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”.
Geometry in Construction