The window sill sits at the bottom of a window, offering a flat place to set small flower pots or decor items and to give form and beauty to a window. It defines the lower aspects of the window and helps determine the proper fit for curtains or shades. Sometimes it's the inside measurement that's important, when a covering needs to be contained within the open space of the window. Outside measurements ensure that a curtain or shade covers the entire window, if that's the desired effect. Take these measurements carefully for a professional-looking window treatment.
Inside Sill Measurement
Locate the inside of the window's bottom. The inside width of a window sill does not exceed the distance from the inside jamb on one side to the inside jamb on the other. This is where you measure for tension rods and other inside-mount treatments.
Place the measuring tape against the left side of the inner sill and run it smoothly across the width of the window until you get to the opposite inside sill on the right.
Read the measurement on the tape and record the result on your paper with a pencil or pen.
Outside Sill Measurement
Locate the left outermost edge of the window sill.
Place the measuring tape at the point where the outermost edge of the window sill lies. The window sill usually extends beyond the actual opening for the window and sometimes past the outer framework of the window. This measurement indicates the width a window treatment must reach to cover the window and surrounding frame completely.
Run the measuring tape across the window sill to the opposite outermost side on the right.
Read the tape and record the measurement. Unless there are added architectural aspects, this is the width of a window treatment needed to cover the whole window.
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