Door Rough Opening Calculator
Calculate preliminary door rough-opening dimensions, opening area, casing length, stock trim pieces and material cost for identical doors.
Default project preview
38 × 81.5 in
Opening area
86 ft²
Casing length
143.7 ft
Trim pieces
18
Side allowance
1 in
Calculator
Build the material order
Your order will appear here
Enter the project details and calculate a rounded material estimate.
Input and output guide
What each value means
The labels and units below are part of this calculator’s visible methodology. Replace every default with measurements or product data from the actual project.
Door slab width
Enter this project assumption in in. The current imperial default is 36 in.
Door slab height
Enter this project assumption in in. The current imperial default is 80 in.
Clearance per side
Enter this project assumption in in. The current imperial default is 1 in. Minimum: 0.
Top clearance
Enter this project assumption in in. The current imperial default is 1.5 in. Minimum: 0.
Identical doors
Enter this project assumption in a unitless value. The current imperial default is 4. Minimum: 1.
Casing faces
Enter this project assumption in a unitless value. The current imperial default is 2. Minimum: 0. Maximum: 2.
Trim stock length
Enter this project assumption in ft. The current imperial default is 8 ft.
Trim waste allowance
Enter this project assumption in %. The current imperial default is 10 %. Minimum: 0. Maximum: 50.
Price per stock trim piece
Enter this project assumption in a unitless value. The current imperial default is 18. Minimum: 0.
How to read the outputs
Opening area
all rough openings. Default preview: 86 ft².
Casing length
2 faces plus waste. Default preview: 143.7 ft.
Trim pieces
8 ft stock. Default preview: 18.
Side allowance
each side. Default preview: 1 in.
Top allowance
above slab. Default preview: 1.5 in.
Trim cost
stock pieces only. Default preview: $324.00.
Methodology
Transparent calculation
BuildMeter converts every input to a consistent internal unit, applies the selected allowance, and rounds products up to whole purchasable units.
Rough width = door slab width + two side clearances. Rough height = door slab height + top clearance. Casing follows two sides plus the head on each selected face.
Reviewed project guidance
How to use this estimate in a real project
Last reviewed: July 30, 2026
Estimate preliminary framed opening dimensions from door slab or unit size and editable installation clearances.
Calculation steps
- 1Enter the actual project dimensions or operating values.
- 2Apply the calculator formula using consistent units.
- 3Review the unrounded result and the editable allowance.
- 4Round only to the product, stock length or equipment increment actually purchased.
Worked numerical example
A 36 in × 80 in prehung door with 2 in width and 2.5 in height allowance suggests a 38 in × 82.5 in rough opening.
Purchasing and rounding
Use the exact door unit or manufacturer's installation sheet before framing. Rough-opening allowances vary between slab-only, prehung, pocket and specialty systems; frame to the required opening rather than adding a universal clearance to every door size.
Interpret the result
The calculated opening is a planning dimension based on the clearances entered. It should be checked against the purchased unit before studs and header are fixed, because jamb thickness, threshold, shimming space and manufacturer tolerances can change the required rough opening.
Page-specific assumptions
- Inputs describe one consistent project condition.
- Entered yields and efficiencies match the selected product or equipment.
- Site-specific code and engineering requirements are checked separately.
Common mistakes
- Mixing imperial and metric inputs.
- Using nominal dimensions where actual coverage controls.
- Rounding intermediate values too early.
- Treating a quantity estimate as design approval.
Primary and technical references
BuildMeter provides planning estimates, not engineering, architectural, electrical, mechanical, plumbing, permit or code approval. Review the methodology and verify final decisions with product documentation and qualified professionals.
FAQ
Door Rough Opening Calculator questions
Is there one universal rough-opening allowance?+
No. Door units, jambs, thresholds and installation systems vary. Use the manufacturer rough-opening specification.
Does this calculate a structural header?+
No. Header size depends on wall type, span, loads and code.
Why is casing calculated on two faces?+
Interior doors often receive casing on both sides, but the entered face count can be changed.