When I talk with customers every day one of the first and most common questions I get is "how do I know what size to order?" and "do I need custom window boxes?" This can be a daunting task, especially for someone who may not be confident in measuring their windows or is afraid to make a mistake. The good news is that it's not as difficult as you might expect and in many cases a handyman can help out.
Architects like for window boxes to be the same size as the window. Everyday I look at blueprints and 99% of the time this is the case. The general rule of thumb is that if there is a wood trim that surrounds the window then you want to include that in your measurement. The biggest reason for this is because there are always studs that frame the window out and when the box goes a little larger than the window and includes the trim around the window then you are sure to be able to catch those studs during the installation. This will make the install go much smoother and with less frustration.
If for some reason you can't include the trim in the measurement don't worry. In most cases you'll need to catch 3 or 4 studs to actually mount the box. A window box that is 5' feet long usually requires 3-4 mounting points to support the weight and a window box that is 10' long could require as many as 6-8 anchor points. This often means you will be locating studs that are directly beneath the window that are either 16" or 32" apart from one anchor point to the next. There are some good videos online for how to install a window box that are worth watching if you need help visualizing the whole process. I usually don't recommend a custom window box if you can find one that is the same size as your window plus trim or even just a few inches long.
Can you go smaller?
Although it is not recommended, unfortunately it is common so the short answer is sometimes. Because locating a source for custom window boxes can sometimes be challenging you may only have access to certain sizes at your local stores. Ideally you want to be as close to the size of the window and surrounding trim as possible. If you're going to be off by a few inches it's better to be a few inches too long, rather than a few inches too short so that you can at least catch the easy studs around the window. If you're going to go smaller than the window then I suggest to stay within the 10% rule. For this don't get a window box that is more than 10% smaller, so for example a 60" long window, try to get a box no more than 6" too short or it may look funny. I will be better to spend a few extra dollars to get a custom window box every time rather than come up significantly short of the window.
If you're not sure what size to get and you do better with visualizations then you can do the paper towel trick. This can be very helpful and keep you from ordering the wrong size window box from online. To do the paper towel trick you take a roll of paper towels and roll them out to a desired measurement. Next, cut with scissors then tape the rectangle beneath the window. This can help you see what the length of the box will look like prior to ordering and might keep you from making a mistake.
Not only should you consider the length, but often you also want to consider the height and depth (front to back) of the flower box as well. For a one story home or a home close to the street you can often get away with an 8" tall window box. However, for a 2-story home or a home far from the street a 10" tall window box is a better proportion. Another advantage of a 10" box is that it retains water better and doesn't dry out as quickly so there's less watering from the owners end. If you are not sure of the height then no sweat, you can still do the paper towel trick, but also fold the paper towel so that it is either 8" or 10" tall. Tape each scenario beneath the window and go to the street to view. Snap a picture of each scenario and compare the two. This should help you lock in the length and height needed for your box. Most of the time once you know the height you typically get a depth (front to back) that is about the same dimension.
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