How Much Is Enough?

Goal:  You will be able to calculate the linear feet of window trim needed for any dimension window.

Interior window trim (casing) is a molding that is nailed to the finished window frame to give the window a “finished” look.  The trim is located on the inside of the home around the perimeter of the window frame and is often cut at a 45º angle.  See picture.   You can think of it as a picture frame around the window.  The same idea is also used around doors.

Problem 1:

Your window frame (inside edge) is 38” wide by 54”.  There is to be a 3/16” reveal on all edges.  Your corners are mitered at 45º.  The trim (casing) is 2 1/4” wide.  How many inches of trim do you need?  You can ignore saw blade width but do need to consider casing profile. 

Problem 2:

Problem 2 is a generalization of problem 1.  What is a formula (shortcut) for finding the length of trim needed if the window size is “w” inches wide and “h” high with a trim width of “t”?  Again, assume you have a casing profile to consider.

Teacher Notes:

Students may need to research terms such as reveal, casing, and casing profile unless you provide a demo.  You will need to decide if you need to give hints regarding 45-45-90 special right triangles.  You can extend this to building a octagonal frame using 30-60-90 special triangles.  Trim carpenters know the rule by heart but do not call it a “formula”.