Planning Aluminum Window Sizes and Openings in a Renovation

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Aluminum window sizes should be based on the clear opening measured on site, not on old drawings or the old window's frame. Measure width and height at several points, allow for the frame profile and installation tolerances, and confirm the final dimensions with the manufacturer before production.

Why Measuring the Opening Comes First

In a renovation, the opening is the only reliable reference. Original plans are often outdated, and walls may not be square or level after decades of use. Measure the rough opening, meaning the space between the wall surfaces that the new window will fit into, rather than the size of the old window frame, which may have been installed loosely or partially hidden.

Record each dimension in the same units and note which side of the opening each figure belongs to. Photos with a tape measure in frame help the workshop understand the situation before production begins.

Measuring Width and Height Correctly

Measure the width at the top, middle and bottom of the opening, and the height at the left, centre and right. Use the smallest reading as the limiting dimension for fitting, and note any large differences, which indicate an out-of-square wall or a previous repair.

Check diagonals to confirm the opening is square.
Note the wall thickness, since it affects the depth of the frame and the reveal.
Look for lintels, sills and obstructions such as shutters or pipes.

Allowing for Frame Profile and Clearances

A window is never ordered to the exact size of the opening. The frame profile takes up space, and a fitting gap is needed for packing, sealing and levelling. The overall size of the aluminum unit is therefore the opening minus the clearance agreed with the installer and the manufacturer.

Clearances depend on the profile system, the anchoring method and whether the window is fixed into a masonry or concrete opening. The workshop should set these values for the specific product rather than relying on a general rule.

Sash Configuration and Opening Direction

The number of sashes and how they open affects the planned size. A wide opening can be split into fixed and operable panels to keep individual sashes manageable and to control weight. Decide early whether the windows will be sliding, casement, tilt-and-turn or fixed, because each type needs different clearances and hardware space.

Also confirm the room for opening: outward-opening sashes need space on the exterior, and sliding sashes need a clear track zone on each side. Check furniture, radiators and external shading before fixing the final layout.

Checking Structure, Fixings and Sealing

Larger aluminum windows place greater loads on the wall and the lintel, so the surrounding structure should be inspected before ordering. Cracked masonry, rotten timber or uneven concrete may need repair before installation. The fixing points must be placed where the wall can carry them safely.

Plan the sealing details at the same time: waterproofing membrane, insulation around the frame and the finish of the reveal. These work determines how well the final installation performs against water and air leakage.

Confirming the Order Before Production

Before production starts, the contractor or owner should receive a written specification with the final sizes, sash layout, opening direction, finish and installation notes, and approve it in writing. A site measurement sheet shared with the drawing gives both sides a common reference, and any change after production usually costs more and delays the project.

Ask the manufacturer to recheck the dimensions against the drawing, and arrange a final site check if the opening was measured before other trades began their work.

Frequently asked questions

Should I measure the old window frame or the opening?

Measure the opening itself, because the old frame may be undersized, damaged or partly hidden. The new window must fit the clear space in the wall, after allowing for the frame and installation clearance.

How many measurements should I take?

Take at least three width readings and three height readings, plus both diagonals. This shows whether the opening is square and which dimension should be used for fitting.

Can aluminum windows be made to a non-standard size?

Yes, aluminum windows are made to measure in the workshop, so the size can follow the actual opening. The final dimensions still need to be confirmed with the manufacturer against the drawing.

What if the wall is not straight or level?

Note the variations on the measurement sheet and let the manufacturer and installer know before production. They can plan the frame, packing and sealing to accommodate the irregularity.

Common mistakes to avoid

Ordering windows from old drawings or from the previous frame without remeasuring the opening is the most common error. Taking only one measurement per dimension also hides out-of-square walls that cause fitting problems later.

Our recommendation

Measure the opening on site after the wall work is finished, take multiple readings, and have the manufacturer confirm the final sizes against a written drawing before production starts.

Summary

Plan aluminum window sizes from careful on-site measurement of the opening, with allowances for the frame, clearances and sash layout. Confirm the specification in writing before ordering.

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