How Windows Are Measured
Cross-manufacturer ordering gotchas
The same-looking call-out number doesn't mean the same thing across brands. One manufacturer's "3060" might be the unit/nominal size, while another's "3060" is the actual rough opening — and impact-rated lines often quote dimensions tip-to-tip (frame edge to frame edge), not the glass or the hole. Before you order or buck anything, confirm WHICH dimension the schedule is giving you, because guessing wrong puts you off by inches. That's exactly why Anchor makes you tag each opening as nominal, R.O., or M.O.: a single mis-tag throws the buck off by about 3 inches.
- Nominal to masonry opening: add 3.5"
- Labeled R.O. to masonry opening: add 3.0"
- M.O. label: used as-is, no adjustment
- Mis-tag error on the buck: ~3"
- Finished hole: window + 1/2" shim
- Example: 36x60 window to 39.5x63.5 masonry opening
- A call-out like "3060" is shorthand (roughly 3'0" x 6'0"), but shorthand for WHAT differs by brand — it can be the nominal unit size or the rough opening.
- Impact / Florida-rated product schedules frequently list tip-to-tip frame dimensions, which are not the same as the hole you need to frame.
- The fix is dirt simple: read the column header and the fine print on the schedule, then tag the opening in Anchor before bucking.
- Because nominal+3.5" = (nominal+0.5")+3" = M.O., a correctly tagged opening always lands on the same masonry opening no matter which basis the brand published.
Pros
- Tagging nominal/R.O./M.O. makes every brand's schedule converge on one correct masonry opening
- Catches the tip-to-tip vs. rough-opening trap before material is cut
- One quick label check prevents re-ordering glass and re-bucking openings
Watch-outs
- Requires reading the actual schedule header/fine print, not just trusting the call-out
- Mixing brands on one job means confirming the basis for each line separately
- A single mis-tag silently mis-sizes the buck by ~3" with no warning