A united inch is a term coined by the window industry to aid in the correct pricing of window frames. A united inch is different than window area in that it only incorporates one-half of the window measurements. A framing chart will cross-reference the measurement of the united inch with the quality of the framing material to estimate the appropriate price. The united inch measurement also allows you to purchase the right amount of molding when building the frame yourself--just remember to double the measurement.
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Measure the length of the window in inches.
Measure the width of the window in inches.
Add together the length and width. This gives you the united inch. For example, if the length is seven inches and the width is nine inches, then the united inch would be 16 inches.