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.
[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.