Replacing a door sounds intimidating, but it is really just a numbers game. If you can read a tape measure, you can handle this project without calling a pro.
I’ll walk you through exactly how to measure a door, whether you are swapping out a simple slab or ripping out the whole frame for a pre-hung unit. Let’s get those numbers right the first time so you don’t end up with a drafty fit.
Key Takeaways
- Know your door type: Determine if you need a simple slab replacement or a full pre-hung system.
- Measure the door, not the trim: For slabs, measure the actual door; for pre-hung, measure the rough opening.
- Check jamb width: Don’t forget to measure the thickness of the wall to ensure the frame fits flush.
- Determine swing direction: Stand with your back to the hinges to figure out if you need a left-hand or right-hand door.
Anatomy of a Door
Before we start measuring, you need to speak the language. Confusing the “jamb” with the “casing” can lead to buying the wrong size.
- Casing: The decorative molding (trim) nailed to the wall around the door frame.
- Slab: The door itself, without any frame or hinges attached.
- Jamb: The vertical and horizontal parts of the frame that the door hangs inside.
- Stop: The thin strip of wood inside the jamb that stops the door from swinging through.
- Stiles: The vertical sides of the actual door slab.
- Rough Opening: The structural opening in the wall (stud-to-stud) that holds the door frame.
- Threshold/Sill: The bottom plate of an exterior door frame that seals against the floor.
- Header: The structural beam running horizontally across the top of the rough opening.
Pre-Hung Door vs. Door Slab
You have two choices when buying a door.
A slab door is just the rectangle of wood or fiberglass. It has no frame, hinges, or handle. You choose this if your existing door frame is in perfect condition and you just want to update the look of the door itself.
A pre-hung door comes attached to a new frame with hinges already installed. You choose this if your current frame is damaged, rotting, or if you are installing a door where there wasn’t one before.
How to Measure for a Door Slab
If you are keeping the existing frame, you need to measure the old door, not the opening.
1. Measure the Width
Run your tape measure horizontally across the door. Measure at the top, middle, and bottom.
- If the numbers vary slightly, and you are buying a pre-made standard door, round up to the nearest inch (usually 24, 30, 32, or 36 inches).
- If you are cutting a door to fit, use the widest measurement, but be prepared to sand it down for a perfect fit.
2. Measure the Height
Measure vertically from the top corner to the bottom corner. Do this on both the left and right sides. Standard doors are usually 80 inches, but older homes might have custom heights.
3. Measure the Thickness
Open the door and measure the edge. Most interior doors are 1 3/8 inches thick, while exterior doors are usually 1 3/4 inches thick. This matters for your handle and lockset.
4. Check Hinge Locations
If you aren’t buying a “blank” slab, you need to measure from the top of the door down to the top of each hinge. This ensures your new hinges line up with the old frame.
Pro Tip
If your old door fits perfectly, it is your best template. Lay the old door on top of the new slab and trace the outline to get the exact dimensions for cutting.
How to Measure for a Pre-Hung Door
For a pre-hung door, you need to measure the rough opening. This means you have to remove the interior casing (trim) to see the studs.
1. Measure the Rough Width
Measure the distance between the inside of the left stud and the inside of the right stud. Measure at the top, middle, and bottom. Record the smallest number. Ideally, your rough opening should be 2 inches wider than the door size you plan to buy.
2. Measure the Rough Height
Measure from the subfloor (the plywood or concrete under your carpet/flooring) up to the bottom of the structural header. Record the shortest measurement. The rough opening is usually 2.5 inches taller than the door height.
3. Measure Wall Thickness (Jamb Width)
This is the step most people forget. Measure the thickness of the wall itself, including the drywall on both sides.
- Standard 2×4 walls usually require a 4 9/16-inch jamb.
- Standard 2×6 walls usually require a 6 9/16-inch jamb.
If you get this wrong, your door frame won’t sit flush with the wall, and your trim won’t fit correctly.
How to Measure for a Sliding Door
Sliding patio doors or bypass closet doors require tight tolerances to roll smoothly.
1. Rough Opening Width
Just like a pre-hung door, measure stud-to-stud at the top, middle, and bottom. Use the smallest measurement. Standard sliders come in 60-inch (5-foot) or 72-inch (6-foot) widths.
2. Rough Opening Height
Measure from the subfloor to the header. Note that if you are installing on top of finished flooring, you need to account for that height difference so the door doesn’t get stuck on the carpet.
3. Check for Square
Sliding doors hate crooked frames. Use a level to check the sides and the top. If the opening isn’t square, you will need shims to level the track, so make sure you have a little wiggle room in your measurements.
Determine Door Swing Direction
Ordering the wrong swing direction is a classic DIY mistake. Here is the foolproof way to get it right.
Stand in the door frame with your back against the hinges so the door swings open away from you.
- If the door is on your left, it is a Left-Hand door.
- If the door is on your right, it is a Right-Hand door.
For exterior doors, you also need to specify “In-swing” (opens into the house) or “Out-swing” (opens to the outside). Most residential entry doors are In-swing for security, while patio doors often swing out.
FAQs
Here are the most common questions I get asked when homeowners start ripping out frames.
What Are Standard Door Sizes?
The most common height for passage doors is 80 inches (6 feet, 8 inches). Standard widths include 24, 28, 30, 32, and 36 inches. If you are building for aging-in-place or wheelchair accessibility, a minimum width of 36 inches is recommended.
How Do You Read a Door Size Code?
Industry pros use a simple code like “3068.” The first two digits are width in feet and inches (30 = 3 feet, 0 inches). The last two digits are height (68 = 6 feet, 8 inches). So, a 3068 door is 36 inches wide by 80 inches tall.
Can I Trim a Door Slab to Fit?
Yes, solid wood doors can be trimmed significantly. However, hollow core doors only have about an inch of solid wood block around the perimeter. If you trim more than that, you will expose the hollow center and ruin the door’s structure.
What if My Door Frame Isn’t Square?
If your frame is out of square (crooked), do not try to cut a crooked door. Instead, buy a slightly smaller door slab to fit the narrowest point of the frame, or better yet, replace the entire unit with a pre-hung door and use shims to level it perfectly.
How Do I Measure for a Storm Door?
Measure the opening of the exterior trim (brick mold), not the door itself. Measure width at the top, middle, and bottom, and height on both sides. Storm doors are designed to mount onto this exterior trim, so those dimensions are what matters.
Bottom Line
Now that you know how to measure a door, the hard part is over. Whether you are dealing with a standard 36-inch entry or a quirky closet slider, accuracy is your best friend.
Always measure twice so you only have to buy once. Grab your pencil and paper, check that wall thickness one more time, and go get that upgrade your home deserves.
