Measure the opening width, height, and depth before you order

Take three width measurements: one at the top of the opening, one in the middle, and one at the bottom. Use the smallest number. Do the same for height — measure on the left side, center, and right side of the opening. Write down the smallest height. Then measure how far the door frame sticks out from the wall on both sides, and how deep the frame itself is from front to back. These three steps take fifteen minutes and prevent ordering a door that won't fit.

Measure in inches, not feet and inches. A door that is 35 and 7/8 inches wide is not the same as 36 inches. Manufacturers build doors in standard sizes — typically 30, 32, 34, or 36 inches wide, and 80 inches tall for interior doors or 82 to 84 inches for exterior doors. If your opening is 35 and 7/8 inches, you will need to shim or trim the frame, not order a custom door.

Take measurements on a day when the door is closed and the frame is not swollen from humidity. Wood frames swell in damp weather and shrink when dry, which can change your numbers by a quarter inch or more. If you are replacing a door in a bathroom or basement, measure on a dry day if possible.

Key Takeaways

  • Measure width at three points (top, middle, bottom) and use the smallest number to avoid ordering a door too wide for the opening.
  • Measure height on the left, center, and right side of the frame and use the smallest measurement, because frames are often not perfectly square.
  • Measure the depth of the existing frame and how far it extends from the wall, because replacement doors must fit within the existing frame pocket.
  • Write measurements in inches and eighths of an inch, not rounded to the nearest whole number, because standard door sizes are fixed and a quarter inch matters.
  • Measure on a dry day if the door is in a damp room, because wood frames swell and shrink with humidity and can throw off your numbers.

Measure width at the top, middle, and bottom of the opening

Open the door fully. Place a tape measure horizontally across the opening at the top, where the frame meets the header. Write down the distance from the inside edge of the left jamb to the inside edge of the right jamb. Repeat this measurement at the middle of the opening and again at the bottom, just above the threshold or floor.

The three numbers will often be different. Older homes and homes that have settled unevenly have openings that are wider at the top than the bottom, or vice versa. Use the smallest of the three measurements — that is the width your new door must fit. If the opening is 36 inches at the top, 35 and 7/8 inches in the middle, and 35 and 3/4 inches at the bottom, your door width is 35 and 3/4 inches.

If the difference between your largest and smallest measurement is more than a quarter inch, the frame is out of square. This is normal in older homes. You will need to shim the frame when you install the new door, or have a carpenter adjust it before installation.

Measure height on the left, right, and center of the frame

Measure vertically from the top of the frame (the header) down to the bottom of the frame (the sill or floor). Measure on the left side of the opening, the right side, and the center. Write down all three numbers.

Again, use the smallest measurement. If the left side is 80 and 1/8 inches, the center is 80 inches, and the right side is 79 and 7/8 inches, your door height is 79 and 7/8 inches. Frames that have settled or shifted often measure taller on one side than the other.

For interior doors, standard heights are 80 inches. For exterior doors, standard heights are 82 or 84 inches. If your opening is 79 and 7/8 inches, you will need to trim the header or add a threshold to fit an 80-inch door, or order a custom door. Most people trim the header rather than pay for a custom door.

Measure the frame depth and wall thickness

Close the door and look at the edge of the frame where it meets the wall. Measure how far the frame sticks out from the wall surface on both sides — the side where the door swings into the room and the side where it opens to the other room. These measurements tell you whether the replacement door frame will fit in the existing pocket.

Also measure the thickness of the frame itself, from the inside edge (where the door closes) to the outside edge (where it meets the wall). Standard frames are 4 and 9/16 inches thick for walls that are 4 and 1/2 inches thick, which is the standard interior wall. If your wall is thicker — common in older homes or exterior walls — the frame may be thicker too.

If you are replacing the door only and keeping the existing frame, these measurements confirm that a standard replacement door slab will fit. If you are replacing the entire frame, these measurements tell you what size frame to order.

Account for the threshold and any trim

If the door is an exterior door, measure the height from the top of the frame down to the top of the threshold, not to the floor. The threshold is a separate piece that sits on top of the sill, and it affects how tall the door opening actually is. Standard exterior doors are 82 inches tall, which assumes a threshold about 2 inches high.

If there is trim around the frame — baseboards, casing, or molding — measure whether the new door will clear it when it swings open. If the trim is in the way, you will need to remove it, cut it, or adjust the door swing. This is especially important for interior doors in tight spaces.

Record your measurements and check them twice

Write your measurements on a piece of paper or take a photo of your notes. Include the date and the room or location. If you are ordering from a supplier or contractor, bring the written measurements with you or send them by email so there is a record of what you measured.

Before you place an order, measure the opening again. Measure on a different day if possible. If your second set of measurements matches the first, you are ready to order. If they differ by more than an eighth of an inch, measure a third time to find the true number.

Many door suppliers and installers will also measure the opening themselves before they deliver or install the door. If they find a significant difference from your measurements, ask them to explain what they found. This is your chance to catch a mistake before the door arrives.

Understand standard door sizes and when you need custom options

Interior doors come in widths of 24, 28, 30, 32, 34, and 36 inches, all at 80 inches tall. Exterior doors come in widths of 32, 34, and 36 inches, at heights of 82 or 84 inches. Bifold doors, sliding doors, and French doors have their own standard sizes. If your opening matches one of these sizes exactly, ordering is straightforward. If it does not, you have two choices: shim or trim the frame to fit a standard door, or order a custom door.

Shimming is the standard approach. If your opening is 35 and 7/8 inches wide, you order a 36-inch door and shim the frame by adding thin wood shims behind the jambs to take up the extra space. This is inexpensive and is how most replacement doors are installed. Custom doors cost significantly more and take longer to arrive.

If your opening is smaller than the smallest standard size — for example, 28 inches wide when the smallest available door is 30 inches — you will need a custom door or will need to widen the opening, which requires removing and rebuilding the frame. This is a job for a carpenter, not a DIY project.

Frequently Asked Questions

Do I measure the inside or outside of the frame?

Measure the inside of the frame — the distance from the inside edge of the left jamb to the inside edge of the right jamb. This is the opening where the door slab will fit. Do not measure the outside of the frame or the wall opening, because those measurements include the frame itself and will give you a number that is too large.

What if my measurements are not exact eighths of an inch?

Round down to the nearest eighth of an inch. If your width is 35 and 15/16 inches, round down to 35 and 7/8 inches. Rounding down ensures the door will fit without binding. If you round up and the door is too wide, it will not close properly or will require planing, which weakens the door.

Should I measure with the door open or closed?

Measure with the door open so you can see the full frame and access all four sides. If the door is closed, you cannot see the top of the frame or measure the full height accurately. Open the door all the way and prop it if needed so it stays in place while you measure.

What if the frame is damaged or warped?

Measure the opening as it is now, not as it should be. If the frame is warped, a new door will fit the warped opening, but it may not close smoothly. If the frame is damaged, you may need to replace the entire frame, not just the door. A carpenter or door installer can tell you whether the frame can be straightened or must be replaced.

Can I measure over the phone and have someone else verify?

It is better to measure yourself or have the installer measure in person. If you must describe measurements over the phone, take photos of the opening from multiple angles and send them along with your written measurements. This gives the supplier a visual reference and reduces the chance of a misunderstanding.