Door rough opening calculator

The framed opening to leave in a wall for a pre-hung door, sized from the slab. Width, height, and the header length — with the jamb and shim allowance broken out instead of hidden inside a round number.

01 How the math works

A pre-hung door is a slab already hung in its own jamb. The carpenter frames an opening a little larger than that jamb, sets the unit in, then shims it plumb and nails it off. The rough opening is the slab size plus room for the jamb and the shims.

Width gets the jamb on both sides plus a shim gap on both sides. Height gets the head jamb across the top plus clearance for the head shims and the gap under the door. Name it as a sum and the round numbers stop being magic.

With standard parts — 3/4 in jamb stock and about 1/4 in of shim each side — the width allowance is 2 × 3/4 + 2 × 1/4 = 2 in. The head jamb plus roughly 1-3/4 in of head play and under-slab clearance makes the height allowance 2-1/2 in. That is the familiar "slab plus 2 inches wide, plus 2-1/2 inches tall" rule, written out so you can see which part moves when the parts change.

02 Worked example

Take a standard interior pre-hung bedroom door: a 32 in × 80 in slab (813 mm × 2,032 mm), on 3/4 in (19 mm) jambs with 1/4 in (6 mm) of shim each side.

Width: 32 + 2 × 3/4 + 2 × 1/4 = 34 in (864 mm). Height: 80 + 3/4 + 1-3/4 = 82-1/2 in (2,096 mm). So the framed opening is 34 × 82-1/2 in. The header above it bears on two jack studs, so it spans the opening width plus two 1-1/2 in jacks: 34 + 3 = 37 in (940 mm). A 36 in slab runs the same way to a 38 × 82-1/2 in opening and a 41 in header.

03 When this calculator is wrong

The flat "+2 inches" survives because a 3/4 in jamb is nearly universal on interior units. But the 2 inches is two jamb legs and two shim gaps, not a constant. A unit with thicker jambs changes it, and a calculator that treats "+2 inches" as fixed sends you to the saw with the wrong opening — the same way nobody tells a homeowner a 2×4 is 1.5 × 3.5 in until it matters.

• Thicker jambs widen the opening. An exterior or custom unit on 1-1/4 in jambs needs 2 × 1-1/4 + 2 × 1/4 = 3 in of width, not 2 — a full inch more than the interior rule. Enter the actual jamb stock.
• Finished floor eats the height clearance. The 2-1/2 in height assumes the jamb legs run near the subfloor. Set the unit, then lay 3/4 in tile over a mortar bed under it, and the under-slab gap you planned for is gone. Frame the opening off the finished floor height, or hold the jamb legs up.
• It sizes the opening, not the header. The header length here (opening width + 3 in) is the board length only. Its depth — 2×6 through 2×12 or built-up — is set by span and load in IRC Table R602.7(1), not by this calculator.
• Slab-only installs are a different job. Hanging a bare slab in an existing jamb is sized to the jamb's inside dimension and the hinge/strike gaps, not to a rough opening. This page is for pre-hung units going into framing.

04 What to do with the result

Frame the opening at the width and height above, measured from the finished floor, not the subfloor, if a thick floor is going in later. Check the opening for square across both diagonals before the unit arrives — a 1/4 in of shim per side only squares a rack that started close. For the header, use the length here and size its depth from the span table.

05 Common questions

What is the rough opening for a 32-inch door?

For a standard interior pre-hung 32 in slab, frame the opening 34 in wide × 82-1/2 in tall — slab width plus 2 in, slab height plus 2-1/2 in. Confirm against the specific unit's spec sheet, since jamb and shim allowances vary.

What is the rough opening for a 36-inch door?

38 in wide × 82-1/2 in tall for a standard interior pre-hung unit. The height is the same 82-1/2 in as other 6'8" doors; only the width tracks the slab.

How much bigger is a rough opening than the door?

About 2 in wider and 2-1/2 in taller than the slab for interior pre-hung doors. The width is two 3/4 in jamb legs plus a 1/4 in shim each side; the height is the head jamb plus head and under-slab clearance.

Do I measure the rough opening from the subfloor or the finished floor?

From the finished floor when a thick floor goes in after framing. The 2-1/2 in height allowance includes the clearance below the door, and tile or thick plank will consume it if you frame off the bare subfloor.

How long is the header over a door opening?

The rough opening width plus 3 in — two 1-1/2 in jack studs, one each side. The header's depth (size) is set by span and load in IRC Table R602.7(1), not by the opening width.