Take measurements in three places on the door frame, not just one

A door that fits perfectly in one spot on the frame can bind or rattle in another. Measure the width at the top, middle, and bottom of the opening. Measure the height on both the left and right sides. Write down all six numbers — the frame is rarely a perfect rectangle, and the smallest measurement is the one that matters when you order.

Use a steel tape measure, not a cloth one. Cloth stretches and gives you a false reading. Measure from the inside edge of the frame on one side to the inside edge on the other side. For height, measure from the inside bottom of the frame (where the threshold sits) to the inside top.

If the opening is wider at the top than the bottom, or taller on one side than the other, note that down too. The door manufacturer needs to know whether the frame is square or out of plumb. Some doors can be trimmed slightly; others cannot.

Key Takeaways

  • Measure width at three heights (top, middle, bottom) and height on both sides; use the smallest number for each dimension when ordering.
  • A steel tape measure gives an accurate reading; cloth measures can stretch and lead to an ill-fitting door.
  • Note whether the frame is square or tilted — the manufacturer needs this information to cut the door correctly.
  • Measure the depth of the frame (how thick the wall is) because the door slab thickness and hinge placement depend on it.
  • If the opening is significantly out of square, a professional installer may need to shim or adjust the frame before hanging the new door.

Measure the frame depth from front to back

The frame has thickness — the distance from the outside of the house to the inside. This is called the frame depth or jamb depth. Measure it by placing your tape measure flat against the inside edge of the frame and extending it toward the outside wall. This number tells you how thick your new door slab needs to be and where the hinges will sit.

Standard frame depth is 4 5/8 inches for a 2-by-4 wall with drywall on both sides. Older homes, thicker walls, or walls with different materials may be 3 5/8 inches, 5 1/4 inches, or something else entirely. If your frame depth does not match a standard size, the door may need to be shimmed (spacers added behind the hinges) or the frame itself may need adjustment.

Check whether the frame is square using the diagonal method

A frame that is out of square will cause a door to swing open or closed on its own, or to bind in one corner. To check, measure diagonally from the top-left corner of the frame opening to the bottom-right corner. Then measure from the top-right corner to the bottom-left corner. If both diagonal measurements are the same, the frame is square. If they differ by more than 1/4 inch, the frame is out of plumb.

Write this down and tell the installer or door company. A slightly out-of-square frame can often be corrected during installation by shimming behind the hinges. A severely out-of-square frame may need the opening adjusted before the new door goes in, which is a carpentry job, not a door-hanging job.

Measure the existing door to understand swing direction and hardware location

Before you remove the old door, note which way it swings. Stand inside the room and observe whether the hinges are on your left or right. This is called the hand of the door — left-hand or right-hand. The replacement door must swing the same way, or it will not fit the hinges and strike plate already in the frame.

Also measure where the doorknob sits on the existing door — typically 36 to 38 inches up from the bottom. If you are replacing the door slab only (keeping the existing frame and hardware), the new door must have the hole in the same spot. If you are replacing the entire unit including frame and hardware, you have more flexibility, but 36 inches is still the standard.

Account for the threshold and weatherstripping thickness

The bottom of the door frame includes a threshold — a raised piece that sits on top of the sill. If you are measuring the height of the opening, measure from the top of the threshold, not from the floor. The door will rest on the threshold when closed.

If the existing door has weatherstripping (foam or rubber seal around the frame edges), note its thickness. A new door with thicker weatherstripping may not close smoothly in a tight frame. The installer can trim weatherstripping, but it is better to know about it before the door arrives.

Record all measurements and note any irregularities

Create a straightforward sketch of the opening and write all six measurements on it: width at top, middle, and bottom; height on left and right; and frame depth. Add notes about anything unusual — a tilted frame, a threshold that is higher on one side, a wall that is not plumb, or existing hardware you plan to keep.

Take a photo of the opening and the frame from the outside and inside. When you call the door company or meet with an installer, you will have everything they need to confirm the door will fit. If measurements vary by more than 1/2 inch across the opening, the installer will likely want to see the opening in person before ordering, because the frame may need adjustment.

Frequently Asked Questions

What if my measurements are different at the top and bottom?

Use the smallest measurement. If the opening is 36 inches wide at the top and 36 1/4 inches at the bottom, order a door for 36 inches. The installer will shim the frame to make it square, or the door will be trimmed slightly to fit the narrower spot.

Do I need to remove the old door before measuring?

No. Measure the opening with the old door in place. You can measure around the edges of the frame without removing anything. If you do remove the old door first, be careful not to disturb the frame itself, because moving it can change your measurements.

What is the difference between measuring for a slab and measuring for a prehung unit?

A slab is the door only; you keep the existing frame and hinges. Measure the width and height of the opening itself. A prehung unit includes the frame and hinges; measure the rough opening in the wall, which is typically 2 inches wider and taller than the finished opening, depending on how the frame will be installed.

Can I order a door that is slightly larger and have it trimmed?

Solid wood doors can be trimmed on the sides and top, but only by 1/4 to 1/2 inch total. Hollow-core and fiberglass doors have less material to work with and should not be trimmed. Always order the correct size rather than oversizing and hoping to trim it down.

What if the frame is more than 1/2 inch out of square?

The frame itself may need to be adjusted or rebuilt before the new door is hung. This is a carpentry job. Contact a professional installer to assess whether shimming during installation will work, or whether the frame needs structural adjustment first.