Take three measurements before you order a new door

Measure the width at the top, middle, and bottom of the door opening. Measure the height at the left, center, and right. Write down all six numbers — if they differ, the opening is out of square, and you need to know that before ordering. Most replacement doors come in standard sizes: 32, 36, or 42 inches wide, and 80 inches tall for interior doors or 82 inches for exterior doors. If your opening is not a standard size, you will either need a custom door or a frame kit to adjust the opening.

Measure the depth of the wall at the side of the door frame — this is called the rough opening depth or wall thickness. Doors come in different depths to fit walls of different thicknesses: typically 4⅝ inches for a standard 2×4 wall, or 6⅝ inches for a thicker wall. If you measure wrong here, the door frame will not sit flush, and you will have gaps or trim problems.

Key Takeaways

  • Measure width at three heights and height at three widths to catch an out-of-square opening before you order.
  • The wall thickness (measured at the side of the frame) determines whether you need a 4⅝-inch or 6⅝-inch deep door frame.
  • Standard door sizes are 32, 36, or 42 inches wide; if your opening is different, you will need a custom door or a frame adjustment kit.
  • Note whether the door swings inward or outward and which side the hinges are on, because this affects which direction the new door opens.
  • If measurements vary by more than ⅛ inch, the opening is out of square and you should photograph it or call a professional before ordering.

Measure the width of the opening

Use a tape measure to find the distance across the inside of the frame at three different heights: near the top, in the middle, and near the bottom. Measure from the inside edge of the left frame to the inside edge of the right frame. Write down all three numbers.

If all three measurements are the same, the opening is square. If they differ by ⅛ inch or less, that is normal and acceptable. If they differ by more than ⅛ inch, the opening is out of square — this happens when the house has settled or the frame has warped. An out-of-square opening means the new door will not close evenly or may stick. Take a photo of the opening and either call a professional to assess it or contact the door supplier to ask whether a standard door will work.

Measure the height of the opening

Measure the distance from the bottom of the frame to the top of the frame at three points: on the left side, in the center, and on the right side. Again, write down all three numbers.

Standard interior doors are 80 inches tall; standard exterior doors are 82 inches tall. If your opening is shorter than standard, you will need a custom door or a frame kit that raises the opening. If your opening is taller, you will need a custom door or you will have to fill the gap above the door with trim or a transom window. Most replacement jobs use standard heights because the cost and complexity jump sharply for custom sizes.

Measure the wall thickness

This is the depth of the wall at the side of the door frame. Stand to the side of the door and measure from the inside face of the frame (the side facing into the room) to the outside face of the frame (the side facing out). This number tells you how thick the wall is and determines what depth of door frame you need to order.

A standard 2×4 wall with drywall on both sides is about 4⅝ inches thick. Older homes or exterior walls with extra insulation may be 6⅝ inches or thicker. If you order a frame that is too shallow, it will not reach the outside of the wall and you will have a gap. If you order one that is too deep, it will stick out past the wall and look wrong. Measure carefully and confirm the number with the door supplier before you order.

Check the hinge side and swing direction

Stand in front of the door and note which side the hinges are on — left or right. Then note which way the door swings: does it open toward you or away from you? This is called the swing direction, and it matters because doors are handed — a left-hand door and a right-hand door are not interchangeable.

If you order a door that swings the wrong way, it will not fit the opening without expensive modifications. Write down "left-hand inswing" or "right-hand outswing" (or whichever applies to your door) and include this in your order. If you are not sure, take a photo of the door from the side where the hinges are visible, and show it to the supplier.

Photograph the opening and existing door

Take a clear photo of the entire door frame from the front, showing the trim and the wall around it. Take a side photo showing the depth of the wall. Take a close-up of the hinges and the hinge side. These photos help a supplier or contractor understand your situation if measurements seem unusual or if the opening is out of square.

If the door is an exterior door, also photograph the threshold (the piece at the bottom) and note whether it is wood, metal, or composite. Different thresholds have different heights and may affect how the new door sits. If the door has a sill pan or water barrier underneath, photograph that too — you may need to reuse it or replace it as part of the door installation.

Know when to call a professional

If your measurements vary by more than ⅛ inch, if the opening is noticeably tilted or twisted, or if the frame is cracked or rotted, call a contractor before you order. A professional can assess whether the opening needs to be squared up before a new door goes in, or whether a custom door is the better choice. This costs less than ordering a door that does not fit.

If you are replacing an exterior door, a professional should also check the sill and the area underneath for rot or water damage. Water damage is common around doors and may need repair before the new door is installed. If you install a new door over rotted wood, the problem will get worse and the door will fail early.

Frequently Asked Questions

What if my door opening is not a standard size?

You have two options: order a custom door made to your exact measurements, or use a frame adjustment kit to make your opening match a standard size. Custom doors cost more and take longer to arrive. Frame kits are cheaper and faster but require some carpentry to install. Ask your supplier which option makes sense for your situation.

Do I need to measure the existing door?

No. Measure the opening, not the door. The opening is what determines what size door you can install. The existing door may be warped, damaged, or installed incorrectly, so it is not a reliable guide.

What does "out of square" mean and why does it matter?

Out of square means the opening is not a perfect rectangle — one corner is pushed in or pulled out. This happens when a house settles or a frame warps. A door installed in an out-of-square opening will not close evenly and may stick or rattle. A professional can assess whether the opening can be squared up or whether a custom door is needed.

Can I measure over the phone and have someone else buy the door?

You can, but it is risky. Measurement mistakes are common and expensive to fix. If possible, take photos and measurements yourself, or have the person buying the door present when you measure. Most suppliers will not accept returns on custom or special-order doors, so accuracy matters.

Should I measure the trim around the door?

Measure the opening itself, not the trim. The trim is decorative and can be removed and replaced. The opening is what the new door frame has to fit into. If you are keeping the existing trim, note its width so the new frame will align with it, but the opening measurement is what determines whether a standard door will work.