened, held in a channel at each end
about 71.8 kg
Bending stress governs; the shelf itself weighs 2.8kg.
Choosing the thickness first? The thickness by use guide gives the trade-standard pick for a shelf by span. Weighing the panel for the brackets? The glass weight calculator does that.
Straight answers
Shelf load questions
How much weight can a glass shelf hold?
It depends on the span between the brackets far more than on anything else: the load a shelf can carry falls in proportion to the span for strength and with the cube of the span for sag. The calculator solves the standard beam formulas for your span, depth and thickness, with a design stress of 20 N/mm² for toughened glass and 7 N/mm² for annealed, and reports the smaller of the strength limit and the span/500 sag limit.
What is the formula?
For a shelf resting on a bracket at each end carrying a load spread evenly along it, the bending moment is W x L / 8 and the centre sag is 5 x W x L³ / (384 x E x I). With a section modulus Z = b x t² / 6 and a second moment I = b x t³ / 12, solving M / Z = design stress gives the strength-limited load and solving the sag equation for span/500 gives the sag-limited load. A shelf held in a channel at each end uses W x L / 12 and W x L³ / (384 x E x I).
Is this a structural certification?
No. It is a planning guide using trade design stresses, not a design to BS EN 16612 or a calculation stamped by an engineer. It assumes a load spread evenly along the shelf, glass with clean polished edges, and brackets that themselves hold the load. A heavy object at the centre, a chipped edge or a weak fixing all reduce what the shelf carries. Ask us for a specification if the load matters.
Why does the sag limit come before the glass breaks?
Glass is stiff but it is far stronger than it looks, so on most shelves a visible bow appears well before the bending stress reaches the design value. The calculator applies span/500 as the sag ceiling, the usual aesthetic guide for shelving, and tells you which of the two limits governed.
Toughened or annealed for a shelf?
Toughened. It carries roughly three times the design stress of annealed glass, so a thinner shelf holds more, and if it ever breaks it crumbles rather than dropping shards. Every glass shelf we cut is toughened unless you ask otherwise.