Choosing the right sheet thickness

It's tempting to just default to 18mm for everything, but the wrong thickness either wastes money and weight, or leaves you with a part that sags, flexes or won't hold a fixing properly.

Start with the job, not the number

Thickness is really a stand-in for three separate questions: how far does this part span unsupported, what load will it carry, and what will be fixed into its edge? Answer those first and the thickness usually becomes obvious.

Unsupported span and sag

A shelf, worktop or panel that's only supported at its ends will sag over time if it's too thin for how far it spans — how much depends on the material, the load, and whether there's a front lip or rail stiffening the front edge. As a working rule, the longer the unsupported span, the thicker the sheet (or the more mid-span support you need to add). If you're not sure and the part matters — a worktop, a long shelf holding books — it's cheaper to add a mid-span support or upgrade a thickness than to redo it once it's sagged.

What typically goes where

• 6–9mm — cabinet backs, drawer bottoms, small box sides, anything light that's fully supported on most of its face.
• 12mm — light shelving with close support, cabinet ends on a small carcass, drawer sides.
• 18mm — the general-purpose thickness for cabinet carcasses, furniture panels and shelving with a reasonable span — it's the default for a reason.
• 25mm — worktops, workbenches, heavily loaded shelving, and anywhere the edge itself needs to look and feel substantial.

These are starting points, not guarantees — a wide, heavily loaded shelf in 18mm can still sag if it spans far enough, and a well-supported 12mm panel can outperform a poorly supported 18mm one. For anything genuinely structural or safety-critical, check a proper span table for the specific material or get a professional opinion rather than relying on rules of thumb.

What's fixed to the edge

Hinges, drawer runners and shelf pins all need enough material to bite into without blowing out the edge. Thin sheet (under around 12mm) makes fixings fiddlier and less secure at the edge — if your design has a lot of edge-mounted hardware, that alone can push you up a thickness even if load isn't otherwise a concern.

Weight and cost work against thickness

Every step up in thickness adds weight (worth considering for anything that gets moved or wall-mounted) and cost per sheet. There's rarely a reason to over-specify "just in case" — pick the thickness the job actually needs, and if you're genuinely unsure between two, the lighter option with a bit of added support usually beats the heavier, more expensive one.

Once you've settled on a thickness, the Boardwise estimator will pack your parts against real UK sheet sizes and show you the quantity and cost.