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How to Measure a Door (3 Methods)

a white door in a hallway

Surprisingly, measuring a door can be quite difficult. Whether you’re replacing or adding a door to your home, figuring out the correct size is essential. Otherwise, you may end up with a door that simply doesn’t fit.

However, there is a very specific method you need to follow to ensure that you’re measuring the door correctly. Otherwise, you’ll end up unable to properly fit and hang your door.

Below, we’ll lay out these steps in a simple measure to make sure you get it right the first time!

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Types of Doors

While you’re likely to open and close many doors throughout your life, there are some specific anatomy terms that are helpful to understand before you jump into measuring. Furthermore, the type of door also matters since some come with extra parts that you need to account for when measuring.

For instance, a pre-hung exterior door often comes with trim, which is necessary to make it an airtight seal. However, be sure you account for this trim when ordering the door, or you’ll end up with a door that is much too big.

Beyond this, measuring interior and exterior doors is the same. Exterior doors without trim do not need to have this extra width added to them.

Measuring a Door Slab

A door slab is a door that doesn’t have any hinges or similar accessories on it. It is just the actual wood portion of the door. If you’re just changing the door slab, then you only need to measure the door slab for reference.

In this section, we’ll show you how to do just that:

1. Take a Width Measurement

Firstly, you’ll want to measure the length of the door. Typically, you can do this with a tape measure. Be sure you close the door before you start measuring. Place one end of the tape measure on the inner side of the door, and then stretch it until you hit the other side. Be sure to measure twice to ensure that you are correct.

Some doors warp after being used for a while. If you get multiple dimensions on different parts of the door—use the biggest one.

2. Take a Height Measurement

Keep the door closed, but this time measure the height. It is often easiest if you stick the end of the measuring tape under the door and then move the measuring tape to the top of the door. You’ll need a step stool or a chair to get an accurate measurement in this case.

Remember to measure the door itself, not the doorway, which should be just a bit bigger than the actual door.

3. Measure the Thickness

Now, you’ll need to take a thickness measurement. To do this, simply place your tape measure along the edge of the door to measure the thickness. You can do this on either side of the door, and they should give you similar measurements. Again, things might be off a bit due to a normal warp. Always use the biggest measurement.

a male customer measuring the thickness of a door
Image By: Sergey Ryzhov, Shutterstock

Some Points of Contention

While measuring a door is quite easy, there are some small problems that can mess up your measurements.

Here are some things to keep in mind:
  • Always measure the back of the door, not the front. If one side of the door has warped, it is usually the front side. Therefore, the front of the door may actually be smaller than the back. For an accurate measurement, always measure the back.
  • Older homes may have doors that don’t fit their doorways very well due to warping. In this case, you’ll need to measure the doorway and then size it down a bit. The actual door won’t provide accurate measurements.

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Measuring a Pre-hung Door

Measuring a pre-hung door is a bit different than measuring other sorts of doors. The process is a bit different depending on exactly what you’re doing. If you have not previously hung a door in that doorway, then the process is going to be a bit different than if you’re replacing an existing door.

For doorways that have never had a door before, here’s what you need to do:

Doorways without Previous Doors

1. Measure the Width

First, you need to measure the width of the doorway. Preferably, you want to take the measurement in multiple places since it isn’t odd for the doorway to be off a little bit in certain locations. Use the smallest figure for the width that you measure.

2. Measure the Height

Next, you want to measure the height. Depending on your flooring, this can be a bit more difficult. Remember, if you have carpet, don’t press into it, as you don’t want the door to press into it—instead, measure just to the top of the carpet.

As always, measure in a few different spots to ensure that you’re getting an accurate measurement. Always take the smallest measurement.

3. Measure the Thickness

Because there is no door to measure, you’ll just need to measure the thickness of the doorway. Generally, this is just a rough measurement. You just don’t want the door to be much thicker than the doorframe, as this will make it look quite strange.

As always, get measurements in multiple areas.

Replacing a Door with a Pre-hung

Now, if you’re replacing an existing door with a new one, the process is going to be a bit different:

1. Remove the Trim

You cannot get an accurate measurement for a pre-hung door with the trim there. For this reason, you’ll need to pop off the trim with a pry bar to ensure that you only measure the door frame.

2. Measure the Width

Not that you’ve removed the trim, but you can accurately measure the inside of the doorframe. You’ll want to measure the actual doorway – not the door that is in the doorway. As always, choose the smallest measurement that you get after measuring in a few different places.

tape measure
Image Credit: idobicollection, Pixabay

3. Take Height Measurements

Next, you’ll need to get the height measurement. Run a tape measure from the very top of the doorway to the bottom. If you’re inside, start at the floor. However, if you’re outside, it is often better to start at the top and move down. Measure in at least three different locations to get an accurate measurement

4. Measure the Thickness

Not that you have the height and width, you’ll need to determine the thickness. You can do this by measuring the inner edges of the side jamb, which is what catches the door on the side of the doorway.

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Simply put, here are the things you need to keep in mind when measuring for a doorway.

If you measure for a door slab, you want to measure the door itself and use the largest dimensions. Be sure to get each measurement in multiple locations, as doors often warp and aren’t usually perfectly shaped. You’ll also want to measure the door’s edge to determine the thickness.

However, if you measure for a pre-hung door, things are a little bit different. Firstly, you’ll want to measure the frame itself—not the door that’s hanging in it. In other words, you want the doorway measurements—not the measurements of the door itself.

By following our simple steps above, you can measure your new door correctly the first time, ensuring that you don’t end up with the wrong-sized door. As always, you’ll need to measure in multiple locations to ensure that you get the correct measurement.

Featured Image Credit: TheDigitalArtist, Pixabay


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