Construction

Door Rough Opening Calculator

The single most common reason a freshly-installed pre-hung door won't latch is the rough opening was framed wrong — either dead-tight to the jamb (no shim space) or so loose the jamb doesn't square. This calculator gives you the rough opening width and height for any standard pre-hung door (interior or exterior), with the right shim allowance baked in, plus the header size you'll need if it's a load-bearing wall. Enter your door's nominal width and height — the calculator handles the rest.

Rough opening

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RO width: —
RO height: —
Header size (typical): —

Why the rough opening is bigger than the door

A pre-hung door comes as a slab + jamb assembly. The jamb is 3/4" thick on each side, plus you need shim space between the jamb and the rough framing to plumb and square the unit. Standard convention:

Width: nominal door width + 2 inches. (3/4" jamb x 2 + 1/2" shim x 2 = 2.5", but most builders round to 2.) A 36" door = 38" rough opening width.
Height: nominal door height + 2.5 inches. Same logic, with a tiny bit more above for shim space and finished flooring. An 80" door = 82.5" RO height.

Some big-box pre-hungs come with their own "RO required" sticker on the unit — use that number if it differs from the calculator's. Custom or imported doors sometimes need a 3" or 3.5" wider RO. The shim-space rule is what matters: you need 1/4 to 1/2 inch between jamb and king stud on each side after the unit is shimmed plumb.

Header sizing for load-bearing walls

If the wall above the door is load-bearing (supports floor joists, ceiling joists, or roof rafters above), the door opening needs a header to carry that load across the gap. IRC prescriptive headers for typical residential 1-story load above:

Up to 3-ft span: 2-2x6 (two 2x6's nailed together with 1/2" plywood spacer)
3 to 5 ft span: 2-2x8
5 to 7 ft span: 2-2x10
7 to 9 ft span: 2-2x12
Over 9 ft: engineered LVL or steel beam

For 2-story load above, bump everything up one size. For non-load-bearing interior walls, a flat 2x6 or even a doubled 2x4 "header" is plenty — it's not carrying anything but its own weight and the wall above.

How to use this calculator

Find your door's nominal size. Interior doors are typically 24", 28", 30", 32", or 36" wide — always 80" tall. Exterior doors are 32" or 36" wide × 80" (or 84" for taller specs).
The calculator returns the rough opening dimensions to frame and the typical header size based on door width.
Always confirm against the door manufacturer's RO spec sheet — the calculator gives the standard, but specialty doors (steel exterior, sliding pocket, French) have their own rules.
Frame the opening with a king stud each side, a jack stud (trimmer) supporting the header, the header itself, and cripples above the header up to the top plate.

Common scenarios

Standard 36" exterior front door, 80" tall, load-bearing wall. RO = 38" wide x 82.5" tall. Header: 2-2x8 (3-ft span). Frame with 2x4 king studs and jack studs on each side, header tight to the jacks, cripple studs above the header.

32" interior bedroom door, non-load-bearing partition wall. RO = 34" wide x 82.5" tall. "Header" is just a flat 2x4 — it's not structural, just framing for drywall to land on.

72" double interior French door (two 36" leaves), load-bearing wall. RO = 74" wide x 82.5" tall. Header: 2-2x12 (6-ft span, 1-story load above). Often spec'd as an LVL for cleaner sizing.

FAQ

Why is 80" tall the standard? +

It's the historical residential ceiling clearance standard — 80" door + 4" header + finished floor lands neatly under an 8-ft (96") ceiling. 9-ft and 10-ft ceilings often use 84" or 96" tall doors for proportion. Garage and exterior doors sometimes spec at 80" or 84" depending on architectural style.

What's a jack stud vs. a king stud? +

The king stud runs full-height from sole plate to top plate, framing the edge of the door opening. The jack stud (also called a trimmer) sits next to the king stud and runs from the sole plate up to the underside of the header — it's what physically supports the header's load. Every header needs jack studs on both ends.

Do I need cripple studs above the header? +

Yes, on any wall over 8 ft tall — cripples are short studs spaced 16" oc above the header, filling the space up to the top plate. They give the drywall something to nail to and continue the load path. On 8-ft walls with a tight header-to-top-plate gap, you can sometimes skip them; check local code.

What header size for a 3-foot span carrying 2 stories above? +

Bump to 2-2x10 instead of 2-2x8. The general rule: 1-story load = standard header; 2-story load = up one nominal size; load-bearing wall with full roof + ceiling = standard header (it's essentially a 1-story load).

Can I use an LVL instead of dimensional lumber for the header? +

Yes — 1-3/4" thick LVLs in various depths (5-1/2", 7-1/4", 9-1/2", 11-1/4", etc.) span longer with less depth. A 9-1/2" LVL can replace a 2-2x12 in many residential applications, freeing up headroom. Use double LVLs (two pieces side by side with 1/2" plywood) for wider walls. LVL headers cost 30-50% more than dimensional lumber but build cleaner.

What about widening an existing door opening? +

Doable but it's structural work. Temporary shoring above the wall holds the load while you cut the existing king and jack studs back, install a longer header, and re-frame. Get a permit and an engineer's review for any load-bearing wall modification — this isn't a weekend DIY for non-builders.

How much can I shim a wonky rough opening? +

You can shim out roughly 1/2" on each side and still have the jamb pull tight to the rough framing with finish nails or pre-hung trim screws. More than that, the trim won't cover the gap and the door starts feeling "hollow" when slammed. If your RO is more than 1.5" over the door+jamb width, re-frame one side.

Does the rough opening change for exterior vs interior doors? +

The +2" wide and +2.5" tall rule is the same. Exterior doors typically have thicker jambs (4-9/16" vs 4-9/16" stock both ways but exterior commonly has a sill assembly built in) — still, RO dimensions match the rule. The biggest difference is that exterior door openings need flashing and a properly-sloped sill underneath — framing math same, water management different.