How To Measure an Octagon Window

D. Laverne O'Neal

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.

An octagon window appears difficult to measure, but isn't.

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.


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 1/2-inch clearance. Less skilled practitioners may need a larger gap. So, for an 18-inch-by-18-inch window, the rough opening should measure between 18.5 inches by 18.5 inches and 19 inches by 19 inches.

Octagon Window Frame

  1. 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.

  2. 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 2-inch 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.

  3. 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

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

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

  3. 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 19-inch 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.