Tools You Will Need
You will need a steel tape measure, a pencil, and paper to note your measurements. Always use a steel tape measure rather than a fabric one for accuracy.
Measuring Width
Measure the width of the opening in three places: at the top, middle, and bottom. Use the smallest measurement as your width, and deduct 10mm to allow for fitting tolerances.
Measuring Height
Measure the height in three places: left side, centre, and right side. Again, use the smallest measurement and deduct 10mm.
Check for Square
Measure diagonally from corner to corner in both directions. If the measurements differ by more than 10mm, the opening may not be square and you should seek advice before ordering.