How To Measure an Octagon Window
An octagon window offers a quaint alternative to the usual rectangular designs. Like circular, oval and hexagonal designs, it is considered a unique, or specialty, window. Octagonal windows are often used in attics or as accent windows set above rectangular windows or doorways.
To install an octagon window, you need to know not only the window frame measurements, but the size of the rough opening at the installation site. Measuring octagon windows may seem a difficult task, but it's really just a matter of determining length and width.
Things You Will Need
 Measuring tape
 Pencil
Tip
Remember, you are measuring the octagon window frame, not the size of the panes. Take all measurements from the outer edge of the frame rather than the point where the pane meets the frame. The rough opening should be 1/2 to 1 inch larger in dimension than the window frame you have measured. The skill of the carpenter and the thickness of the wall are factors in determining the size of the clearance. If your carpenter is experienced, he may be able to work within a 1/2inch clearance. Less skilled practitioners may need a larger gap. So, for an 18inchby18inch window, the rough opening should measure between 18.5 inches by 18.5 inches and 19 inches by 19 inches.
Octagon Window Frame

Measure the length of the octagon's third side. Counting from the top of the figure, the third side would be that which corresponds to the number 3 on an analog clock.

Divide the figure you obtained in step 1 by 2 to find the midpoint of the third side. For example, if the third side measures 4 inches, the midpoint is at the 2inch mark. Mark the midpoint of the third side with a pencil. Mark the midpoint of the octagon's seventh side  the side opposite the third side  with a pencil.

Measure the distance between the midpoints of sides three and seven. Record the figure; for example, 18 inches. Perform the same series of steps for sides one and five  i.e., find the midpoint and measure the distance between the top and bottom sides of the octagon. In a standard octagonal window, the length and width measurements are identical; for example, 18 inches by 18 inches. An elongated window might measure 20 inches by 24 inches, for example. Window sizes are expressed in terms of length and width, but you do not need to calculate the size of the window in square inches.
Rough Opening

Place your tape measure against the wall. Make a mark at the "zero" point. In this example, the task is to measure a 19inchby19inch rough opening.

Measure horizontally and make a mark at 19 inches. Trace a straight line between the zero point and the 19inch point. You have just measured the top of the 19inch square.

Place the tape measure at the leftmost point  the zero point  of the line marking the top of the square, and run the tape measure downward vertically to 19 inches. Draw a straight line between the zero point and the 19inch point. You have just marked the left side of the square. Repeat this process for the bottom of the square and the right side of the square until you have marked and traced all four sides of the square.
The Drip Cap
 An octagon window offers a quaint alternative to the usual rectangular designs.
 Like circular, oval and hexagonal designs, it is considered a unique, or specialty, window.
 Octagonal windows are often used in attics or as accent windows set above rectangular windows or doorways.
 Mark the midpoint of the octagon's seventh side  the side opposite the third side  with a pencil.
 Perform the same series of steps for sides one and five  i.e., find the midpoint and measure the distance between the top and bottom sides of the octagon.
 Trace a straight line between the zero point and the 19inch point.
References
 FixHome: Octagon Window Installation Instructions
 Mark Smith, Owner; Nu Englander Octagon Windows; Greenland, New Hampshire
Writer Bio
D. Laverne O'Neal, an Ivy League graduate, published her first article in 1997. A former theater, dance and music critic for such publications as the "Oakland Tribune" and Gannett Newspapers, she started her Webwriting career during the dotcom heyday. O'Neal also translates and edits French and Spanish. Her strongest interests are the performing arts, design, food, health, personal finance and personal growth.
Photo Credits
 Hemera Technologies/AbleStock.com/Getty Images
 Hemera Technologies/AbleStock.com/Getty Images
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