Whether you're retrofitting an old bay window or you'd like to replace that large picture window with something a little fancier, you can take the initial measurements for your bay window yourself. However, unless you're a professional, you'll need to have a professional bay window installer check your measurements before you make your purchase. Although most bay windows are measured in a similar fashion to standard windows, the measurements needed and where the measurement is taken from can vary based on the manufacturer and the type of window being installed.
Use a pry bar to gently remove any trim around the old window, if necessary.
Locate the boards that make up the actual window opening or frame around the window itself; these are typically 2-by-4s.
Measure the width and length of the rough opening from the inside. Measure the length of the rough opening three times, once at each end and once in the middle. Follow the same process to measure the width of the window.
Place one end of your measuring tape on the inside of the window where it meets the sill. Measure from this point to the outside of the window; this is the jamb width. If a bay window is not currently installed, measure from the interior, outside of the sill edge where the window would sit, out to the edge of the window opening on your home's exterior.
Place an angle finder on the outside of each angle of your bay window to discover the angle of the window, if you're retrofitting a window.
If your bay window is made up of three smaller windows with three separate window frames, measure each window separately in the same way you would a standard window. Always err on the side of caution when it comes to window measurement. It is far better to purchase a window that is slightly smaller than your opening than to purchase a window that is too large.
Always have a bay window installer double-check your measurements before purchasing a bay window. The measurements required and where they're taken from can vary based on the window.