How to Measure an Interior Door

Almost every interior door that arrives wrong was ordered from a bad measurement. The failure is rarely arithmetic. It is measuring the wrong thing, because a door project contains three different measurements that all get called “door size,” and they differ by more than an inch.

This guide breaks down what each measurement actually is, how to take it, and how to read a manufacturer’s sizing chart without buying the wrong door. It is worth reading before you buy, not after, because a door that does not fit is a return, and a slab you cannot return is a piece of firewood.

If you already know what you are installing, how to install a prehung interior door and how to install an interior door slab both start from the numbers this page produces. Before you order either, the prehung vs slab door guide compares the two formats so you buy the one that fits your frame and your skill.

The Three Measurements, and Why They Are Not Interchangeable

Here is the single most useful thing to understand before you measure anything.

A door project involves three numbers, and each one is bigger than the last. The slab size is the door panel itself. The unit dimension, sometimes called the outside frame or OD frame size, is the slab plus the jambs hanging on it. The rough opening is the framed hole in the wall that the unit drops into.

Here is a real example from a Dyke Door sizing chart, using a nominal 2/4 door, which means 24 inches:

Notice the pattern, because it resolves most of the confusion. The nominal call-out size is what gets printed on the box. The slab is about 1-1/4 inches narrower than its nominal number. The unit adds the jambs back on. And the rough opening is the largest of the three, typically nominal width plus 2 inches and nominal height plus 2-1/2 inches.

So if you measure your opening at 30 inches wide and then order “a 30 inch door,” you have made a 2 inch error. That door needs a 32 inch rough opening. You would be installing a door that is 2 inches too big for the hole.

The inverse mistake is just as common. Someone measures the existing slab, gets 28-3/8 inches, and rounds up to “a 29 inch door.” There is no 29 inch interior door. The nominal sizes are a fixed set, which is what makes doors interchangeable across a house.

Measuring the Existing Slab

When you are replacing a door in a frame that is staying, you are measuring the slab. Start by removing the door from its hinges, which takes two or three minutes with a screwdriver and buys you an accurate measurement that is impossible to get with the door in place.

Here is the procedure that the measuring guides agree on.

1. Measure the width at three points: about 6 inches from the top, at the middle, and about 6 inches from the bottom. Use a tape measure on the door itself, not the gap in the frame.
2. Measure the height at two points: on each side, from the top edge to the bottom edge, skipping the hinge side where the hardware and hinge mortices live.
3. Use the smallest measurement. This is the rule that matters most and the one most often skipped. Doors are rarely perfectly rectangular, and the smallest reading is the one that tells you whether the door will clear the frame.
4. Measure the thickness. Standard hollow-core interior slabs are 1-3/8 inches and solid-core slabs are 1-3/4 inches. If your new slab is thicker than the old one, it will not close against the stop.

One measurement rule deserves special emphasis: measure the door, not the space between the door stops. The stops are there to hold the door closed, and the gap between them is smaller than the door is. Measuring the stops and buying to that number is a classic way to end up with a slab that is an inch too wide.

Measuring a Rough Opening

When the frame is being replaced or you are starting from a bare opening, you are measuring the hole in the wall. Take the width at the top, the middle, and the bottom of the opening, and the height at both sides, and again use the smallest figure.

From there, you have a decision to make. If the opening is a framed hole that has already been built, the safe approach is to work from the manufacturer’s rough opening table for the specific unit you want.

If you are cutting a new opening in a stud wall, you get to choose, and the rule is different. The rough opening is nominal door width plus 2 inches and nominal door height plus 2-1/2 inches.

That second rule lines up with what most retailers list. Menards product pages, for instance, list a 24 inch door with a 26 by 82-1/2 inch rough opening, and a 36 inch door with a 38 by 82-1/2 inch opening. That is plus 2 inches on width and plus 2-1/2 inches on height, every time.

Here is where the two big instruction sheets appear to contradict each other, and it trips up almost everyone. Home Depot’s prehung instructions say the rough opening must be 2 inches wider than the door. The Lowe’s Masonite sheet says it must be 1/2 inch wider than the unit.

They are both right, because they measure from different points. The 2 inches is added to the slab. The 1/2 inch is added to the unit, which already includes the jambs. Use whichever one matches the number in your hand, and read your own manufacturer’s chart before you order.

Height, and the case of the 96 inch door

The standard interior door height is 80 inches, which gives a nominal rough opening of 82-1/2 inches. But 80 inches does not fit everywhere, and the exceptions are where measurement mistakes hide.

A 96 inch door, sometimes sold as an 8 foot door, is the standard in many basements, in ranch homes with slab foundations, and in rooms where the ceiling is low enough that a standard door would look wrong against it. Its rough opening is roughly 98-1/2 inches tall.

There is a second reason a standard door will not fit that has nothing to do with height: the floor. If the finished floor sits higher than the subfloor the frame was built on, a door trimmed for the subfloor will drag on the flooring.

This is why the floor finish needs to be decided before the door is ordered, and why the Lowe’s entry door instructions tell you to determine the actual finish floor height before installation and adjust the rough opening if needed.

The safe rule for a replacement door: measure the existing opening and the existing slab, and order the same nominal height the door you are replacing. If the floor has changed, that is a framing problem to solve separately.

Jamb depth and wall thickness

This is the measurement people forget entirely, and it is the one that cannot be fixed after the door arrives. The window and door buying guide maps the wider family, but for this cluster the three measurement rules below are the ones that decide whether the door fits.

Jamb depth is the thickness of the frame itself, measured from one face to the other. Standard jamb stock is 4-9/16 inches for a 2×4 wall and 6-9/16 inches for a 2×6 wall, both assuming drywall on both faces. A 2×4 wall with half-inch drywall on each side measures 4-1/2 inches of finished thickness, which is what the jamb has to fill.

If the jamb is shallower than the finished wall, the casing will not sit flat against the wall and you will have a gap around the door trim. If the jamb is deeper than the wall, the door will sit back inside the opening and the trim will look wrong. Neither is fixable by trimming the door.

Some cases need a different solution entirely. A wall that is unusually thick, or one that is plaster and lath rather than drywall, may call for a split jamb, which is cut lengthwise so you can adjust the fit on site. A wall that is thinner than a 4-9/16 inch jamb is not a standard residential situation and you should confirm before ordering.

Swing and Hand: Getting the Door to Open the Way You Want

The second thing you need to know before you order is which way the door should swing, and which hand it is.

The standard test, and the one the major manufacturer sheets use, is simple. Stand outside the room with your back to the hinge side and push the door open.

The knob is on your right, so it is a right-hand door. If the knob is on your left, it is a left-hand door. Masonite and Home Depot both define it this way: when the door opens toward you and the knob is on the left, it is a left-hand door.

Here is the trap, and it is worth knowing before you order. A second convention exists in the trade that names the same door by its hinge side instead of its handle side.

Under that convention you stand on the side the door swings away from you, face it, and read the hinges: hinges on your left means left-hand. For the same physical door, that convention gives you the opposite answer from the handle-side one.

Some suppliers add a second word to specify the hinge side while others use it for the lock side, and the same supplier may apply different conventions on different product lines. Between the two conventions and the extra words, the term “left-hand door” does not identify a specific slab on its own.

So order by description rather than by the label. Instead of answering “left-hand or right-hand,” tell the supplier which side you want the knob on when you open the door toward you.

That sentence removes the ambiguity every time, because it describes the door instead of naming a category that means different things to different sellers.

If you are matching an existing door, there is a shortcut. Leave the old slab on its hinges, open it, and look at which side the knob is on. That answer is already correct for the frame you have, and you can hand it over in words.

Two swing decisions are worth making deliberately rather than by default. A door that swings into a room where it will hit furniture is a daily annoyance, and a door that swings into a hallway is worse.

If the frame is staying and the existing door swings a particular way, matching it is usually right. If the frame is new, this is a free choice. A door that opens against a wall, away from the traffic path, tends to be the one you stop noticing, which is the point.

There is also a hardware detail that follows from the hand. The lock set must match the hand of the door, because the latch and the internal mechanism are handed. A left-hand lock set in a right-hand door will not work properly, and this is a mistake you can only make once the door is on the hinges.

A Pre-Order Checklist

Everything above collapses into a short list you can run before you click buy. The value of the list is that each item is something that cannot be corrected on the day of installation.

• Which number do I have? Slab, unit, or rough opening. Write it down, and label it.
• Width: the smallest of three measurements, and the nominal size it corresponds to.
• Height: 80 inches or 96 inches, decided by the opening and the floor finish.
• Thickness: 1-3/8 inch hollow core, or 1-3/4 inch solid core.
• Hand: which side the knob goes when the door opens toward you.
• Swing: in or out, and does it hit anything.
• Jamb depth: 4-9/16 or 6-9/16 inches, measured against the finished wall.
• Lock set: the backset that matches your existing strike, and the hand that matches the door.
• Bored or not: does the slab come with a lock hole, and does a hollow-core slab have a lock block stamp?

If you are replacing an existing door and want the new one to look identical, one more trick is worth knowing. Take the old slab off its hinges and use it as your measuring template.

Lay the new slab on top of it, align the edges, and mark the hinge locations directly through the old hinge holes. That single step replaces most of the guesswork in a replacement job.

And if you are choosing between product formats rather than just sizes, the difference between a slab and a prehung unit decides how much of the work is yours. The windows and doors terms explained glossary covers the frame and finish vocabulary that comes up in the rest of the cluster.