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Window Rough Opening Size Chart: How to Measure for Custom Aluminum Windows
Created on 07.29
Accurate rough opening dimensions help prevent ordering errors, production delays, and unnecessary adjustments during installation.
For custom aluminum windows and doors, the rough opening is not the same as the finished outside frame size. Installation clearance must be deducted from the measured opening before the preliminary frame dimensions are confirmed.
This window rough opening size chart explains how to measure the structural opening, calculate the preliminary frame size, and prepare accurate project information for manufacturer review.
It is intended for builders, contractors, developers, dealers, importers, and project buyers working in Australia, the United States, Canada, Africa, the Middle East, and other international markets.
AMJ Window Rough Opening Size Chart
The following allowances apply when AMJ calculates the preliminary outside frame size from the measured rough opening.
Unit Type | Left | Right | Top | Bottom |
Standard Window | 10 mm | 10 mm | 10 mm | 10 mm |
Floor-to-Ceiling Window or Door | 10 mm | 10 mm | 10 mm | No deduction |
Standard Window Calculation
For a standard window, allow 10 mm on all four sides. Frame Width = Rough Opening Width − 20 mm
Frame Height = Rough Opening Height − 20 mm
The total width reduction is 20 mm because 10 mm is allowed on both the left and right sides.
The total height reduction is also 20 mm because 10 mm is allowed at both the top and bottom.
Floor-to-Ceiling Window or Door Calculation
For a floor-to-ceiling window or door, allow 10 mm on the left, right, and top.
No deduction is made at the bottom.Frame Width = Rough Opening Width − 20 mm
Frame Height = Rough Opening Height − 10 mm
The bottom of the unit must follow the approved floor, sill, and threshold detail.
Important: These formulas are used for preliminary AMJ project sizing. Final production dimensions must be confirmed on the approved shop drawings.
Window Rough Opening vs. Actual Window Size
Rough opening dimensions, nominal window sizes, actual frame sizes, and glass sizes describe different measurements.
Term | Meaning |
Nominal Window Size | A general size description used in drawings, schedules, product references, or early planning. |
Actual Window Frame Size | The finished outside width and height of the manufactured frame. |
Window Rough Opening Size | The clear structural opening prepared to receive the window. |
Glass Size | The dimensions of the glass unit installed inside the frame. |
Daylight Opening | The visible glass area or clear opening after installation. |
A window described as 24 × 36 may refer to a nominal size, an actual frame size, or a rough opening. That description alone is not enough to confirm the correct production dimensions.
Always confirm what the stated measurement represents before approving the order.
Why Does a Window Need Installation Clearance?
A window frame should not be manufactured to fit tightly against the structure on every side.
Controlled installation clearance gives the installer space to:
- Level and square the window frame
- Position shims at the fastening points
- Correct minor variations in the opening
- Apply perimeter insulation and sealant
- Coordinate flashing and waterproofing materials
- Complete final sash and hardware adjustments
If the rough opening is too small, the frame may be difficult to install or may become distorted.
If the opening is too large, fastening, sealing, waterproofing, and finishing become more complicated.
How to Measure a Window Rough Opening
1. Identify Every Opening
Match each opening to the architectural elevation, floor plan, BOQ, or window schedule.
Give every opening a clear reference number, such as W01, W02, or W03.
Do not combine openings simply because they appear similar. Their dimensions, opening directions, glass configurations, frame systems, or installation conditions may be different.
2. Measure the Width in Three Locations
Measure the clear structural width at:
- The top
- The middle
- The bottom
Record all three measurements.
Use the smallest width for the preliminary frame-size calculation.
3. Measure the Height in Three Locations
For a standard window, measure from the structural sill to the underside of the header at:
- The left side
- The center
- The right side
Record all three measurements.
Use the smallest height for the preliminary frame-size calculation.
For a floor-to-ceiling window or door, measure from the approved bottom reference level to the top of the structural opening.
4. Check Both Diagonals
Measure from the top-left corner to the bottom-right corner.
Then measure from the top-right corner to the bottom-left corner.
These measurements are called Diagonal A and Diagonal B.
If the two diagonal measurements are significantly different, the opening may not be square. It should be corrected or reviewed before the window or door is released for production.
5. Check Level and Plumb
Confirm that:
- The top is level
- The bottom is level
- Both sides are plumb
- The opening is not visibly twisted or distorted
A correctly calculated frame may still be difficult to install if the rough opening is not level, square, and plumb.
6. Confirm the Wall Type
Tell the manufacturer whether the opening is prepared in:
- Wood framing
- Light-gauge steel framing
- Concrete
- Concrete block
- Brick or masonry
- A prepared subframe
Wall construction affects anchoring, flashing, perimeter sealing, waterproofing, and finishing details.
7. Confirm the Type of Unit
Identify whether the opening is for a standard window or a floor-to-ceiling window or door.
This distinction changes the frame-height calculation.
For a standard window:
- Deduct 10 mm from the left
- Deduct 10 mm from the right
- Deduct 10 mm from the top
- Deduct 10 mm from the bottom
For a floor-to-ceiling window or door:
- Deduct 10 mm from the left
- Deduct 10 mm from the right
- Deduct 10 mm from the top
- Do not deduct 10 mm from the bottom
8. Submit the Measurements for Shop-Drawing Review
Do not release a custom window or door order for production from a handwritten size alone.
The final shop drawings should confirm:
- Rough opening dimensions
- Actual outside frame dimensions
- Window or door type
- Opening direction
- Panel layout
- Frame depth
- Glass configuration
- Installation method
- Sill or threshold detail
Window Rough Opening Calculation Examples
Example 1: Standard Window
Measured rough opening:
Width: 1200 mm
Height: 1500 mmFrame Width = 1200 − 20 = 1180 mm
Frame Height = 1500 − 20 = 1480 mm
Preliminary outside frame size: 1180 × 1480 mm
Example 2: Larger Standard Window
Measured rough opening:
Width: 1800 mm
Height: 1600 mmFrame Width = 1800 − 20 = 1780 mm
Frame Height = 1600 − 20 = 1580 mm
Preliminary outside frame size: 1780 × 1580 mm
Example 3: Floor-to-Ceiling Window or Door
Measured rough opening:
Width: 3000 mm
Height: 2400 mmFrame Width = 3000 − 20 = 2980 mm
Frame Height = 2400 − 10 = 2390 mm
Preliminary outside frame size: 2980 × 2390 mm
These examples calculate the preliminary outside frame size, not the glass size.
Window Rough Opening Measurement Worksheet
Builders and contractors can record project openings using the following format.
Opening ID | Unit Type | Rough Opening Width | Rough Opening Height | Wall Type | Quantity |
W01 | Casement Window | ||||
W02 | Sliding Window | ||||
W03 | Awning Window | ||||
W04 | Tilt-and-Turn Window | ||||
W05 | Floor-to-Ceiling Unit |
The following information should also be recorded when available:
- Project location
- Delivery country
- Window or door type
- Opening direction
- Wall thickness
- Glass requirements
- Frame color
- Screen requirements
- Installation method
- Sill or threshold detail
- Required certification or performance criteria
Project drawings may use millimeters or inches. Use one consistent unit throughout the window schedule and clearly confirm the final production dimensions on the approved drawings.
Common Window Rough Opening Mistakes
Measuring Only Once
An opening may be narrower at the top or bottom.
Always measure the width and height in three locations.
Using the Largest Measurement
The largest dimension may cause the manufactured frame to be too large for part of the opening.
Use the smallest recorded width and height for preliminary sizing.
Confusing the Rough Opening and Frame Size
The rough opening is the structural opening in the building.
The actual frame size is calculated after the required installation clearance has been deducted.
Deducting 10 mm Only Once
For a standard window, the 10 mm allowance applies to every side.10 mm left + 10 mm right = 20 mm total width reduction
10 mm top + 10 mm bottom = 20 mm total height reduction
Deducting 10 mm From the Bottom of a Floor-to-Ceiling Unit
For an AMJ floor-to-ceiling window or door, deduct 10 mm from the left, right, and top only.
No deduction is made at the bottom.
Measuring From Decorative Finishes
Do not measure from:
- Interior trim
- Exterior casing
- Drywall returns
- Siding
- Removable finishing materials
Measure the clear structural rough opening.
Ordering Before Drawings Are Confirmed
Window type, wall structure, installation method, threshold, panel layout, and glass configuration may affect the final production dimensions.
Approve the shop drawings before manufacturing begins.
What to Send AMJ for Review
For a faster project review and more accurate quotation, send:
- Project location and delivery country
- Architectural drawings or elevations
- Window schedule or BOQ
- Rough opening width and height
- Window and door types
- Opening directions
- Quantity
- Wall construction
- Glass requirements
- Frame color
- Screen requirements
- Installation, sill, or threshold details
ViewAMJ custom aluminum window and door solutionsor send your drawings and project information through theAMJ project contact form.
Frequently Asked Questions
What is the typical rough opening for a window?
The rough opening depends on the required outside frame size.
For an AMJ standard window, allow 10 mm on all four sides.Frame Width = Rough Opening Width − 20 mm
Frame Height = Rough Opening Height − 20 mm
What is the rough opening for a 24 × 36 window?
First confirm whether 24 × 36 refers to the nominal size, actual outside frame size, or measured rough opening.
These measurements are not interchangeable.
When 24 × 36 represents the measured rough opening, use one consistent measurement unit and apply the standard-window allowance before confirming the preliminary frame size.
What is the rough opening for a 36 × 60 window?
The same rule applies.
First confirm whether the dimensions describe the nominal size, actual frame size, or rough opening.
The final production size should be confirmed on the approved shop drawing.
How much bigger should a window rough opening be?
For an AMJ standard window, the rough opening allows 10 mm on each of the four sides.
This creates a total difference of 20 mm in width and 20 mm in height.
For a floor-to-ceiling window or door, the total difference is 20 mm in width and 10 mm in height.
No deduction is made at the bottom.
What is the difference between a window rough opening and actual size?
The rough opening is the clear structural opening in the wall.
The actual window size is the outside dimension of the manufactured frame after the installation allowance has been deducted.
Should I use the smallest rough opening measurement?
Yes. Measure the width and height at three points and use the smallest recorded values for preliminary sizing and manufacturer review.
How do I check whether the opening is square?
Measure both diagonals.
If Diagonal A and Diagonal B are equal or very close, the opening is generally square.
A significant difference should be corrected or reported before production.
Does the rough opening calculation determine the glass size?
No.
The calculation determines the preliminary outside frame size.
The glass dimensions are calculated separately according to the selected aluminum window or door system.
Can I submit measurements in inches?
Yes.
Project drawings can use millimeters or inches, but one consistent unit should be used throughout the window schedule.
Final dimensions will be confirmed on the approved shop drawings.
Do floor-to-ceiling doors use the same calculation as standard windows?
No.
Standard windows deduct 10 mm from all four sides.
Floor-to-ceiling windows and doors deduct 10 mm from the left, right, and top only.
No deduction is made at the bottom.
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