Measure the opening, not the old gate
If you're replacing an existing gate, measure the actual structural opening (the gap between posts or pillars), not the old gate panel itself — the old gate may have been undersized or fitted with shims that hide the true opening dimension.
Check for level and square
Openings settle and shift over years — measure both diagonals of the opening, not just width and height. If the diagonals differ significantly, the opening isn't square, and that needs to be communicated to your fabricator rather than discovered at installation.
Account for clearance
A gate needs a small operating clearance on all sides to swing or slide freely, especially important for sliding gates that need clear track space — this clearance amount depends on the gate type, so it's worth confirming with your fabricator rather than assuming a flush fit.
Don't skip the site visit
For anything beyond a simple, small gate, our site visit checklist applies here too — a fabricator taking their own measurements on-site catches problems a homeowner's tape-measure numbers might miss.