By their nature bay windows are more complicated to deal with.
Measure the distances between points A to B, B to C and C to D. The track or pole is usually positioned 12 to 15 cm above the window (if space allows) and 15 to 30cm (excluding the finials) either side of the window to allow the curtain to hang clear of the edge of the window.
For Pencil, Standard gathered, Pinch Pleat or Goblet headings if you have a pole, measure from the bottom of the curtain ring (see dotted line in the diagram). If you have a track, decide whether you want the curtains to hang below the track or to cover the track – and measure accordingly.
For Eyelet and Tab Top headings measure from the top of the pole and note the diameter of the pole (we need it to work out the size of the channel for the tabs).
Decide where your curtain will finish:-
To the window sill: measure 1cm above sill (distance E)
Below sill: measure at least 15cm below sill
To clear the floor: measure 1cm above floor level (distance F)
To break on the floor: measure to the floor and add 3 to 5cm
To gather on the floor: measure to the floor and add 5-15cm
And finally measure from the underside of the window recess or frame (distance G) to the point where your curtains will finish.
Download the Guide to Measuring Bay Windows.