The formula
area = width × height, and perimeter = 2 × (width + height)
Where it comes from
Area is a multiplication, but the figure that explains the price is a different one: the metres of frame per square metre of opening. A square window one metre by one has four metres of perimeter for one square metre; a long one, two metres by half a metre, has the same area and five metres of perimeter. Since the frame and hardware are paid for by the linear metre and the glass by area, the second one costs more even though the quote is agreed "per square metre".
How to work it out by hand
- Convert both measurements to metres
- Multiply them: that is the area of the opening
- Add width and height and double it: that is the perimeter
- Multiply the area by the price per square metre
What is worth knowing
The measurements asked for here are those of the structural opening, which is what you measure on the wall. The finished window is made a centimetre or two smaller on each side so it can be plumbed and sealed, so never order joinery at the exact size of the opening. Another point for comparing quotes: on small windows the price per square metre shoots up, because much of the manufacturing cost is fixed per unit. Comparing two quotes "per square metre" only makes sense when the openings are of similar size.