How to Measure for Custom Doors (Shop Method)

Bad measurements are how good wood becomes an expensive problem. Custom doors don’t forgive “about right.” Here’s the shop method I use so the slab fits the opening the first time — not the third.

What you’re actually measuring

There are two different jobs people mix up:

• Rough opening (RO) — the framed hole in the wall (stud to stud, floor to header). This is for pre-hung units or building a jamb.
• Door slab — the door leaf itself, if you’re hanging into an existing jamb or replacing only the door.

Write down which one you’re ordering before you touch the tape.

Tools (keep it simple)

• 25′ tape (or a quality 16′)
• Combination square or level
• Pencil + notepad (phone notes get lost on job sites)
• Optional: laser for tall openings, but tape still wins for jambs

Rough opening — width

1. Measure width in three places: top, middle, bottom — stud face to stud face (or finished drywall opening if that’s what’s framed).
2. Record the smallest number. Walls aren’t square; the tight spot rules.
3. Note if the opening is out of plumb (difference left vs right).

Typical allowance: pre-hung interior doors often want roughly 2″ over slab width and 2–2½” over slab height for jamb + shims (varies by jamb thickness and floor finish — confirm your jamb detail). Don’t invent the allowance; match the unit you’re building or buying.

Rough opening — height

1. Measure left and right from subfloor (or finished floor if it’s already in and final) up to the underside of the header.
2. Again: take the smaller height.
3. Call out floor finish: carpet, hardwood, tile change the reveal at the bottom.

Check square (don’t skip this)

Measure both diagonals of the opening corner-to-corner. If they differ more than about ¼”, the opening is racked. You can still hang a door, but you’ll shim more on one side — or fix framing first on a new build. Write the diagonal numbers on the same page as width/height.

Existing jamb — measuring for a slab only

1. Width: inside jamb to inside jamb at top, mid, bottom. Order to the tight width minus your side reveals (commonly ~1/16–1/8″ per side — match how you hang).
2. Height: top jamb down to finished floor at both sides. Subtract bottom clearance (often ~½–¾” over hard floor; more over carpet).
3. Thickness: measure the old door edge or jamb stop depth so the new slab matches.
4. Hinge side / swing: stand on the side the door swings toward you; note left-hand or right-hand. Photo the hinges and latch bore.
5. Hinge locations: measure from top of slab to top of each hinge, and hinge size. Custom work loves a hinge template that matches the old door.

Reveals and “the last inch”

A door that looks expensive usually isn’t magic wood — it’s even reveals and a clean swing. Plan:

• Even gap hinge side and latch side
• Consistent head gap
• Bottom clearance that clears flooring and doesn’t whistle

If the opening is ugly, the slab can’t save it alone. Fix the hole or build the jamb true.

What to send your builder (or your future self)

• RO width (3 places) + RO height (2 places) + diagonals
• Floor finish now / floor finish final
• Swing and hinge side
• Jamb thickness / wall thickness
• Photos: full opening, each corner, floor transition, existing hinges if replacing a slab

Shop note

I cut my teeth in a door shop. The jobs that went smooth always had the same boring paperwork: three widths, two heights, diagonals, swing, floor. Fancy drawings don’t fix a tape that only got used once in the middle.

Measure twice. Cut when the smallest number is honest.

— Built by a King · Fit for royalty, crafted to rule.