Measure the door frame opening, not the door itself
The opening in your wall—called the rough opening—is what you measure, not the existing door. Replacement doors come in standard sizes, and you need to know whether your opening matches one of those sizes or whether you need a custom door. Most interior doors come in widths of 24, 28, 30, 32, or 36 inches, and exterior doors typically start at 32 inches. Heights are usually 80 inches for standard interior doors and 80 or 84 inches for exterior doors.
The frame itself—the wood or metal border around the door—sits inside the rough opening. When you measure, you are measuring the opening the frame occupies, which tells you what size door will fit. If your frame is damaged or you are removing it entirely, you measure the rough opening in the wall. If you are keeping the existing frame and replacing only the door panel, you measure the opening inside the frame.
Key Takeaways
- Measure the opening width at three points—top, middle, and bottom—because walls are often not perfectly square, and you need the smallest measurement.
- Measure the opening height at three points—left, center, and right—for the same reason, and record the smallest number.
- Standard door sizes come in 2-inch increments for width (24, 26, 28, 30, 32, 36 inches) and 80 or 84 inches for height, so your measurements should match one of these.
- If your opening is between standard sizes, you will need a custom door, which costs more and takes longer to arrive than stock doors.
- Measure twice and write down all three measurements for each dimension so you can compare them and catch mistakes before ordering.
Gather the right tools and clear the space
You need a tape measure at least 10 feet long—a 25-foot tape is better because you can stretch it without kinking. A level is essential to check whether the frame is plumb (perfectly vertical) and square, because an out-of-plumb frame will make a new door hang crooked. A pencil and paper or a phone with a notes app will hold your measurements so you do not forget them between the frame and the store.
Clear the area in front of and behind the door so you can move freely and reach all the spots you need to measure. If the door is currently closed, open it fully and prop it so it stays out of your way. If there is furniture, boxes, or clutter blocking access to the frame edges, move it aside. You need clear sight lines to the top corners and edges of the frame.
Measure the width at three heights
Measure the width of the opening from the left edge of the frame to the right edge of the frame. Do this at the top of the frame, in the middle, and at the bottom. Write down all three numbers. Walls settle and shift over time, so the opening is often wider at one height than another. The smallest of the three measurements is the one you use, because that is the widest door that will fit through the entire opening.
Place your tape measure flat against the frame, not at an angle. If you are measuring an interior door and the frame is still in place, measure from the inside edge of the left frame piece to the inside edge of the right frame piece. If you are measuring a rough opening with no frame, measure from the inside edge of the left wall stud to the inside edge of the right wall stud. Take your time and read the tape carefully—a mistake of half an inch here means the door will not close properly or will not fit at all.
Measure the height at three widths
Measure the height of the opening from the bottom to the top. Do this at the left side of the frame, in the center, and at the right side. Again, write down all three numbers and use the smallest one. Floors are often not level, and the frame may have settled unevenly, so the opening height can vary across the width.
For an interior door with the frame in place, measure from the top inside edge of the frame to the bottom inside edge. For a rough opening, measure from the top of the header (the horizontal beam above the opening) to the floor or subfloor. If the floor is uneven, measure to the highest point of the floor in the opening, because that is where the door will sit. A door that is too tall will bind against the floor or the header and will not swing freely.
Check whether the frame is plumb and square
Place your level against the left side of the frame from top to bottom. The bubble should stay centered. If it drifts to one side, the frame is leaning—it is not plumb. Do the same on the right side. Then place the level across the top of the frame horizontally. If the bubble is not centered, the top is not level, which means the frame is twisted.
An out-of-plumb or twisted frame will cause a new door to hang unevenly and may prevent it from closing or opening smoothly. If the frame is significantly out of plumb—more than a quarter inch over the height of the door—you may need to shim the new door during installation, or the frame may need to be straightened or replaced. Note any out-of-plumb measurements on your sheet so you can mention them to the door supplier or installer.
Record your measurements in a clear format
Write down your measurements in a way you can read later without confusion. A straightforward format is:
| Dimension | Top/Left | Middle/Center | Bottom/Right | Use This |
|---|---|---|---|---|
| Width | ___ | ___ | ___ | ___ (smallest) |
| Height | ___ | ___ | ___ | ___ (smallest) |
Take a photo of your measurements with your phone so you have a backup. Also take a photo of the door itself—the style, color, and hardware—so you can match it or choose a replacement that fits your home. If the frame is out of plumb, note that too. When you go to the store or call a supplier, you will have everything you need to order the right door the first time.
Match your measurements to standard door sizes
Once you have your smallest width and height measurements, compare them to the standard sizes your supplier carries. Most home improvement stores stock interior doors in 24, 26, 28, 30, 32, and 36-inch widths, all at 80 inches tall. Exterior doors usually start at 32 inches wide and may go up to 36 or 42 inches, also at 80 or 84 inches tall.
If your measurements match a standard size exactly or are within a quarter inch, you can order a stock door and have it in days or weeks. If your opening is between standard sizes—for example, 31 inches wide—you will need a custom door, which takes longer and costs more. Some suppliers can also trim a slightly oversized door to fit, though this is not always possible depending on the door style and construction. Ask your supplier what options are available for your specific measurements before you order.
Frequently Asked Questions
Do I measure the door or the opening?
Measure the opening, not the door. The opening is the space in the wall where the door frame sits. The door itself is smaller than the opening so it can swing freely. If you measure the existing door, you will order a door that is too small.
What if my three measurements are all different?
Use the smallest measurement. If your width measurements are 32.5, 32, and 31.75 inches, order a 30-inch door because it is the only size that will fit through the entire opening at all three heights. A larger door will bind where the opening is narrowest.
Can I order a door that is slightly larger and trim it down?
Some doors can be trimmed, but not all. Solid wood doors and doors with glass panels are harder to trim than hollow-core interior doors. Ask your supplier whether trimming is possible for the specific door you want before you order. If trimming is not an option and your opening is between standard sizes, you will need a custom door.
What does it mean if my frame is out of plumb?
Out of plumb means the frame is leaning or twisted instead of standing straight up and down. A new door will hang unevenly in an out-of-plumb frame. You may be able to shim it during installation to make it work, or the frame may need to be straightened. Tell your installer about any out-of-plumb measurements so they can plan ahead.
Should I measure the frame opening or the rough opening?
If the frame is still in place and you are replacing only the door panel, measure the opening inside the frame. If you are removing the frame or it is already gone, measure the rough opening in the wall. Your supplier can tell you which measurement they need based on what you are replacing.