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Rough Opening Calculator

Calculate precise framing dimensions for windows, interior doors, and exterior doors based on nominal unit sizes and jamb allowances.
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The Shim Space 📏

The Rough Opening (RO) is always larger than the unit itself to allow the carpenter to level, plumb, and squarethe unit within the wall using shims. For doors, we also account for the thickness of the jambs and the necessary clearance for flooring or thresholds. Check your local manufacturer's specs if they deviate from these standard architectural rules.

RO Width

32.000"
Framing clear width

RO Height

81.750"
Framing clear height

Framing Note

Includes jambs, 1/2" shim, and 1/2" floor clearance.

HEADERRO HEIGHT: 81.750"RO WIDTH: 32.000"
For estimation purposes only. Always consult a licensed professional before beginning work. Full Trade Safety Notice →
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What is Framing Clearances and Rough Openings?

Rough openings (ROs) are the clear spaces left in a framed wall to accept the installation of windows and doors. Framers must accurately size the RO to ensure the pre-hung unit fits comfortably while leaving precisely enough gap to insert cedar shims. This shim space allows the carpenter to plumb, level, and square the unit independently of the wall framing.

Mathematical Foundation

Interior Door RO Width
WRO=WNom+(2×J)+SW_{RO} = W_{Nom} + (2 \times J) + S
WNomW_{Nom}= Nominal door slab width (e.g., 30 inches for a '2-6' door).
JJ= Jamb thickness on each side (typically 0.75 inches).
SS= Shim allowance for plumbing and squaring (typically 0.5 inches total, or 1/4 inch per side).
Exterior Door RO Height
HRO=HNom+J+S+CH_{RO} = H_{Nom} + J + S + C
HNomH_{Nom}= Nominal door slab height.
JJ= Head jamb thickness.
SS= Top shim allowance.
CC= Bottom clearance for the sill/threshold unit and finished floor (standard 1.5 to 2 inches).

Laws & Principles

  • The 'Plus 2, Plus 2' Interior Rule: A fast industry rule of thumb for standard interior pre-hung doors is to add 2 inches to the nominal width and 2 inches to the nominal height. A 32x80 door (2-8/6-8) requires a 34x82 inch rough opening. This perfectly covers two 3/4-inch jambs and provides 1/2-inch of shim space.
  • Exterior Sill Clearances: Exterior doors require a taller rough opening than interior doors. The sill (threshold) assembly is thick, and the door must clear the interior finished flooring (hardwood, tile, or carpet) plus a doormat. It is common to add 2.5 inches to the nominal exterior door height.
  • Window Sizing Rules: Unlike doors, most modern flanged windows are manufactured 1/2-inch smaller than their 'call-out' size. A 3-0 x 4-0 (36 x 48 inch) window unit actually measures 35.5 x 47.5 inches. Framing the RO to the exact 36x48 call-out size automatically creates the perfect 1/4-inch shim gap on all four sides.

Step-by-Step Example Walkthrough

" A carpenter is framing an interior wall for a '2-6 x 6-8' (30-inch by 80-inch) pre-hung solid core door with standard 3/4-inch pine jambs. "

  1. 1. Identify Nominal Dimensions: Width = 30 inches, Height = 80 inches.
  2. 2. Calculate RO Width: 30 + 0.75 (left jamb) + 0.75 (right jamb) + 0.5 (shim space) = 32 inches.
  3. 3. Calculate RO Height: 80 + 0.75 (head jamb) + 0.5 (top shim space) + 0.75 (floor clearance for carpet) = 82 inches.
Final Result: The framer sets the jack studs to an exact width of 32 inches and sets the bottom of the header at 82 inches above the subfloor.

Quick Answer: How big should a rough opening be?

For standard interior doors, add 2 inches to the door width and 2 inches to the door height. (A 30"x80" door needs a 32"x82" opening). For exterior doors, add 2 inches to the width and 2.5 inches to the height to accommodate the thicker threshold. For windows, frame the opening to the exact geometric 'call-out' size on the plan; modern manufacturers physically build the window 1/2-inch smaller than the call-out size to automatically create your 1/4-inch shim gap all the way around.

Common RO Rules of Thumb

Interior Door RO = (Door W + 2") × (Door H + 2")

Exterior Door RO = (Door W + 2") × (Door H + 2.5")

Bifold Closet Door RO = (Exact Door W + 1.25") × (Door H + 1.5")

Note: Always measure height directly from the bare subfloor, not the finished floor.

Standard Interior Door RO Chart

Call-Out Actual Door Slab Width RO Height RO Common Use
2-0 x 6-8 24" × 80" 26" 82" Small closets, utility pantries
2-4 x 6-8 28" × 80" 30" 82" Bathrooms, laundry rooms
2-6 x 6-8 30" × 80" 32" 82" Standard bedroom doors
2-8 x 6-8 32" × 80" 34" 82" Master bedrooms, common exterior
3-0 x 6-8 36" × 80" 38" 82" Entry doors, ADA compliant routing

Framing Mistakes to Avoid

The Finished Floor Disaster

A framer is given door heights of 82" RO. They snap their chalk lines on top of 3/4" hardwood flooring instead of the subfloor. When the custom pre-hung doors arrive, they are 3/4" too tall to slide under the header. The crew has to pull the drywall, cut the jack studs, and raise the headers on 14 doors — causing $2,000 in labor overruns. Always measure rough openings from the bare subfloor.

The Exact-Fit Window Mistake

A framer orders a custom window that measures exactly 48" x 48" physically. Without reading the spec sheet, they frame the RO exactly to 48" x 48". The window binds against the studs. When the house settles over the first winter, the downward deflection of the wall header crushes down directly onto the glass, shattering the insulated pane. A 1/2" gap (1/4" per side) allows the house to flex without exploding glass.

Window & Door Installation Best Practices

Do This

  • ✓Check ROs diagonally for square. Measuring width and height isn't enough. Place a tape measure from the top left corner down to the bottom right, and then top right to bottom left. If the two measurements differ by more than 1/4", the RO is a parallelogram and the door will bind.
  • ✓Wait for the spec sheet on custom units. Custom multi-slide exterior doors or european tilt-turn windows do not follow the standard 'plus 2 inches' rule. Demand the manufacturer's exact rough framing specifications before cutting lumber.

Avoid This

  • ✗Don't over-shim headers. Only shim the side jambs. You should NEVER drive shims tightly between the door head-jamb and the lumber header above it. If the lumber header sags from structural load, it must not press into the door frame.
  • ✗Don't forget drywall thickness on pocket doors. A pocket door RO requires a massively different calculation. The width must accommodate twice the door width plus 1 inch, while the height must account for the upper track hardware.

Frequently Asked Questions

What does "2-8 x 6-8" mean on a floor plan?

This is standard architectural shorthand for feet and inches. 2-8 means 2 feet 8 inches (32 inches). 6-8 means 6 feet 8 inches (80 inches). Be careful not to confuse this with feet and decimal form (e.g., 2.8 ft). A 2-8/6-8 door calls for a 34" x 82" rough opening.

Why do windows have a different RO rule than doors?

Doors are named by their physical slab size, so you must ADD the thickness of the door frame (jambs) to figure out the opening size. Windows are named by the architectural opening size. The manufacturer builds the actual physical window frame slightly undersized (usually by 1/2 inch) to provide clearance.

What if my rough opening is framed too large?

If the RO is slightly too large (1/2" to 1"), you can pack it out by ripping strips of plywood or dimensional lumber and nailing them to the jack studs. If the RO is massive (e.g. 3 inches too wide), you cannot span it with standard door casing (trim). You will have to frame in an entire additional jack stud on one side.

What are jack studs and king studs?

The king stud runs continuously from the bottom plate to the top plate of the wall. The jack stud (sometimes called a trimmer) is nailed against the king stud, but it is cut shorter so the structural header can physically rest on top of it. The rough opening width is measured between the two inward-facing jack studs.

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