How to Measure Standard Sizes of Exterior Doors

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Typical single exterior doors should be what is known as a 36 x 80 door. Some of the side or rear doors may be smaller in width up to 32 x 80. 

We will see what that means until the necessary measures. For now, let's talk about simple 36 x 80 pre-hung doors. You can buy different sizes of doors at China WDMA  door.

The nominal width of the door is 36 inches. The nominal height of the door is stated as 6/8, which is 6 feet 8 inches or just 80 inches. Since these doors are pre-hung with the door jamb frame, the actual size of the pre-hung door is 37 ½ inches by 81 5/8 inches. Note that the width is 1½ inches wider than the actual door and the height is 1 5/8 inches taller than the actual door. Keep this in mind.

Now we need to get to the approximate measurements of the size of the opening. In other words, the entry area "before any door is installed" must be larger than the size of the pre-hung door. 

The approximate opening size is the size of the construction with the side wood (studs) and header. For 36 x 80 exterior doors, this rough opening should be 38 ¼ inches by 82 1/8 inches. As you can see, the standard exterior door size rough opening is 2¼ inches wider than 36 inches and 2 1/8 inches taller than 80 inches. Keep this in mind.

If you are using brick mold openings, the dimensions would be 40 inches by 82 7/8 inches. As you can see by using brick trim, the width of the opening is 4 inches wider than the door and 2 7/8 inches taller than the door itself. Keep this in mind.

So suppose here now you are working with new construction or replacing an existing door, you will need to measure the approximate opening size.

Trim may need to be removed to obtain this measurement on existing doors. Here's how to determine the approximate opening size for your home. For the width of the rough opening, you will measure between the studs at the top then the center, and then at the bottom. You should take the smallest of those three numbers as the width. 

For the approximate height of the opening, measure from the floor to the bottom of the header on the left and right sides of the opening. Use the smaller of the two measurements.

These two measurements are your approximate opening size. If you have anything other than 38 ¼ inches x 82 1/8 inches (or very close), you may not be able to move through this. 

You may have to choose a different size door or go for a special order door. Time to call in the professionals!

There are several good reasons besides the obvious one for placing the pre-hung door in the rough opening that this opening has to be larger than the pre-installed door itself. 

You need extra space to ensure that the installed pre-hung door can be plugged in and squared properly. Door manufacturers have slightly different sizes for their pre-hung doors, so you will need additional space to fit most manufacturers.

You need extra space because doors expand and contract with changes in temperature and seasons. It is not a large number, but it is enough for a tight fit to create problems during this expansion and contraction. We could give you a good chart of the different door sizes, but the chart may vary slightly by manufacturer.

So let's go with a formula that you can use no matter what door size you have. 

• Approximate Opening Width Size = Nominal Door Width + 2 ¼ inches (or + 4 inches if brick mold is used).

• Approximate Opening Height Size = Nominal Door Height (80 inches) + 2 1/8 inches (or + 2 7/8 inches if using a brick mold).

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