A bay window juts out from the side of a house, but fitting bay windows is not much more of a challenge than fitting a pair of regular curtains. Because of the window's odd shape (a bay window often features several angles and requires a special angled rod), the curtain hardware and rod should be in place to take an accurate measurement.
Hang the curtain rail before you take any measurements. This will ensure you get the most accurate measurements.
Measure the curtain rail (from the far left of the rail to the far right) to get your curtain and valance width.
Measure the distance from the rail to the floor. This will be your curtain length.
Measure the distance from the rail to the spot where you would like the valance to hang (valances are typically around 12 to 18 inches). This is your valance length.
If you want a bay window curtain to go straight across the opening (it's a cheaper option), measure slightly above the opening to the floor to get the curtain length and measure across the width from one side to the other to obtain the curtain and valance width.