Technical diagram of a window with dotted-line measurements
The R.O. should be 7/8″ larger than the OSM.

What is R.O.?

R.O. stands for “rough opening.” It is the roughed-out space or opening where a window is to be installed. The R.O. should be approximately 7/8“ larger than the outside measurement (OSM) of the window frame.

When measuring the R.O., it is important to have accuracy and consistency. If the R.O. is less than 1/2 “ or greater than 1 “ you may be leaving too little or too much wiggle room. Be sure to measure the R.O. along both plumb (vertical) and level (horizontal) surfaces, to ensure the measurement is consistent.

You should consider hiring a professional to help install your window if you are having trouble obtaining these exact measurements.

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