Understanding Window Sizing: RO vs NET (New Construction & Replacement)

 If you are familiar with construction, then you may know that measuring and ordering windows can be complicated. Whether they’re for a new construction house or a replacement project, it’s crucial to know the difference between “RO” sizing and “net” sizing. 


Rough opening (RO) size is simply the framed opening size. Meaning after a window is framed by the framer, the width and height of the stud to stud measurement is the RO dimension.


Net size (also called frame size or heel size) is the actual width and height of the window from outside edge to outside edge.

New Construction:

If you’re building a new house then windows are always part of the equation. Looking at blueprints it may seem obvious what the sizes are - especially if there is a window schedule. However, there can be some confusion about what the ACTUAL size of the windows will come. The important thing to know is that all specs related to window sizing on house plans is always the RO size. 


2850 = 2'8" x 5' ROUGH OPENING



Ordering windows for new construction:

Since the sizes on the plans are always the RO size, the windows will be quoted out/ordered in the same manner. This means that when the windows come, they will be made smaller to fit within the framed openings. Usually 1/2” smaller width and height, but depending on manufacture it is sometimes 3/4” smaller. 

Replacing Windows

Other outside edge. 23 3/4" is the exact size 
however a standard 23 1/2" wide would fit fine.
Standard Aluminum Window. Measure outside
frame to outside frame.




































When measuring for replacement windows, it can be a difficult task knowing what size to order. I do this for a living so trust me when I say that it can be a challenge. Knowing how the window was originally installed helps guide you how and where to measure too. But what’s important to know is that compared to new construction, often times the sizes you measure/order will be the NET SIZE. This means the window will be manufactured to your exact specifications - no downsizing. 


More often than not, you will be ordering replacement windows based off their net size. This is more true in certain instances… for example:

  1. If you have a house pre 1960 (before standard aluminum windows were manufactured).
  2. If you have wood windows (as often times those are funky sizes).
  3. If you have stucco or masonry exteriors as the replacement windows will need to be downsized typically to fit within the openings).


If you have aluminum or vinyl windows in a house that is built from 1960-present that you are measuring for replacement, there is a good chance you can order the new windows based off the RO size. 

Old Double Hung window opening. Net sizes will be used in this instance to measure/
order the new windows.


In Conclusion:

RO window sizing means the window will be downsized 1/2”-3/4” in width and height to fit within the rough opening. 

Net window sizing means the window will be made to your exact dimensions.


Ordering windows can be complicated but understanding these differences will help architects, builders, framers and carpenters alike for any projects that include ordering windows. Less mistakes will be made because there is no confusion. The last thing anyone wants is to have to reorder a window after months of waiting for them in the first place.

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