04/25/2025 Posted By Siarhei Kanushok

High-quality and stylish doors are a beautiful finishing touch to a renovation and a long-term investment in your home or flat. They zone space, create privacy, protect from outside sound, and functionally decorate the doorway, becoming an integral part of the interior.

Whether you are planning to replace an exterior or interior door, the most essential step before purchasing is to take the correct measurements. Here, you will find a detailed step-by-step guide to accurately measuring all the necessary dimensions so that the door will last for many years after installation.

Necessary tools

You only need a few simple things that everyone has at home:

  • Tape measure.

  • Paper.

  • Pen or pencil.

If convenient, you can record all your measurements in your phone's notes so they're always at your fingertips.

How to measure your door when replacing the door (slab) only?

Replacing the slab means the existing door frame has already been installed in the opening. Accordingly, all measurements must be made exclusively for the slab and can be made while it is still hanging on its hinges.

Measuring slab width

Using a tape measure, determine the width of the slab exactly from the left edge to the right edge in three places: at the bottom, in the middle, and at the top of the door. Write down the largest value if they are different from each other.

Standard slab widths:

  • 18, 24, 28, 30, 32, or 36 inches for interior doors.

  • 36, 38, 48, 50, 62, 64, or 68 inches for exterior doors.

Measuring slab height

Then, measure the height of the slab three times from the bottom to the top. Measure from the left, middle, and right edges of the door. Write down the largest value if they are different from each other.

Standard slab heights:

  • 80, 84, 92 ½, or 96 inches for interior doors.

  • 80, 82, or 96 inches for exterior doors.

Measuring slab thickness

Open the door ajar to measure its thickness.

Standard slab thickness:

  • 1 ¾ or 1 9/16 for interior doors.

  • 2 1/16 inches for exterior doors.

How to measure your door when replacing the entire door system (door unit)?

Replacing the door unit is a more labor-intensive process, which is reasonable if the house or apartment is to be renovated globally or if the door frame is worn or damaged. Note that in this case, not all measurements will concern the slab, only the rough opening. In general, there is no more than that in the previous situation; however, you should be consistent and careful to avoid calculation errors.

Step 1: Preparing the rough opening

First, remove furniture, decor, and other items around the opening to allow full access. Next, check the possibility of installation, i.e. make sure that the configuration of the door you have chosen is suitable: for example, for swing doors, you need enough space to work with, for the pocket system, it is necessary to have a wide wall where the slab will be hidden when opened, and for barn doors there must be no switches and sockets on the sides, as well as the ability of the wall to withstand the track with the slab.

The better the condition of the rough opening, the more accurate the measurements will be because gaps, unevenness, chips, and other defects can affect the final result.

Step 2: Width measurement

Measure the width of the doorway in three places: top (left edge to right edge), middle, and bottom, using a tape measure. Write down the largest value if they are different from each other.

Step 3: Height measurement

Similarly, measure the height of the rough opening three times: on the left, in the middle, and on the right. If the exterior or interior door has a threshold, the measurements should be taken from its bottom (from the floor). Write down the largest value if they are different from each other.

Step 4: Depth measurement

Measure the depth of the opening, which will be the width of the wall without finish, casings, and other elements.

Step 5: Door dimensioning

Please note that each configuration has a specific ratio of rough opening to door dimensions. Use our detailed instructions to calculate the required height and width of the door correctly.

Useful materials: