Measure in at least three places as indicated in image above.
Take the smallest height and smallest width, then deduct 10mm (see example below).
Example:
Smallest Width: 1200mm Smallest Height: 1100mm (height is from the top of the cill)
Deduct 10mm from each
You require a window which is: Width: 1190mm, Height: 1090mm
Measure in at least three places as indicated in image above
WIDTH : Measure on the out side from reveal to reveal than plus 14mm this is so the new window will have 7mm cover behind each reveal
Height : Measure from the out side from the finished plaster (x in photo) to the top of the cill
If necessary chip away a small bit of the window or the plaster to find the top of the cill and to check how much cover there is behind the reveals. (On older houses with stone cills there might be no step up on the cill)
To measure your bay window
Widths : Measure along the outside edges at the top of the cill (see image)
Height : Measure from the top of the cill to the heads than deduct 10mm
