Measure the opening width, height, and depth before you order
A replacement door has to fit the frame that's already in your wall. If you measure wrong, the door won't close, won't open fully, or won't fit at all. You need three measurements: the width of the opening, the height, and how deep the frame sits in the wall. Take each measurement in three places—top, middle, and bottom for width and height—because older frames are often slightly out of square. Use the smallest number you get, because the door has to fit the tightest spot.
You'll need a tape measure at least 6 feet long, a level, a notepad, and a helper to hold the tape steady. Measure in inches, and write down every number. Don't round up or down yet—your supplier will tell you what to do with the exact measurements.
Key Takeaways
- Measure the opening width at the top, middle, and bottom, and use the smallest number because frames are rarely perfectly square.
- Measure the opening height on both the left and right sides, plus the middle, and again use the smallest measurement.
- Measure how far the frame sticks out from the wall on both sides, because the replacement door frame has to fit into that space.
- Check whether the frame is plumb (perfectly vertical) and level (perfectly horizontal) with a level tool, because a crooked frame means the door won't hang straight.
- Bring all three measurements and the plumb/level check to your supplier before you order, because they will tell you what size door to buy.
How to measure the width of the opening
Close the existing door if there is one. Measure across the opening from the inside edge of the left frame to the inside edge of the right frame. Do this at three heights: near the top, in the middle, and near the bottom. Write down all three numbers. The smallest one is your opening width.
If the frame is very crooked and the three numbers are more than half an inch apart, you may need to have the frame straightened or replaced before the new door will hang properly. Your door supplier can tell you whether the frame is salvageable when you show them the measurements.
How to measure the height of the opening
Measure from the bottom of the opening straight up to the top. Do this on the left side of the opening, the right side, and the middle. Write down all three numbers. The smallest one is your opening height.
Measure from the floor to the bottom of the frame opening, not from the floor to the sill (the threshold). The sill is often raised and will throw off your measurement. If the floor is uneven, measure at the lowest point where the door will swing.
How to measure the frame depth
The frame sits in the wall like a box. You need to know how far it sticks out on both sides of the wall, because the new door frame has to fit into that same space. Measure from the outside edge of the frame on one side to the outside edge on the other side. This is your frame depth, also called the frame thickness.
Measure this in at least two places—top and bottom—because walls are sometimes thicker at different heights. If the two numbers are different, bring both to your supplier. Also check whether the frame is flush with the wall surface or recessed (set back) on one or both sides, because that affects how the new frame will fit.
Check whether the frame is plumb and level
A door that hangs crooked will bind (stick) when you try to close it, or it will swing open on its own. Before you order a replacement, check whether the existing frame is plumb (straight up and down) and level (straight side to side). Place a level against the left side of the frame, then the right side. The bubble should sit in the center. Do the same across the top and bottom of the frame.
If the frame is out of plumb or level by more than a quarter inch, tell your supplier. They may recommend shimming (adding thin wedges) during installation, or they may suggest that the frame needs to be straightened first. A new door in a crooked frame will never close smoothly.
Measure the existing door to check your frame measurements
If the existing door is still hanging, measure it as a sanity check. Measure the width of the door itself (not the opening) from the outside edge of one side to the outside edge of the other. Measure the height from top to bottom. A standard interior door is 32 inches wide and 80 inches tall. A standard exterior door is 36 inches wide and 80 inches tall. Your opening should be roughly 2 inches wider and 2 inches taller than the door itself, because the frame takes up space.
If your opening measurements don't match this pattern, double-check them. Measure again in a different spot, or have your helper measure while you watch. A mistake here means ordering a door that won't fit.
Document everything before you go to the supplier
Write down all your measurements on a single sheet of paper: opening width (top, middle, bottom), opening height (left, right, middle), frame depth, and whether the frame is plumb and level. Take a photo of the opening and the frame from the side so the supplier can see how it sits in the wall. Bring the existing door with you if you can remove it, or at least bring a photo of it.
Tell the supplier whether you want to keep the existing frame or replace it. If you're keeping the frame, the new door has to fit inside it exactly. If you're replacing the frame, you have more flexibility, but the new frame opening has to match the hole in the wall. The supplier will use your measurements to tell you what size door to order and whether any shimming or frame repair is needed during installation.
Frequently Asked Questions
What if my measurements are different at the top, middle, and bottom?
That means the frame is out of square, which is common in older homes. Use the smallest measurement, because the door has to fit the tightest spot. Tell your supplier the range (for example, "32 inches at the top, 31.75 at the middle, 31.5 at the bottom"). They will advise whether shimming will work or whether the frame needs straightening.
Do I need to remove the existing door before measuring?
No. You can measure around it. Measure the opening from inside edge to inside edge of the frame, not from the door itself. If the door is swollen or warped, remove it so you can see the actual frame opening clearly.
What if the frame is recessed into the wall on one side?
Measure the depth on both sides separately and bring both numbers to your supplier. A recessed frame means the new frame may need to be shimmed out on one side to sit flush, or the new frame may be built to fit the recess. Your supplier will know which approach works for your situation.
Can I order a door before I measure the frame depth?
No. The frame depth determines how thick the new door frame can be. If you order without measuring, the new frame may not fit in the wall opening, and you'll have to return it. Always measure depth before you order.
What if the floor is uneven under the door opening?
Measure the height at the lowest point where the door will swing. If the floor slopes more than a quarter inch across the opening, tell your supplier. They may recommend a threshold that slopes to match the floor, or they may suggest shimming the frame during installation.