Thickness
How thick does an acrylic panel need to be?
Span, load and edge support decide it, and the relationship is not linear — stiffness scales with the cube of thickness, which is why a rib often beats a millimetre.
Thickness
Span, load and edge support decide it, and the relationship is not linear — stiffness scales with the cube of thickness, which is why a rib often beats a millimetre.
Stiffness of a flat panel scales with the cube of its thickness. Double the thickness and the panel becomes eight times as stiff, not twice.
This has a practical consequence that surprises people: the difference between 6 mm and 8 mm is much larger than it looks on a price list. A shelf that visibly sags at 6 mm is usually fine at 8 mm, and going to 10 mm is often overkill.
It also explains why a rib is so effective. A 20 mm upstand along the front edge of a 5 mm shelf adds stiffness out of proportion to the material it uses, because it moves material away from the neutral axis. Gluing a rib onto a thin panel can be cheaper than buying a thicker panel — and lighter.
| Thickness | Relative stiffness | Relative weight |
|---|---|---|
| 3 mm | 1.0× | 1.0× |
| 5 mm | 4.6× | 1.7× |
| 6 mm | 8.0× | 2.0× |
| 8 mm | 19× | 2.7× |
| 10 mm | 37× | 3.3× |
| 12 mm | 64× | 4.0× |
A panel supported on all four edges is dramatically stiffer than the same panel supported on two, and both are stiffer than a cantilever. The support condition often changes the answer by a factor of three or more, which is more than a thickness jump would give.
| Support | Effective stiffness | Typical use |
|---|---|---|
| All four edges fixed | Highest | Recessed panel, framed window |
| All four edges simply supported | High | Shelf in a frame, display panel in a groove |
| Two opposite edges | Medium | Shelf on two brackets |
| Two adjacent edges | Low | Corner-mounted signage |
| One edge (cantilever) | Lowest | Divider panel fixed along one side |
For light loads — display items, small products, signage — these spans are a reasonable starting point with all four edges supported. Halve the allowable span if only two edges are supported.
| Thickness | Comfortable span (4-edge) | Max span (4-edge) | Typical use |
|---|---|---|---|
| 3 mm | 200 mm | 300 mm | Small sign, backing panel |
| 4 mm | 250 mm | 400 mm | Brochure holder, small shelf |
| 5 mm | 300 mm | 500 mm | Display shelf, light-duty riser |
| 6 mm | 400 mm | 600 mm | Retail shelf with product |
| 8 mm | 500 mm | 750 mm | Loaded shelf, partition panel |
| 10 mm | 600 mm | 900 mm | Heavy shelf, machine window |
| 12 mm | 700 mm | 1000 mm | Structural panel, guard |
| 15 mm | 850 mm | 1200 mm | Large guard, heavy display |
| 20 mm | 1000 mm | 1500 mm | Architectural, thick-sheet work |
Acrylic creeps under sustained load. It does not fail suddenly; it deflects slowly over months, and a shelf that was straight on installation can show a visible sag a year later carrying the same weight.
The creep behaviour depends on temperature and stress level. At 20 °C and moderate stress it is gradual; at 40 °C — inside a sunny shop window — it accelerates sharply.
Practical implications: for a shelf that will carry load permanently, size for about two-thirds of the short-term capacity, or add a rib, or support the middle. For anything in a hot environment, treat the allowable stress as substantially reduced.
This is also why we ask where the part will be used rather than just what it carries. A display shelf in an air-conditioned store and the same shelf in a south-facing window are different engineering problems.
When a panel needs more stiffness, there are two routes and one is usually much cheaper.
| Thicker sheet | Rib or return | |
|---|---|---|
| Stiffness gain | Cube of thickness ratio | Large — moves material from neutral axis |
| Material cost | Rises linearly with thickness | Small added piece |
| Weight | Rises linearly | Minimal |
| Labour | None extra | Bonding or bending, plus finishing |
| Appearance | Clean flat panel | Visible edge, needs design intent |
| Best when | Short spans, clean look matters | Long spans, weight matters, edge can be designed in |
A folded front return on a shelf is the classic example: it stiffens the panel, hides the front edge thickness and creates a natural place for a price strip. It is often better design as well as cheaper.
1. Sizing by span alone. Load and support condition matter as much. A 600 mm span with four-edge support carries more than a 400 mm span cantilevered.
2. Ignoring creep. Sizing for short-term strength then leaving the part loaded forever. Add 50% to the effective load for permanent loading, more in heat.
3. Over-thickening to fix a support problem. If the panel is sagging because it is only supported at two edges, adding thickness is expensive. Adding a support is cheap.
4. Forgetting the transport minimum. Below about 3 mm, large panels are fragile in transit regardless of load. A 2 mm panel that is strong enough in service can still arrive cracked.
For light loads with four-edge support, thickness in millimetres roughly equal to span in millimetres divided by 60 is a workable starting point: 300 mm span → 5 mm, 600 mm → 10 mm.
For two-edge support, divide by 40 instead: 300 mm span → 7.5 mm, round to 8 mm.
For a cantilever, divide by 25.
These are starting points for light display loads, not engineering calculations. For anything carrying real weight, in heat, or over a long period, send the span, the load and the support condition and we will size it properly — it takes less time than discovering the sag in service.
For a shelf on two brackets carrying display product, 12 mm is a reasonable starting point; for a light load you might get away with 10 mm plus a front return. If the shelf is supported at the back and both ends, 8 mm may be adequate. Send the load and the support arrangement and it can be sized properly.
Because stiffness scales with the cube of thickness — 8 mm is about 2.4 times as stiff as 6 mm, not 33% stiffer. That is why thickness changes feel disproportionate.
Yes, noticeably. Creep accelerates with temperature, so a shelf in a sunny window at 40 °C will deflect far more than the same shelf at 20 °C. For hot environments, treat the allowable load as substantially reduced or add support.
Often yes, and it can be cheaper and stiffer. A 5 mm panel in an aluminium frame behaves better than a 10 mm panel unsupported, and weighs much less. The trade-off is that the frame becomes a visible design element and a separate component to source.
Those three numbers decide the thickness faster than any drawing. We will size it and tell you whether a rib would be cheaper than a thicker sheet.