Wondering how to measure a door for replacement like a pro. Get accurate steps to measure width, height & depth for a perfect fit.
Before you even think about picking out your new door style, there’s one step you can’t to skip is getting accurate door measurements. This step is what determines whether your new replacement door will slide right in or leave you fighting gaps, misalignment, or worse, a door that won’t close all the way.
When a door doesn’t fit properly, you risk air leaks, energy loss, and even issues with your door jamb or hinges over time. And it’s not just about looks. That tight seal between the door slab and the existing door frame is what keeps your home comfortable and secure.
To get that proper fit, you’ll want to measure the width and height of your existing door, check the jamb depth, and note whether you’re replacing only the door slab or the entire door unit. Small mistakes at this stage can turn into costly problems later.
But don’t worry, it’s a straightforward process once you know what to look for. And of course, if you need help figuring out whether your opening is standard or something a bit custom, US Windows is always here with expert advice and friendly support.
Tools You’ll Need Before You Start
Before you dive into how to measure a door for replacement, it helps to have a few simple tools on hand.
Start with a tape measure, you’ll need it to record the width and height of your existing door and the rough opening. A pencil and notepad will help you jot down the smallest dimension from each set of measurements (trust us, you’ll want these handy).
If you’re planning to remove the interior trim or casing to measure the door frame, grab a flat pry bar. Just take it slow to avoid damaging the surrounding wall or exterior trim.
A level is optional but useful, especially if you’re checking for any warping along the top jamb or side jamb. And here’s a bonus tip: take a few photos of the old door, door panel, and existing frame before you start. It’s a simple way to document hinge placement, door swing, or anything unique about the frame size that you might want to match with your new door.
Having these basic tools nearby will make the process smoother, and give you confidence that your door measurements are right the first time.
Understand the Type of Door You’re Measuring
Before you pull out the tape measure, it helps to know exactly what kind of door you’re dealing with. That alone shapes how you take your measurements and what kind of replacement door you’ll need.
Door Slab, Door Jamb, Door Swing vs. Prehung Door Unit
If you’re replacing just the door slab, you’ll focus only on the door width, height, and thickness. But if you’re installing a prehung door unit, you’ll also need to measure the door jamb, frame, and rough opening. These extra measurements help ensure the new unit fits the space and swings properly once installed.
Interior or Exterior Door?
Interior doors tend to be lighter and slightly thinner than exterior doors for replacement. Measuring for interior doors is often simpler, but accuracy still matters to avoid issues with door swing or friction. With exterior doors, including front doors, you’ll also need to account for thresholds, jamb depth, and exterior trim for a weather-tight fit.
Storm, Patio, or Screen Doors
If you’re working on a storm door replacement, screen door, or a patio door, take note of where the door mounts are inside or outside the existing frame. You’ll often need to measure the outside edge of the opening and be extra mindful of interior trim and any protruding siding or framing that might interfere with a snug fit perfectly.
Understanding the type of door you have upfront saves time and ensures the measurements you take are exactly what your new door needs to fit and function perfectly.
Step-by-Step: How to Measure a Door for Replacement
Before you move forward with choosing a replacement door, you’ll want to take a few key measurements. This process helps you figure out if you’re replacing just the door slab or the full unit, and ensures the new door will fit the opening properly without extra work or surprises later.
Measure the Width
Start by measuring the width of the door opening in three spots across the top, middle, and bottom. You’re measuring between the inside top edges of the side jambs, not including the trim. Doors and frames can shift over time, so if those numbers vary, go with the smallest one. That’s the best way to ensure your new door fits the opening without binding.
Measure the Height
Next, measure from the top jamb down to the floor or the threshold. Again, take three measurements left, center, and right, recording the smallest height. This step is especially important for exterior doors where tight sealing against weather matters.
Check the Door Thickness
Use your tape measure to check how thick the current door is. Most interior doors are 1-3/8 inches, and most exterior doors are 1-3/4 inches. Knowing this helps you choose a replacement that fits into the existing frame without sticking out or sitting too deep.
Measure the Jamb Depth
Now measure from the interior edge of the door frame to the exterior edge. This number helps ensure that your new prehung door or frame won’t look too bulky or too narrow for your wall.
Measure the Rough Opening (if replacing the frame)
If you’re replacing the entire frame or installing a prehung door, you’ll also need the rough opening measurements. That means removing the trim and measuring the space between the studs. Measure both the width and the height. It’s a little more work, but it’s what gives you the most accurate fit for a full replacement.
Measuring for Specific Scenarios (Mini-Guides)
Not all doors are measured the same way. Whether it’s a front entry doors or a sliding patio door, or a full double door setup, the approach shifts a bit. Here’s how to tackle a few of the most common door types.
Measuring a Front Door for Replacement
If you’re working on replacing your front door, start with the basics. Measure the width in three places top, middle, and bottom, then write down the smallest number. Do the same for the height, from the top jamb to the threshold, at the left, center, and right side. Make sure to also check the thickness of the door and the jamb depth. These extra details help ensure your new dream door fits securely and seals properly.
Measuring for a Sliding Glass Door Replacement
Sliding doors are usually full units, not just slabs. To measure one, start by recording the width from edge to edge of the existing frame, in three spots across the top, middle, and bottom. Do the same for the height left, center, and right. These measurements help you order a sliding glass door that fits into the opening without gaps or uneven tracking. If there’s any trim or siding overlapping the frame, make a note of that too.
Measuring a Double Door or French Door for Replacement
For double or French doors, you’ll measure the full width of the opening and the height from the top jamb to the floor. The key here is to double-check that both sides are even. If you’re replacing just the panels, you’ll also want to measure the width and height of each individual door slab. Don’t forget to confirm door swing and hinge placement, especially if only one side opens.
Measuring a Door Slab vs. a Prehung Door
The easiest way to tell which you’re working with is by checking the frame. If you’re keeping the existing frame, you’re just replacing the door slab and you’ll only need to measure the width, height, and thickness of the current door. If the frame is damaged or you’re changing the door style, you’ll likely need a prehung unit. That means measuring the rough opening behind the trim from stud to stud, and from the subfloor to the top framing board.
Frequently Asked Questions
What’s The Difference Between A Door Slab And A Prehung Door?
A door slab is just the panel itself, no hinges, no frame. It’s a good option if your existing frame is still in great shape. A prehung front door comes already mounted in a new frame. This is a better choice if your old frame is damaged, out of square, or you’re changing the door size or swing.
How Do I Know If I Need A New Frame Or Just The Door?
Take a close look at the door jamb, side casing, and the interior trim. If there’s visible damage, rot, or warping, or if the door never closed right to begin withit’, s usually worth replacing the entire unit with a prehung front door.
Can I Replace A Single Door With A Double Door?
In some cases, but it depends on the width of your opening. If your existing frame isn’t wide enough, the wall may need to be re-framed to fit a double door or French door setup. It’s a bigger project, but doable with the right planning.
Do I Need To Remove The Trim To Measure Accurately?
If you’re ordering a prehung door, then yes, you’ll want to remove the interior trim to get accurate rough opening measurements. But for a slab replacement, you can usually measure without taking anything apart.
What If My Opening Isn’t Square?
Many older homes settle over time, and it’s common to find that the top jamb or sides aren’t perfectly even. If the difference between diagonal measurements is more than ½ inch, it’s a good idea to have a pro take a look. It might still be fixable with minor adjustments, but you’ll want to catch it before placing an order.