Door Rough Opening Calculator Formula
Understand the math behind the door rough opening calculator. Each variable explained with a worked example.
Formulas Used
Rough Opening Width
ro_width = door_width + 2 * side_gapRough Opening Height
ro_height = door_height + top_gap + threshold_heightRO Width in Feet
ro_width_ft = (door_width + 2 * side_gap) / 12RO Height in Feet
ro_height_ft = (door_height + top_gap + threshold_height) / 12Variables
| Variable | Description | Default |
|---|---|---|
door_width | Door Width(inches) | 36 |
door_height | Door Height(inches) | 80 |
side_gap | Side Shim Gap (per side)(inches) | 1 |
top_gap | Top Gap(inches) | 0.5 |
threshold_height | Threshold / Subfloor Allowance(inches) | 1.5 |
How It Works
How to Calculate a Door Rough Opening
Formula
RO Width = Door Width + 2 x Side Gap
RO Height = Door Height + Top Gap + Threshold Allowance
The side shim gap (typically 1 inch per side) gives room for the door jamb and shims. The top gap allows clearance for the head jamb. The threshold allowance accounts for the finished floor and threshold assembly.
Standard Sizes
Most interior doors are 80 inches tall. Common widths are 24, 28, 30, 32, and 36 inches. Exterior doors may be 36 inches wide and 80 or 96 inches tall.
Worked Example
A standard 36-inch wide by 80-inch tall door with 1-inch side gaps, 0.5-inch top gap, and 1.5-inch threshold allowance.
- 01RO Width = 36 + 2 x 1 = 38 inches
- 02RO Height = 80 + 0.5 + 1.5 = 82 inches
- 03The framed opening should be 38 in wide by 82 in tall.
Frequently Asked Questions
Why is the rough opening wider than the door?
The extra width accommodates the door jamb material (typically 3/4 inch per side) and shim space (1/4 inch per side) needed to plumb and level the jamb within the framed wall.
What is a standard rough opening for a 36-inch door?
The standard rough opening for a 36-inch pre-hung door is 38 inches wide by 82 inches tall, though you should always check the manufacturer specifications for the exact door you are installing.
Do exterior doors need a larger rough opening?
Exterior pre-hung doors often include a thicker jamb and integrated brick mold, so the rough opening may differ from interior doors. Always consult the door specification sheet.
Ready to run the numbers?
Open Door Rough Opening Calculator