What is a window rough opening?

It is the framed opening prepared to receive the window unit, including the installation tolerances required by the selected window and installation system.

How do you calculate a window rough opening?

A simple estimate adds the side clearances to the window unit width and the head and sill clearances to the unit height.

Are window rough openings standardized?

No. Rough-opening requirements vary by window design, frame system, manufacturer and installation method.

Should the manufacturer's opening dimension be used?

Yes. A published product-specific rough-opening requirement should override a generic calculation.

Window framing geometry

Window Rough Opening Calculator | Framing Size

Estimate a practical framed opening around a window unit while keeping the side, head and sill clearances visible. This makes the result easier to compare with the manufacturer's installation requirements.

Window unit and clearance inputs

Window unit width

Window unit height

Side clearance per side

Head clearance

Sill / bottom clearance

Dimension unit

Window rough openings are product-specific. If the window supplier provides an exact rough-opening dimension, use that documented dimension rather than relying on a generic allowance.

Enter window dimensions and clearances.

Estimated window rough opening

Opening size

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Width × height

—Opening width

—Opening height

—Opening area

—Total clearance

Input / allowance	Value	Applied to

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Window rough-opening formula

Opening width = window width + 2 × side clearance
Opening height = window height + head clearance + sill clearance

Why the clearance is explicit

A window may need space for shimming, alignment, sealing or frame movement. Those requirements differ by product and installation method, so the calculator makes the assumptions editable rather than hiding them in a fixed multiplier.

Do not confuse glazing dimensions with unit dimensions

The visible glass area, sash size and full window-unit dimensions are different measurements. Rough-opening calculations should normally begin with the dimension the window supplier specifies for the unit being installed.

Planning versus final framing

This result is a planning aid. It does not determine headers, sill framing, structural support, flashing, waterproofing details or code-specific requirements around the opening.

Example

A 1,200 mm × 1,500 mm window with 10 mm side clearance at each side, 10 mm at the head and 10 mm at the sill produces:

Width = 1,200 + 20 = 1,220 mm
Height = 1,500 + 20 = 1,520 mm

Always compare that estimate with the selected window system's published installation requirement before framing.

Frequently Asked Questions

Is the window rough opening larger than the window?

Normally the prepared opening needs additional clearance around the unit, but the exact amount is product-specific.

Do both sides need clearance?

The calculator assumes yes and lets you set the clearance per side independently from the head and sill allowances.

Does this include the structural header?

No. The tool sizes the opening only.

Can I use feet or inches?

Yes. The calculator converts all dimensions internally to metres.