Recommended rough opening summary
Results appear here after calculation and stay above the form.
Calculator inputs
Enter the door slab size and installation allowances to estimate a reliable framing opening.
Plotly graph
The chart compares slab dimensions, total additions, and final rough opening dimensions.
Formula used
Rough opening width = total door slab width + (2 × side jamb thickness) + (2 × side shim allowance) + meeting gap for paired doors + extra width allowance.
Rough opening height = door slab height + head jamb thickness + head shim allowance + bottom clearance + threshold thickness + extra height allowance.
Opening area = rough opening width × rough opening height.
Diagonal check = √(rough opening width² + rough opening height²). This helps compare both diagonals on site when checking squareness.
How to use this calculator
- Choose inches or millimeters.
- Select single or double door configuration.
- Enter door leaf width and height.
- Fill in jamb thicknesses and shim allowances.
- Add bottom clearance, threshold thickness, and extra tolerances.
- Press Calculate rough opening to show the result above the form.
- Review the graph, framing note, and example table.
- Use CSV or PDF export to save the calculation.
Example data table
| Door type | Leaf width | Leaf height | Key allowances | Estimated rough opening |
|---|---|---|---|---|
| Single interior | 30 in | 80 in | 0.75 jamb, 0.5 shim, 0.25 extra | 32.75 in × 82.00 in |
| Single exterior | 36 in | 80 in | 0.75 jamb, 0.5 shim, 0.5 bottom | 38.75 in × 82.00 in |
| Double pair | 36 in each | 80 in | 0.125 meeting, 0.75 jamb, 0.5 shim | 74.88 in × 82.00 in |
| Metric single | 900 mm | 2100 mm | 20 jamb, 12 shim, 10 extra | 964 mm × 2148 mm |
Frequently asked questions
1. What is a rough opening?
A rough opening is the framed wall opening prepared to receive a door unit. It includes extra space for jamb material, shimming, leveling, and final adjustment during installation.
2. Why is the rough opening larger than the door slab?
The opening must be larger so the installer can fit the jamb, add shims, plumb the unit, and account for floor variation, thresholds, and small manufacturing tolerances.
3. Should I enter one leaf width or total width for double doors?
Enter the width of one leaf. The calculator multiplies by the number of leaves and adds the meeting gap when a paired door setup is selected.
4. Does this work for prehung doors?
Yes. It is useful for prehung layouts when you know the slab size and jamb-related allowances. Still compare your result with the manufacturer’s specified rough opening.
5. How much shim space should I allow?
Many installers allow around 1/4 to 1/2 inch per side, but the right amount depends on wall straightness, unit size, and manufacturer guidance. Use site conditions to decide.
6. Do finish floors affect door rough opening height?
Yes. Floor finish thickness, underlayment, thresholds, and desired undercut can all change the needed height. Include those allowances before framing or trimming studs.
7. Why does the calculator show a diagonal value?
The diagonal provides a quick geometry check. Matching corner-to-corner diagonal measurements on site helps confirm the framed opening is square before installation proceeds.
8. Can I use millimeters instead of inches?
Yes. Switch the unit system to millimeters. The calculator keeps all values, results, exports, and chart labels in the selected unit system.