How to measure a door opening before buying a door yourself: step by step
Short answer
To measure a door opening before buying a door, follow the 8 steps below, from “Do this after the finished floor is down” to “Write one line per opening and photograph it”. Difficulty 2 of 5; for a first-timer, plan on about half a day. Read “When to stop” below before you start.
Quick review by a licensed contractor on Oct 7, 2026. This is not a certification. Follow product labels where they differ.
A floor that slopes across the opening throws the head jamb out of level and leaves the bottom gap uneven. Half an hour of measuring here decides whether door day goes smoothly or turns into a fight with the framing.
Difficulty 2 of 5 · about half a day · you can do this
Photo of your room, the look you pick, then this guide with the materials counted for your room.
What you need
- Jamb extension strips
- Composite or cedar shims
Tools
- 25 ft tape measure
- 4 ft level (a 2 ft level plus a straight board also works)
- Torpedo level
- Framing square
- Pencil and a notepad (or your phone) to write every number down
Step by step
Step 1 of 8
Do this after the finished floor is down
Measure with the finished floor already in the room, because the height from floor to header is what sets the door's bottom gap. If the floor is not down yet, measure now and write down the flooring thickness you will add later.
Before you measure
- Finished floor is down, or you know its exact thickness
- Old casing and old jamb are removed from the opening
- You have a notepad and will write every number down
- Building is from after 1978, or you are not sanding or breaking any painted trim
Step 2 of 8
Strip the opening back to framing
Pry off the casing and pull the old jamb so you can see bare studs and the header. Score the paint line with a utility knife first so you do not tear the drywall paper. In buildings from before 1978, treat the old paint as lead unless a test says otherwise: use lead-safe practices (pry, do not sand, keep it wet, bag the debris), or hire an EPA lead-safe certified firm.
Step 3 of 8
Measure the rough opening width and height
Width: stud to stud, at the top, middle, and bottom. Height: finished floor to the underside of the header, on both sides. Write down the smallest width and the smallest height — those are the numbers that have to fit. A prehung door normally needs a rough opening about 2" wider than the door (a 30" door needs about 32") and about 2-1/2" taller, measured from the finished floor (an 80" door needs about 82-1/2"). The door unit's own sheet or box has the exact size — go by that.
Step 4 of 8
Measure the wall thickness
Measure from finished wall face to finished wall face at the top, middle, and bottom of the opening. A 2x4 wall with 1/2" drywall on both sides usually takes a 4 9/16" jamb, but measure rather than assume — plaster and double drywall are common in older buildings.
Step 5 of 8
Check the floor under the opening for level
Set a level on the floor across the opening. Note which side is high and by how much: you will trim that much off the bottom of the jamb leg on the high side so the head jamb ends up level. More than about 1/2" out and you should get a carpenter to look at the floor first.
Step 6 of 8
Check both sides for plumb and the opening for square
Hold the level against each stud, front face and side face. Then measure both diagonals of the opening: equal diagonals mean the opening is square. Note any lean — you will correct it with shims, and you need to know now how much room you have.
Step 7 of 8
Decide the hand and which way the door swings
Stand on the side where the door swings toward you. Hinges on your left is a left-hand door; hinges on your right is a right-hand door. Also decide which room the door should open into — usually into the room, not into a hallway.
Step 8 of 8
Write one line per opening and photograph it
Room, width, height, wall thickness, low side of the floor, hand. Mark any fire-rated door (the entry door, a door from the garage into the house) — those are not an interior prehung swap. Tape the note next to the opening or keep it in your phone. Repeat for every opening before you go shopping.
Quality check
- Width measured in three places; smallest number recorded
- Height measured on both sides from the finished floor
- Wall thickness measured top, middle, and bottom
- Floor across the opening checked with a level, high side and the amount noted
- Diagonals measured; you know how square the opening is
- Hand and swing direction written down for every opening
Common mistakes
- Measuring the finished opening instead of the rough opening. If old casing or an old jamb is still in place, you are measuring the wrong thing. A prehung door needs the rough framing size: pull the casing and the old jamb first, then measure stud to stud and floor to header. Mistake #1 in this task — it is how people come home with a door that does not fit.
- Not checking the floor under the opening. A floor that slopes across the doorway throws the head jamb out of level and leaves an uneven gap under the door, and no amount of shimming fixes it once the jamb is nailed. Check it now so you know how much to trim off the jamb leg on the high side.
- Measuring wall thickness in one spot. Walls vary. Measure at the top, middle, and bottom of both sides. If the wall is thicker than the jamb anywhere, you need a jamb extension — better to know today.
- Guessing the swing (hand) of the door. Ordering the wrong hand means returning the whole unit. Stand where the door swings toward you: if the hinges are on your left it is a left-hand door, on your right it is a right-hand door. Write it down with a sketch.
- Measuring one opening and assuming the rest match. In older apartments no two openings are the same. Measure every opening separately and label the notes by room.
Questions
How long does it take to measure a door opening before buying a door?
Difficulty 2 of 5. For a first-timer, plan on about half a day.
What do I need to measure a door opening before buying a door?
Jamb extension strips, Composite or cedar shims. Tools: 25 ft tape measure, 4 ft level (a 2 ft level plus a straight board also works), Torpedo level, Framing square, Pencil and a notepad (or your phone) to write every number down.
What are the most common mistakes when you measure a door opening before buying a door?
Measuring the finished opening instead of the rough opening. If old casing or an old jamb is still in place, you are measuring the wrong thing. A prehung door needs the rough framing size: pull the casing and the old jamb first, then measure stud to stud and floor to header. Mistake #1 in this task — it is how people come home with a door that does not fit. Not checking the floor under the opening. A floor that slopes across the doorway throws the head jamb out of level and leaves an uneven gap under the door, and no amount of shimming fixes it once the jamb is nailed. Check it now so you know how much to trim off the jamb leg on the high side. Measuring wall thickness in one spot. Walls vary. Measure at the top, middle, and bottom of both sides. If the wall is thicker than the jamb anywhere, you need a jamb extension — better to know today.
When should I stop and call a pro?
Stop and get a carpenter to look if: the rough opening is smaller than the door unit you need (enlarging an opening means cutting framing, and in a load-bearing wall that is not DIY); the floor under the opening is off by more than 1/2" across the doorway; the header or the studs are cracked, rotten, or visibly bowed; or the opening is out of square by more than about 1/4" and you cannot tell why. Anything that means opening up the wall is a pro job.
You may also need
- Baseboard and trim calculator
- How to install a prehung interior door yourself: step by step
- How to install door casing yourself: step by step
- How to replace a front entry door yourself: step by step
- How to install baseboards yourself: step by step
- How to fill nail holes and caulk trim yourself: step by step