In most homes, the wall consists of an interior wall and an exterior wall with insulation in between. The opening on the inside is larger than on the outside. In this case, the window is installed behind the exterior wall.

Step 1: Measure the clear opening

• Measure the width between the exterior walls in 3 places (top, middle, bottom).
• Measure the height in 3 places (left, middle, right).

Always take the smallest measurement, this is your clear opening size.

Step 2: Calculate the manufacturing size

To ensure your window fits correctly behind the exterior wall, it needs to be larger than the clear opening.

• Add 25 mm to each side (left, right, top). This is your manufacturing size.
• The extra 25 mm is a guideline.

Step 3: Check the Bottom

• If your window sits on a sill: you’re good, no further adjustment needed.
• If the window does not sit on a sill but also extends behind the wall at the bottom: add 25 mm at the bottom.

Attention!

If you have an interior windowsill closer than 25 mm, you must adjust your measurement accordingly. 

Step 4: Consider the type of window

• For tilt-and-turn windows, check that there is enough space to open the window.
• For fixed windows, this does not matter.

Example

Clear opening = 700 mm wide × 1000 mm high

Calculation:

• Width: +50 mm = 750 mm
• Height: +25 mm = 1025 mm

Manufacturing size = 750 mm × 1025 mm

If there is no exterior sill and the window also needs to go behind the wall at the bottom:

• Height: +50 mm = 1050 mm

Final manufacturing size = 750 × 1050 mm

Curious how your window will look? Discover it instantly with our window configurator!

Step 1: Measure the clear opening

• Measure the width at 3 points.
• Measure the height at 3 points.

Always take the smallest measurement. This is your clear opening size.

Step 2: Calculate the manufacturing size

To allow sufficient space for installation, it is important to reduce the dimensions to account for clearance.

Reduce each side by:

• Left: 6-10 mm
• Right: 6-10 mm
• Top: 6-10 mm

The exact reduction depends on the desired clearance.

Step 3: Consider the type of window

• Tilt-and-turn windows need enough space to open properly.
• Fixed windows do not require extra clearance.

Example window measurement:

Clear opening = 700 mm wide × 1000 mm high

Calculation:

• Width: -16 mm = 684 mm
• Height: -8 mm = 992 mm

Your manufacturing size is therefore 684 × 992 mm.