How Windows Are Measured
Sizing the buck: +3.5 / +3 / as-is
The buck frames the masonry opening, so you have to grow the window size out to that opening before you cut. Anchor handles three label types so you always land in the same place: from a NOMINAL size add 3.5", from a labeled R.O. add 3.0", and a labeled M.O. you use as-is. All three converge on the same masonry opening, and the finished hole always comes back to window + 1/2" shim. Example: a 36x60 nominal window grows to a 39.5x63.5 masonry opening, and after the buck and shim you have a 36.5x60.5 finished hole.
- From NOMINAL: add 3.5" each dimension
- From a labeled R.O.: add 3.0"
- A labeled M.O.: use as-is
- Buck thickness: 1.5" per side
- Shim gap: 0.25" per side (1/2" total)
- Example 36x60 → 39.5x63.5 M.O. → 36.5x60.5 finished hole
- Why +3.5 from nominal: each side gets 1.5" of buck plus 0.25" of shim, so 1.5 + 0.25 = 1.75" per side, times two sides = 3.5" added to width and to height.
- Why only +3.0 from a labeled R.O.: a rough opening already bakes in the 1/2" shim allowance, so you only add the two 1.5" bucks (3.0" total).
- Why M.O. is as-is: a masonry opening is the hole the buck frames, so there is nothing left to add — Anchor uses the number straight.
- They all converge: nominal+3.5 = (nominal+0.5 R.O.)+3 = the labeled M.O. — three labels, one masonry opening.
- The finished hole is always window + 1/2" total (1/4" shim each side), which is what lets you plumb and square the unit. For 36x60 that is 36.5 x 60.5.
Pros
- One rule per label type — no guessing which allowance is already included
- Self-checking: if the three label paths don't land on the same M.O., a measurement is wrong
- Leaves the correct 1/4" shim space per side to plumb and square the unit
Watch-outs
- Mislabeling the input (calling an R.O. a nominal, or vice versa) throws the opening off by 1/2"
- Assumes a standard 1.5" buck per side — non-standard buck stock needs the allowance adjusted