Door Rough Opening Calculator | Frame Opening Size
Estimate a practical rough opening from the actual door dimensions and explicit side, head and floor allowances. Keeping each allowance visible makes the opening easier to review before framing.
Door and opening inputs
Recommended rough opening
| Component | Value | Method |
|---|
How a door rough opening is estimated
The rough opening must be large enough to receive the door assembly while leaving the installation clearance required by the chosen system. Because doors and frames vary, a useful calculator should not hide the allowances inside a single magic number.
Rough opening height = door unit height + head allowance + floor clearance
Why the door-unit dimension matters
A nominal door leaf size is not always the same thing as the overall frame or prehung-unit dimension. Before framing, identify exactly which dimension your supplier or manufacturer calls the door-unit size.
Installation clearance is project-specific
Shims, frame tolerances, floor finishes and installation method can affect the required opening. The allowance fields are therefore explicit inputs rather than hidden universal constants.
What this calculator does not decide
It does not select the door frame profile, fire-rating requirement, structural header, hinge side, accessibility compliance or manufacturer's installation dimensions. Those items must be resolved separately from the actual project documentation.
Worked example
For a 900 mm wide by 2,100 mm high door unit with 15 mm side allowance on each jamb, a 15 mm head allowance and 10 mm floor clearance:
Height = 2,100 + 15 + 10 = 2,125 mm
The result is an estimating dimension. The actual rough opening should follow the selected door assembly documentation when it gives a different requirement.
Frequently Asked Questions
Is a rough opening the same as the door size?
No. The rough opening is the framed opening prepared to receive the door assembly and normally includes installation allowances.
Do both jamb sides get an allowance?
Usually the width calculation uses an allowance at both sides, but the actual requirement depends on the door-frame system.
Should floor clearance be included?
It can be included when the opening height needs to accommodate the door assembly, threshold or finished-floor condition.
Should I trust a generic rough-opening formula over the manufacturer?
No. A product-specific rough-opening requirement should take precedence.