If you’ve ever stood in a home improvement store staring at stacks of wood, MDF, or composite door casing, you know how easy it is to grab a length or two and end up with gaps too tight to fit or leftover trim that’s the wrong size. As a door casing supplier, I’ve seen this mistake more times than I can count—customers come to us frustrated because their first measurement was off, or they skipped a small detail that makes a huge difference when installing trim. Door casing isn’t just a decorative afterthought; it’s the frame that covers the gap between your interior door and the wall, seals out drafts, and adds a polished, finished look to any room. Getting the measurement right isn’t just about saving time or money—it’s about making sure your trim fits like it was meant to. Today, I’m walking you through the exact, step-by-step way to measure for door casing, using the tips I’ve learned from working with homeowners, contractors, and renovators for over a decade. No confusing jargon, no vague rules—just practical, science-backed steps that work every time. Door Casing
First, let’s start with the basics: what door casing actually is, and what measurements you’ll need to take. Most interior door frames have two side jambs (the vertical pieces that hold the door) and one head jamb (the horizontal piece at the top). Casing is the trim that covers the outer edges of these three pieces, wrapping around the entire door opening. That means you’ll need two side casing pieces (each for a vertical jamb) and one head casing piece (for the horizontal top jamb). Before you pick up a tape measure, here’s the first rule: take every measurement twice. And I mean twice. Doors and walls shift over time, especially in older homes, and a single tape measure run can miss small imperfections like a slightly warped jamb or a wall that’s not perfectly plumb (straight up and down). I once had a homeowner send me a photo of a door opening where one side jamb was 1/8 inch taller than the other, and their first measurement missed that small gap—so they ordered casing that was too short for one side. Don’t let that be you.
Now, let’s get to the actual measurements. The first measurement you need is the width of your door opening at the head jamb. This is the distance between the two side jambs, measured from the very outer edge of each jamb. Stand inside the room the door leads to, hold your tape measure against the left side jamb, and extend it straight across to the right side jamb, right at the top where the door closes. Write that number down—this will be the length of your head casing. For most standard interior doors, this is around 36 inches, but don’t assume that’s the case. I’ve worked with openings as narrow as 30 inches and as wide as 48 inches for custom doors, so never go off the standard size listed on the door itself.
Next, the side casing measurements. This is where things get tricky, because you need to measure from the head jamb down to the floor. But wait—there’s a catch here: the floor might not be level. In older homes, floors can slope, so if you just measure straight down from the head jamb to the floor, you might get a side casing that’s too long on one side or too short on the other. The correct way is to measure from the edge of the head jamb straight down to the point where you want the bottom of the casing to sit. For most interior doors, that’s 1/4 inch above the finished floor—this leaves a small gap so the door can swing freely without scraping the floor, and it makes cleanup easier if you ever refinish the floor later. To get this measurement right, use a level if you have one. Mark a point 1/4 inch above the floor on both side jambs, then measure from the head jamb down to that mark. This ensures both side casings are the same length, even if your floor has a gentle slope. If you have baseboards that run along the floor, don’t measure down to the baseboard—measure down to the finished floor, then account for the 1/4 inch gap when marking your cut.
Here’s a pro tip most suppliers will never tell you: add a 1/16 inch overlap to each length. Why? Because even the most careful cuts can be off by a hair, and a 1/16 inch overlap lets you trim the casing slightly during installation to get a perfect fit without gaps. I learned this early on when a contractor client of mine kept returning casing because his cuts were too short. He started adding that extra 1/16 inch, and he’s never had to order a replacement length since. It’s a small adjustment, but it saves a lot of headache.
Wait a second—what about exterior doors? Exterior door casing is a little different, because it needs to be thicker to stand up to weather, and it often has a wider profile to cover more gaps between the door and the exterior wall. The measurement process is almost the same, but with one key difference: instead of measuring to the finished floor, you’ll measure down to the top of the exterior threshold. Exterior doors need to sit on a threshold that seals out water and air, so the bottom of the exterior casing will rest just above or flush with the top of that threshold. For side casings, add a 1/8 inch overlap at the bottom instead of 1/16, because exterior trim is more prone to shifting with temperature and humidity changes.
Now, what if you have a non-standard door? Maybe you have a French door, a pocket door, or a double door opening. For French doors, you’ll have two sets of jambs, so you’ll need two head casings and four side casings, plus a small piece of casing for the middle where the two doors meet. For pocket doors, the casing wraps around the entire opening, so you’ll need three vertical side casings and two horizontal pieces—one at the top of the opening and one at the bottom, since pocket doors slide into the wall. The measurement rule still applies: take each measurement twice, account for gaps, and add that small overlap. I once worked with a historic home that had a 48-inch double door opening; the homeowner thought it needed standard side casings, but after measuring, we found the middle gap was too wide, so we added a custom narrow casing piece to cover it, and it turned out perfect.
Now, let’s talk about common mistakes that lead to bad measurements, based on years of working with customers. First, measuring from the door itself, not the frame. The door is usually slightly smaller than the frame, so if you measure the door’s width and use that for your head casing, you’ll end up with casing that’s too short by an inch or more. Second, not accounting for wall thickness. Wait—hold on, did I forget that? Oh no, that’s a big one! If you’re installing casing on an exterior wall or a wall with thick drywall, the outer edge of the jamb might be recessed. Let me explain: the door frame is installed inside the wall, so the edge of the jamb is usually 1/2 inch to 3/4 inch back from the finished wall surface. That means when you measure the width of the opening, you need to add twice the wall thickness to that number for your head casing length. For example, if your opening’s inner width is 36 inches, and your wall is 1/2 inch thick, add 1 inch (1/2 inch on each side) to get 37 inches for your head casing. Most people miss that part, and they end up with casing that’s too narrow, leaving a gap between the casing and the wall. That’s why measuring the wall thickness with a tape measure is a quick, important step I always include in my process.
Another common mistake is not accounting for baseboard overlap. If you have baseboards in the room, the side casing should meet the baseboard perfectly, not overlap or leave a gap. When you mark the 1/4 inch gap above the floor, make sure the bottom of the side casing lines up exactly with the top of the baseboard. If the baseboard is thicker than standard, you might need to adjust your gap slightly, but always measure the baseboard’s height first to avoid issues.
Once you have all your measurements, double-check them three times. Write them down on a piece of paper, label each length (head, left side, right side, exterior if applicable), and keep them with you when you order your casing. When you order, tell your supplier if you’re using standard or custom casing, and mention any unique details about your door opening—like a sloped floor, thick walls, or a double door. A good supplier will ask you these questions, so don’t be afraid to speak up about any irregularities in your space.
At the end of the day, measuring for door casing isn’t rocket science, but it does require attention to detail. It’s the difference between a trim job that looks DIY and one that looks professional, and it saves you time and money by avoiding unnecessary returns or extra trips to the store. If you’re still unsure about your measurements, or if you have a custom door opening that you need help with, don’t hesitate to reach out. We’ve worked with homeowners and contractors of all skill levels, and we’re here to help you get the perfect fit for your door casing. Whether you need standard MDF casing for a basement remodel, solid wood for a living room upgrade, or weather-resistant composite for an exterior door, we have the products and the expertise to make your project a success.
Don’t let a bad measurement derail your home improvement project. Taking the time to measure correctly the first time will ensure your door casing looks great, fits perfectly, and lasts for years to come. Reach out today to discuss your project and get the high-quality door casing you need at a price that fits your budget.
Exterior Door References
- Fine Homebuilding. (2021). "Measuring and Installing Door Casing for Interior Walls." Fine Homebuilding Publications.
- American Wood Council. (2019). "Residential Door Trim Standards and Best Practices." American Wood Council.
- DIY Network. (2020). "How to Measure for Door Trim: A Step-by-Step Guide for Homeowners." Scripps Networks Interactive.
- International Code Council. (2022). "Residential Window and Door Framing and Finishing Guidelines." International Code Council.
KS Builders Supply LLC
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