Shelving
How to measure for a glass shelf
The number that decides a glass shelf is not the width of the alcove. It is the span between the supports, because sag grows with the cube of the span.
Before you start
What you are measuring
The dimensions that decide the job
- The span between the supports, centre to centre.
- The alcove width at three heights, if the shelf sits in one.
- The depth from the wall to where you want the front edge.
- What the shelf has to carry, and whether the load sits in the middle.
What to have in your hand
- A steel tape measure
- A spirit level
- A pencil
The printable measuring pack carries the record sheet these steps fill in, plus a tracing grid for shapes a tape cannot describe.
In this order
The steps, in the order the workshop takes them
Measure the span between the supports
Bracket centre to bracket centre, or the clear distance between the two channels. This is the figure that sets the thickness, and it is often 100mm or more shorter than the alcove.
Measure the alcove at three heights
If the shelf sits in an alcove, measure across it at the height the shelf will sit, then 100mm above and below. Plastered walls lean. Order to the smallest of the three.
Decide the depth
From the wall to the front edge. Depth changes what the shelf holds but not how much it sags between supports, so choose it for what goes on it rather than for strength.
Allow the clearance it needs to go in
A shelf that drops into an alcove between two channels needs a few millimetres to get in at all. Give us the opening and say it is an opening, and we take the clearance off.
Say what it will carry
Books along the whole length, a single heavy object in the middle and a row of bottles are three different loads. The shelf load calculator turns your span, depth and thickness into the weight the shelf will take.
Note how it is held
Brackets under the ends, a channel along the back, or pins into the side walls. A shelf glazed into channels at both ends is stiffer than the same shelf resting on two brackets, and we size it differently.
What we work to
The tolerance, and where to start
We cut to plus or minus 2mm, and every order is confirmed with a dimensioned drawing before anything is made. An alcove needs real clearance on top of that tolerance: order 3mm to 5mm under the smallest opening measurement, or the shelf will not go in at all.
The usual specification
8mm toughened
8mm toughened for spans of 500–800mm.
The configurator link below opens with an alcove shelf at 800 x 250mm in that specification. It is a starting point the workshop cuts often, not a guess at your opening: change the numbers to the ones you have just measured and the price and the drawing follow.
What costs people a panel
The common mistakes
Measuring the alcove instead of the span
The alcove is what the shelf has to fit into. The span between the supports is what it has to bridge, and only one of the two decides the thickness.
Choosing the thickness by eye
A 6mm shelf looks identical to a 10mm one on a phone screen and sags four times as much over the same span.
Forgetting the walls are not parallel
Three measurements across the alcove, every time. The narrowest is the one that matters.
No clearance at all
A shelf cut to the exact opening cannot be manoeuvred into it. It needs a gap to go in, and the brackets or channels cover it.
When you order
What to send us
- The span between supports and the alcove width at three heights, in millimetres.
- The depth you want.
- How the shelf is held: brackets, channels or pins.
- Roughly what it will carry.
Write them on the record sheet as you go: download the three-page measuring pack PDF, or read the same sheets on the measuring pack page.
Next
Settle the rest before you order
Measuring something else? Every product we cut to size has its own guide on the measuring pack page.