Aluminium Window Sizes: How They Are Measured and Ordered

Aluminium window sizes are quoted width first, then height, always in millimetres. Get that one convention right and most of the confusion around ordering disappears. This guide explains how sizes are written, where each measurement is taken, and the difference between buying a stock size and having a window made to your opening.

How aluminium window sizes are written

A window described as 1200 x 900 is 1200mm wide and 900mm high. Width always comes first. It sounds obvious until you are looking at a list of aluminium window sizes and a 900 x 1200 appears two rows below, which is a completely different window.

The figure quoted is normally the frame size, not the hole it goes into. The structural opening has to be slightly larger so the frame can be positioned, packed square and sealed.

The four measurements that matter

The structural opening is the hole in the brickwork. The frame is what sits in it. The sash is the part that opens, inside the frame. The glass is smaller again, because it sits in a bead and a gasket. When people disagree about aluminium window sizes, it is almost always because two of them are quoting different ones of these four.

Measuring an existing opening

Measure the structural opening, not the frame currently in it, and not the plaster reveal. Take the width at the top, middle and bottom, and the height at the left, centre and right. Work to the smallest figure in each direction.

Then measure both diagonals. If they differ, the opening is out of square, and you need to know that before a frame is made rather than on the day it arrives. Our measuring guide sets out the full method.

Stock sizes or made to size?

On a new build you can work either way, because the opening has not been cast yet. Choosing from standard aluminium window sizes first, then building the opening to suit, is usually the cheaper route.

On a replacement the opening already exists, so the window is made to it. That is the normal case for anyone replacing old steel or timber frames.

You can browse what is available by dimension on our shop by size tool, or see the full range under aluminium windows.

A note on glass and sizes

Standard glazing is 6.38mm clear laminated safety glass. Larger panes are heavier, which affects the hardware a sash needs, so very large opening sashes are not always the best answer to wanting more light. A fixed light beside a smaller opening sash often gives the same glass area with less to go wrong. For glazing standards, SAGGA, the South African Glass and Glazing Association publishes the industry guidance.

Common aluminium window sizes and where they suit

There is no single national standard, but some proportions recur because they suit how rooms are built.

Wide, low windows sit above kitchen counters and in bathrooms, where you want light and ventilation without losing wall space.

Taller windows suit bedrooms and living areas, where the glass runs closer to floor level and the view matters.

Narrow vertical windows work in passages, stairwells and next to doors, often as fixed lights rather than opening sashes.

Why the opening size decides the price

A larger frame uses more profile and more glass, so size drives cost directly. That part is obvious.

What is less obvious is that the split matters as much as the overall size. Two opening sashes cost more than one opening sash and one fixed light of the same total area, because each sash carries its own stays, handle and seals.

If you are working to a budget, reducing the number of opening sashes usually saves more than shaving 100mm off the width.

Getting aluminium window sizes onto an order

Write every opening down as width x height in millimetres, label it by room, and photograph it.

Send that list rather than a description. “The big window in the lounge” is not a size, and it is the most common reason a quote has to be redone.

Frequently asked questions

Are aluminium window sizes quoted width or height first?

Width first, then height, in millimetres.

Is the quoted size the frame or the opening?

Normally the frame. The structural opening needs to be slightly larger to allow for packing and sealing.

What if my opening is out of square?

Measure the diagonals and tell us. A frame can be made to suit, but only if we know before manufacture.

Can I get a window made to a size that is not standard?

Yes. Replacement windows are routinely made to the opening you already have.

Does Coetz fit the windows?

No. We supply aluminium windows and doors nationwide across South Africa; fitting is arranged by you.