Door Rough Opening Calculator
Introduction: Door rough opening basics
A door rough opening is the framed hole prepared before the prehung door, jamb, shims, and trim are installed. It is usually larger than the door slab so the installer has room to square the jamb, shim the sides, adjust for imperfect framing, and leave clearance at the floor or threshold.
For many residential prehung interior doors, a practical planning allowance is about 2 inches wider and 2 to 2.5 inches taller than the nominal slab. Exterior doors, fire-rated doors, pocket doors, thick jambs, and specialty thresholds can require different clearances, so the manufacturer's installation sheet always wins.
Formula
The calculator keeps the math transparent:
Rough opening width = door width + total extra width
Rough opening height = door height + total extra height
Plain-text formula: roughWidthIn = doorWidthIn + totalExtraWidthIn; roughHeightIn = doorHeightIn + totalExtraHeightIn. Default planning allowance is door width + 2 in and door height + 2 to 2.5 in.
The “total extra” values are the full opening allowance, not per-side allowance. For example, entering 2 inches of extra width means about 1 inch of side clearance on each side before shimming.
How to choose inputs
- Door width and height: use the nominal slab size or the actual prehung unit instructions. A 30-by-80 door usually means a 30 inch wide, 80 inch tall slab.
- Total extra width: 2 inches is a common interior starting point. Use more only if the jamb kit, wall condition, or instructions call for it.
- Total extra height: 2 to 2.5 inches often covers top clearance, jamb head, flooring, and threshold planning. Exterior units may need more because of sill details.
- Framing context: measure between framing members, not finished trim. Header size, jack studs, and load path are separate framing decisions.
Worked example
For a 32 inch by 80 inch interior prehung door with 2 inches of width allowance and 2.5 inches of height allowance:
Width: 32 + 2 = 34 inches
Height: 80 + 2.5 = 82.5 inches
That gives a planning rough opening of 34 in wide by 82.5 in tall. Before cutting or framing, compare that against the exact door package, especially if the floor is unfinished or the opening needs a threshold.
Common planning sizes
| Door slab | Typical rough opening with 2 in width / 2.5 in height allowance | Common use |
|---|---|---|
| 28 in x 80 in | 30 in x 82.5 in | Closet or small interior room |
| 30 in x 80 in | 32 in x 82.5 in | Interior passage door |
| 32 in x 80 in | 34 in x 82.5 in | Bedroom or wider passage door |
| 36 in x 80 in | 38 in x 82.5 in | Entry, accessibility, or utility space |
Limitations
This tool is a planning aid, not a framing specification. It does not size headers, evaluate load-bearing walls, decide fire-rating clearances, or account for every threshold and jamb system. Confirm final dimensions with the product instructions, local code requirements, and a qualified carpenter or inspector when the wall is structural.
How to Use
- Select the planning mode that best matches the door package: interior prehung, exterior prehung, slab-only note, or custom manufacturer allowance.
- Choose a common slab size or enter the actual door width and height in inches.
- Enter the total extra width and total extra height from the door instructions or your planning allowance. These are total opening allowances, not per-side values.
- Run the calculation, then compare the result with the manufacturer instructions before framing, cutting, or ordering material.
Arcade Mini-Game: Door Framing Layout Run
Catch useful framing inputs and avoid assumptions that throw off the rough opening.
Start the game, then use your pointer or arrow keys to catch useful inputs and avoid bad assumptions.