Door Rough Opening Calculator Formula

Understand the math behind the door rough opening calculator. Each variable explained with a worked example.

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Formulas Used

Rough Opening Width

ro_width = door_width + 2 * side_gap

Rough Opening Height

ro_height = door_height + top_gap + threshold_height

RO Width in Feet

ro_width_ft = (door_width + 2 * side_gap) / 12

RO Height in Feet

ro_height_ft = (door_height + top_gap + threshold_height) / 12

Variables

VariableDescriptionDefault
door_widthDoor Width(inches)36
door_heightDoor Height(inches)80
side_gapSide Shim Gap (per side)(inches)1
top_gapTop Gap(inches)0.5
threshold_heightThreshold / 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.

door_width = 36door_height = 80side_gap = 1top_gap = 0.5threshold_height = 1.5
  1. 01RO Width = 36 + 2 x 1 = 38 inches
  2. 02RO Height = 80 + 0.5 + 1.5 = 82 inches
  3. 03The framed opening should be 38 in wide by 82 in tall.

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