Name the four dimensions
A dimension needs both a number and a meaning. A 1200 mm width does not tell the supplier whether you are describing the wall opening, a frame or the glass. Record the meaning in a separate size-basis field so it remains attached to the number when the schedule is copied or exported.
| Size basis | What it describes | What it does not establish |
|---|---|---|
| Rough opening | Opening in the wall prepared for the assembly | Final frame size without the relevant sizing requirements |
| Unit / frame | Outside dimensions of the window unit | Visible glass size or usable open area |
| Visible glass | Exposed glass area within the surrounding profiles | Frame dimensions or clear passage |
| Clear opening | Unobstructed opening available with the operable product open | Compliance from frame width and height alone |
The drawing is conceptual. It does not specify how a particular product fits into a wall or how much clearance it needs. Clear opening also depends on the actual operation and geometry of the product.
Find the size basis in your source
Read the schedule header, dimension notes and relevant section or detail before interpreting a width and height. A size code needs the applicable product chart. A field headed only Width is incomplete when the underlying dimension basis is not stated.
Andersen sizing resources distinguish unit, rough-opening and other dimensions. Marvin rough-opening measuring instructions separately label frame, casing and rough opening. These manufacturer references support the distinction; they do not define one allowance for every window.
For the wider drawing context, read the schedule together with its notes. Where the plan and schedule disagree, retain both references and ask the responsible designer to identify the intended basis.
Record the basis beside the number
Keep an unknown basis explicit instead of choosing the most likely interpretation. The original teaching rows below show why two equal numeric widths do not yet describe interchangeable items. They are not product dimensions or installation recommendations.
| Mark | Width x height | Basis | Next decision |
|---|---|---|---|
| A01 | 1200 x 1500 mm | Rough opening | Obtain product-specific frame sizing |
| A02 | 1200 x 1500 mm | Frame | Confirm installation requirements |
| A03 | 1200 x 1500 mm | Unknown | Ask what the source dimensions measure |
Changing units does not change the basis. A rough opening converted from inches to millimeters is still a rough opening. Use the feet, inches and millimeter examples to check the arithmetic separately.
Confirm the installation-specific requirement
Use the sizing and installation instructions for the selected product, configuration and installation condition. Confirm the dimension convention with the supplier before turning a planning value into an order size. Do not add or subtract a default gap based on a different window family.
Record the product reference, instruction revision, source dimension and any outstanding question in the handoff. The schedule template includes a size basis and unresolved-decision fields. A preliminary takeoff remains useful while a dimension is unresolved, provided the uncertainty stays visible.