Fully opened curtains still occupy space. Their gathered fabric, rings or pleats form a stack at the side of the window. If the pole ends close to the glass, that stack may cover more daylight than the room plan anticipated.
Measure beyond the frame
Record the window width and the available wall on both sides. Note adjacent cupboards, walls, radiators and switches. The room may allow a generous stack on one side but very little on the other.
Then identify the proposed curtain heading, fullness, lining and fabric weight. These influence the space the gathered curtain needs. A sheer panel and a heavily lined curtain should not be given the same assumed stack width.
Ask for a realistic stack estimate
Use the curtain maker’s or track supplier’s guidance for the actual design. Where possible, inspect a sample with the same heading and similar fabric. A calculation based only on the finished curtain width can miss how bulky the heading becomes when drawn back.
Mark the proposed pole or track position temporarily on the wall. Imagine both states: curtains closed across the window, and curtains open with the fabric gathered. Check door movement and access to handles in each state.
For an asymmetrical window, a one-way draw may be worth discussing, but it concentrates the stack on one side. Motorised and specialist tracks also have their own end clearances and installation requirements.
Do not extend a pole without checking the required supports and the wall fixing method. Have a suitable installer assess uncertain substrates or heavy assemblies.
The final measurement should describe the whole arrangement, not just the glass. When the curtains are open, the result should match the reason you chose them: light, view and a room that still works around the window.