There are two types of exterior shutters: functional and nonfunctional. It's important to purchase the proper size no matter which type of shutter you choose, but sizing will vary depending on your tastes and the type of shutter you install.
Functional shutters are designed for ventilation, light control and protection from the wind and rain. They are typically installed at the sides of your windows and hinged so that you can open and close them. Since functional shutters are made to cover your home's windows, you should measure your windows precisely to get the right size. Often, functional shutters must either be custom-ordered or trimmed at installation time because windows vary so widely in size.
According to Custom Shutter Co., the traditional formula you can use as a sizing guideline is "exterior shutter width is equal to the width of the window opening minus any clearances divided by the number of windows in the opening." To get your width and height measurements, measure your window in three places both horizontally and vertically. This is helpful because not all windows are square, so measuring in just one place may not provide you with an accurate measurement. Use the smallest measurement to dictate the size of shutter you buy.
If you have more than one window in a single window opening, you will need bi-folding hinges for your shutters. These shutters fold back on one another so that they appear to be only one shutter when open, but are able to close over your entire window.
Nonfunctional shutters are attached to the sides of your windows but no not move. They are primarily for decoration. Sizing for nonfunctional shutters is easier than for functional shutters because measurements for nonfunctional shutters do not have to be precise.
The measurement formula for nonfunctional shutters is the same as for functional shutters, but in the case of nonfunctional shutters, this formula is just a guide. The most common way to take measurements for nonfunctioning shutters is to measure your window trim and install shutters that are the entire height of the trim. For the width measurement, take into consideration your window's width and how far the windows on your home are spaced apart. Choose a width that works for all of your windows. Usually, people order shutters that are 25 to 33 percent of the total width of their windows.