WIDTH = Overall width from brick to brick. ie from outside of jamb to outside of jamb
HEIGHT = Overall height from brick to brick. ie from underside of cill to top of head.
For Example
It is always more accurate to measure from outside as, internally, part of the frames are hidden behind the internal plastering and architraves
Check the distance from the bottom of the glass to the floor. If it is within 800mm (32 inches), safety glass is required under document N of the Building Regulations
a) If existing window is Crittal Type. b) If existing window is Box sash type. c) If existing window is part of a bay.