[header] How to measure your windows [title] Measure the width at three points and use the smallest measurement. [step] Use a tape measure or extendable slide rule to measure the width of the window opening at the base, middle, and top. Write down the smallest of these measurements as the width. 
Be sure to measure from the surface of the jambs or frame, not from the short extensions that hold the window in place. [substeps] A slide rule with extension may result in more accurate interior measurements.