Door rough-opening calculator

Turn a pre-hung door slab size into the framed rough opening — the labeled jamb-and-shim allowance the carpenter frames to.

Measure each opening and confirm sizes and clearances against the exact product you buy. Take three width and three height measurements and use the smallest of each; allow extra for custom sizes and waste. Sizes, clearances and rough-opening allowances vary by product and brand — read the spec sheet and the manufacturer’s data.
Your result
Rough opening (pre-hung)38.0 in × 82.5 in
Door slab size36.0 in × 80.0 in
Pre-hung adds+2.0 in W, +2.5 in H (jamb + shims)

A 36 × 80 in pre-hung door needs a rough opening of about 38.0 × 82.5 in — about 2 in wider and 2½ in taller than the slab for the jamb and shims; a slab-only swap reuses the existing jamb. Confirm the exact rough-opening on the manufacturer’s spec sheet.

Calculator inputs

in
The nominal slab width, e.g. 36 in for a 3'0'' door.
in
Standard exterior slab is 80 in (6'8'').

The rough opening is the framed hole in the wall — the space between the king studs and under the header — that a pre-hung door assembly drops into. It has to be a little larger than the finished door so the jamb clears the framing and there is room to shim the unit plumb, level and square. Frame it too tight and the jamb binds; too loose and you run out of shim and trim coverage.

For a pre-hung door — slab already hung in its jamb — the labeled planning allowance is about +2 in on width and +2.5 in on height over the slab, covering the jamb thickness plus a shim gap. A slab-only swap reuses the existing jamb and needs no framing change at all.

Formula

Add the labeled pre-hung jamb-and-shim allowance to each slab dimension:

rough width  = slab width  + 2.0 in
rough height = slab height + 2.5 in

These are LABELED planning typicals for a standard residential pre-hung unit. The exact figure varies by jamb depth and manufacturer — always confirm the rough opening on the door’s spec sheet before you frame.

Worked example

A standard 36 × 80 in (3'0'' × 6'8'') pre-hung door:

rough width  = 36 + 2.0 = 38.0 in
rough height = 80 + 2.5 = 82.5 in

So you frame a 38 × 82.5 in opening. A 32 in bath door frames to 34 × 82.5 in; a 6'0'' (72 in) double or patio-swing pair frames to 74 × 82.5 in on the same allowance.

Measure before you frame

  • Slab or pre-hung? This allowance is for a pre-hung unit. A slab-only replacement reuses the jamb, so the opening does not change — the slab vs pre-hung reference helps you choose.
  • Confirm the spec sheet. Jamb depth (for 2×4 vs 2×6 walls), sill type and manufacturer tolerances all nudge the number. The spec sheet wins over any rule of thumb.
  • Level the sill. The rough sill under an exterior door must be level and fully supported so the threshold seats and sheds water.
  • Windows use a different gap. For window units the allowance is about ½ in per side; use the window rough-opening calculator for those.

Reference table

Standard exterior slabs are 6'8'' (80 in) tall; 8'0'' (96 in) and double-door (2×36 in) units are common too. Pre-hung rough openings add the labeled jamb-and-shim allowance:

Slab (W × H)Pre-hung rough opening
28 × 80 in30.0 × 82.5 in
30 × 80 in32.0 × 82.5 in
32 × 80 in34.0 × 82.5 in
36 × 80 in38.0 × 82.5 in
72 × 80 in74.0 × 82.5 in
36 × 96 in38.0 × 98.5 in

Add +2.0 in width and +2.5 in height for a pre-hung unit; a slab-only swap reuses the jamb and needs no rough-framing change. Confirm the exact rough opening on the manufacturer’s spec sheet.

Frequently asked questions

What is the rough opening for a 36-inch door?
For a standard 36 × 80 in pre-hung door, the rough opening is about 38 in wide and 82.5 in tall — the slab size plus roughly 2 in of width and 2.5 in of height for the jamb and shims. Always confirm the exact figure on the manufacturer’s spec sheet before framing.
Why is the rough opening bigger than the door?
The pre-hung assembly includes the jamb around the slab, and it has to be shimmed plumb, level and square inside the framing. The extra width and height gives room for the jamb and the shim gaps so the door hangs and swings correctly without binding.
Does a slab door need a bigger rough opening?
No. A slab-only swap reuses the existing jamb, so the framed opening does not change — you are only fitting a new door to the current jamb. The rough-opening allowance applies when you install a complete pre-hung unit.
Is the allowance the same for every door?
It is a labeled planning typical of about +2 in width and +2.5 in height for a standard residential pre-hung door. Jamb depth, sill type and manufacturer tolerances change it, so treat the number as a starting point and confirm the spec sheet.

Related calculators

Rough-opening calculatorSize the rough opening for a window: unit size plus a shim gap per side (typically ½ in). Worked example, a shim-gap chart and why to confirm the spec sheet.Slab vs pre-hung referenceDecide between a slab and a pre-hung door: the jamb condition drives the choice, with the trade-offs, a cost-and-fit comparison and what to check first.Entry door replacement costEstimate entry and exterior door replacement cost from your own prices: door, labor, add-ons, discount and contingency, with formula, example and labeled bands.