Calculators
Rough Opening Calculator
Calculate the framed rough opening for a door or window from the unit size and clearances, plus header length.
- Runs in your browser
- Nothing uploaded
The tool
Enter your numbers below. Results update as you type, and nothing leaves your browser.
Everything you enter stays on your own machine. Nothing is sent to us or to anyone else, which you can confirm in your browser network tab.
What people use it for
- Framing door and window openings
- Ordering replacement windows
- Checking existing openings
- Teaching framing basics
About this tool
What the Rough Opening Calculator does
A rough opening is the framed hole a door or window fits into. It is slightly larger than the unit to leave room for shims and levelling. A common allowance is about 2 inches wider and 2½ inches taller than a prehung door; windows usually need about ½ inch each way. Always check the manufacturer’s instructions.
Formula
- Rough opening width = unit width + width allowance
- Rough opening height = unit height + height allowance
- Header length = rough opening width + 3 in (two 1½ in jack studs)
Worked example
Using the starting values in the calculator above:
| Input | Value |
|---|---|
| Unit type | Prehung door |
| Unit width | 36 in |
| Unit height | 80 in |
| Width allowance (0 = typical) | 0 in |
| Height allowance (0 = typical) | 0 in |
| Result | Value |
|---|---|
| Rough opening | 38 × 82.5 in |
| Rough opening width | 38.00 |
| Rough opening height | 82.50 |
| Header length | 41.00 |
How to read the result
Manufacturers specify rough openings for their units; their figure always takes priority over a rule of thumb.
Measure finished floor height when framing door openings so the door clears flooring and thresholds.
Quantities are estimates. Check product data sheets and local codes, and confirm with your supplier before ordering.
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Questions
Questions about this tool
The things clients actually want to know — costs, timelines, ownership and what happens after launch.
Typically about 38 inches wide by 82½ inches high for a standard prehung door.
Commonly about ½ inch wider and taller than the window unit.
It rests on jack studs on each side of the opening.
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- UK, USA & the Gulf
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- Working since
- 2023