Knowing how to measure a interior door correctly saves you a massive headache at the hardware store and keeps you from having to return a heavy slab that just won't fit. It sounds like a simple enough task—you just pull out a tape measure and call it a day, right? Well, not exactly. If you've ever tried to hang a door only to realize it's a quarter-inch too wide or that the hinges don't line up, you know how quickly a "five-minute job" can turn into an all-day ordeal.
Most people don't realize that houses settle over time. That perfectly rectangular doorway from 1994 might be a slightly funky trapezoid today. That's why we don't just take one measurement and hope for the best. We're going to look at the right way to get those numbers so your new door swings perfectly every single time.
Getting your tools together
Before you even touch the door handle, make sure you have a few things handy. You don't need a fancy laser level or anything high-tech. A standard steel tape measure is your best friend here. Don't use a fabric one from a sewing kit; they stretch, and being off by an eighth of an inch actually matters when you're dealing with door clearances.
You'll also want a notepad and a pen. It's easy to think you'll remember "30 and 1/4," but by the time you walk to the computer or the store, your brain might turn that into "31 and 1/4," and then you're in trouble. If you're measuring multiple doors in the house, label them clearly, like "Guest Bath" or "Pantry," so you don't mix them up.
Slab vs. Pre-hung: Which are you doing?
Before you start pulling the tape, you need to decide what you're actually replacing. This is the biggest fork in the road.
If you're just swapping out the "slab"—that's just the wooden door itself—and keeping the existing frame and trim, you'll be measuring the door that's already there. This is common if your current door is just beat up or if you want a different style but the frame is still in great shape.
On the other hand, if you're ripping everything out down to the 2x4 studs, you're looking for a "pre-hung" door. This comes with the door already attached to a new frame. In this case, you aren't measuring the old door; you're measuring the "rough opening" (the hole in the wall). Knowing which one you need changes how to measure a interior door significantly.
Measuring an existing door slab
Let's say the frame is fine and you just want a new look. To get the right size for a replacement slab, you should measure the old door directly.
Measure the width in three spots
Don't just measure the middle. Measure the width at the top, the center, and the bottom. Why? Because doors can warp, or the house can shift. If the numbers are slightly different, use the largest measurement as your guide, but keep in mind you might need to shave a little off the sides of a new wood door to get a custom fit. Most standard interior doors come in widths like 24, 28, 30, 32, or 36 inches.
Get the height
Just like the width, measure the height on both the left and right sides. If you've got a thick carpet or you're planning on installing new hardwood floors soon, keep that in mind. You need enough clearance at the bottom for the door to swing without dragging. If the floor is uneven, use the shortest measurement so the door doesn't get stuck on the floor.
Don't forget the thickness
This is a step a lot of people skip. Most interior doors are 1 3/8 inches thick. However, some heavy-duty or older doors might be 1 3/4 inches. If you buy a door that's too thick, it won't sit flush with the jamb, and the latch won't line up. It's a small detail that makes a huge difference.
Measuring the rough opening
If you're doing a full renovation and taking out the frame, you're measuring the "rough opening." This is the space between the wooden studs once the old door and its frame are completely gone.
To do this right, you'll want to measure the width between the studs in three places (top, middle, bottom) and take the smallest measurement. Then, measure the height from the floor to the header (the top beam) in two or three places and take the smallest measurement there, too.
The reason we take the smallest measurement for a rough opening is that you need a little bit of "wiggle room" to level and plumb the new door frame. You can always fill extra space with shims, but you can't easily make a hole in a wall bigger without a lot of extra work.
Determining the door swing (Handing)
Once you have the dimensions, you have to figure out the "handing." This is just a fancy word for which way the door swings. Getting this wrong is a classic DIY mistake.
The easiest way to figure this out is to stand in the doorway with your back against the hinge jamb (the side where the hinges are). If the door swings to your right, it's a right-handed door. If it swings to your left, it's a left-handed door.
Another way to think about it: if you're pulling the door toward you, which hand are you using to grab the handle? If you use your right hand to pull it open and it swings to your right, it's right-handed. It sounds confusing when you read it, but once you're standing there, it usually clicks.
Measuring for hardware placement
If you're buying a "blank" slab (one that doesn't have holes pre-drilled for the handle and hinges), you'll need to know exactly where the old hardware was located if you want it to match the existing frame.
Measure from the top of the door down to the center of each hinge. Usually, there are three hinges on an interior door. You'll also want to measure from the top of the door to the center of the hole for the doorknob (the "bore hole").
Also, check the "backset." This is the distance from the edge of the door to the center of the doorknob hole. Most modern doors have a backset of either 2 3/8 inches or 2 3/4 inches. If you get this wrong, your latch won't reach the strike plate on the frame.
Double-checking the jamb width
If you're buying a pre-hung door, you need to make sure the door jamb is the right width for your wall. A standard interior wall made of 2x4 studs with half-inch drywall on both sides usually requires a 4 9/16-inch jamb. If you have an older house with thicker plaster walls or you're using 2x6 studs, you might need a wider jamb.
To measure this, just measure the total thickness of the wall where the door is going, including the drywall or plaster. Don't include the decorative trim (casing) in this measurement—just the wall itself.
Common pitfalls to avoid
One thing that trips people up is measuring the trim instead of the door. The trim (the decorative wood around the door) covers the gap between the frame and the wall. It doesn't tell you anything about the size of the door you need. Always pull the tape measure across the actual door or the space where the door sits.
Another tip: if you're measuring a door that isn't there anymore (maybe it was kicked in or removed years ago), measure the inside of the door frame where the door is supposed to stop. Subtract about 1/8 of an inch from the width and height to allow for a gap so the door can actually move without rubbing.
Wrapping things up
Taking the time to figure out how to measure a interior door is really about precision and double-checking your work. It's much better to spend an extra ten minutes measuring three times than to spend three hours trying to sand down a door that's too big or staring at a door that's too small.
Write everything down, take photos of your measurements if you need to, and don't be afraid to pull that tape measure out one last time before you head to the store. Once you have those numbers locked in, you can shop with confidence, knowing that your new door is going to look great and, more importantly, actually close properly. Good luck with the install!