A “Transom Light” or “Transom Window” is a piece of glass that is placed above the frame of the door. This window is designed to allow more natural sunlight into the home without sacrificing privacy.
This window above the door can take many shapes which causes them to not be standardized in the industry. They are commonly seen in half oval / circlular shape but are also seen as rectangles. The size of the transoms also varies greatly based on other aspects of the home: door size (width), if there are sidelights present, and the overall design of the home / entrance way. All these can play factors on the size and shape of the transom light recommended for your home. There also needs to be adequate space between the top of the door frame and the roof of the home for a transom light to fit.
Considering these other elements when selecting the design for your transom window is important to complementing the rest of the entrance way and home.
Measuring a rectangle transom is a little more straight forward than measuring an oval / circular shaped transom. With the rectange units, you can simply measure the width and the height of the glass starting from the outside edge of the frame. If you have sidelights, a common width of the transom would extend from the outside left frame edge from the left sidelight all the way to the outside right frame edge of the right sidelight.
A = Transom Glass Width including Frame
B = Transom Glass Height including Frame
If you have an oval / circular shapped transom, the measurement can be more difficult, as there are other variables. Having an installation professional help you with your measurement is recommended for all transoms. Since there are not defined industry standards for transom lights, most transoms need to be custom ordered.