How to Measure Bay Windows for Custom Roman Shades

bay windows on a sunny day
What You'll Need
Metal self-locking tape measure
4-foot carpenter's square, or 6-foot square for tall windows
Carpenter's level
Pencil and notepad

Because of the curvature of bay windows, custom Roman shades are a better choice than ready-made Roman shades to provide the most precise fit and function in them. These suggestions and tips will help you obtain the most accurate measurements of your bay windows.

Step 1 - Determine Where You Wish to Hang Your Roman Shades

The standard depth of the hanging headrail for Roman shades is 2 inches. You can place Roman shades so they nearly touch the glass, or move them outward for easier operation. If you have a window seat in the center of the bay window, hang the Roman shades as close to the window glass as possible. Draw a line with the yardstick on the top inside edge of the window frame, marking the placement of the headrail. Draw a line at the same distance from the window on the lower window bay. This horizontal line will be your guide for measuring the window width.

Step 2 - Measure the Width of the Center Window

Obtain measurements of the window width at three locations: 6 inches down from the top of the window, at the midpoint of the window, and 6 inches up from the bottom of the window. If the bay window has pane dividers, use these as guides to keep your tape measure level while measuring. Measure to the nearest 1/8 inch or 0.1 cm. If the measurements differ, select the smallest width measurement for your Roman shades. Write down the width measurement on your notepad.

Step 3 - Measure the Window Height

As you did for the width, select three points at which to measure the height of the bay window. Use the pane dividers to select measuring locations or measure at 6 inches in from each side of the window and at the midpoint. Choose the smallest of the three measurements as your window height. Record this dimension on your notepad.

Step 4 - Check the Window Frame is Level at the Top

Use the carpenter's level and the carpenter's square to check that the frame is level and perpendicular on all sides. If there is a slight variation from 90 degree angles, you can adjust the placement of the header rail with 1 or more thin wooden shims when attaching it.

Step 5 - Measure the Width and Height of the Side Bay Windows

If you are planning to hang Roman shades in all three parts of a large bay window, measure to determine the width and height of the two side bays. Remember to record the smallest measurement of width and height for each side bay window.

Step 6 - Measuring Square Bay Windows

If the side panels of your bay windows are at 90 degrees to the center bay instead of 45 degrees or less, measure the width and depth of each bay as described above. Add a separation of 1/2 inch (1.2 cm) at the edges of each set of side window shades closest to the center window Roman shades to allow for raising and lowering.