Door Rough Opening Size Calculator
Calculate the required rough opening (RO) dimensions for framing a door, accounting for the door slab, jamb thickness, shimming space, and clearance allowances.
Standard sizes: 24", 28", 30", 32", 36"
Standard sizes: 80" (6'8"), 84" (7'0"), 96" (8'0")
Typically 3/4" (0.75") for pre-hung doors
Typically 1/2" (0.5") per side for shimming and leveling
Pre-hung includes frame; slab requires separate frame installation
Formula
Rough Opening Width:
RO Width = Door Slab Width + (2 × Jamb Thickness) + (2 × Shim Allowance) + 2"
Rough Opening Height:
RO Height = Door Slab Height + Jamb Thickness + Shim Allowance + 2.5"
The +2" in width accounts for framing clearance on both sides.
The +2.5" in height accounts for the sill plate, threshold, and head clearance.
These allowances follow standard IRC (International Residential Code) and NAHB framing guidelines.
Assumptions & References
- Standard jamb thickness is 3/4" (0.75") for pre-hung interior doors.
- Standard shim allowance is 1/2" (0.5") per side to allow for plumb and level adjustment.
- The 2" width clearance and 2.5" height clearance are industry-standard framing allowances per NAHB and IRC guidelines.
- For exterior doors, additional weatherstripping and threshold height may require adjusting the height allowance.
- Pre-hung doors include the frame; slab-only doors require a separately installed frame.
- Always verify rough opening requirements with the specific door manufacturer's installation instructions.
- References: IRC Section R311, NAHB Residential Construction Performance Guidelines, AWI (Architectural Woodwork Institute) standards.