Most gate problems start before any steel is cut: a wrong measurement. If you order a gate to the raw opening width, it will bind, scrape, or leave an ugly gap. This guide shows exactly how to measure a gate opening, what clearances to subtract, and how to account for posts and hinges so your custom gate fits the first time.

Measure the opening, not the gate you imagine

Start with the clear structural opening: the distance between the two fixed points the gate will span, whether those are brick pillars, steel posts, or a wall edge. Measure at three heights: bottom, middle, and top. Openings are rarely perfectly parallel, and the smallest of the three is your working width.

Why three heights matter

Pillars lean over time and walls bow. If you measure only at the top and it is 10 mm wider than the bottom, a gate built to the top figure will jam near the ground. Always build to the tightest point, then plan gaps around it.

Subtract the clearances

A gate never equals the opening. You must leave room to move and swing. Typical allowances for a swing gate:

These are practical starting points, not fixed rules; confirm with your fabricator because hinge type and finish thickness change the numbers. Powder coating adds a fraction of a millimeter per side, which matters on tight tolerances.

Ground clearance depends on your surface

On flat concrete, 50 mm is often enough. On a gravel or sloped driveway, raise it so the leaf never catches at the highest point of its travel. A gate that scrapes will chip its coating and start rusting exactly where you cannot see it.

Account for posts and foundations

If you are installing new posts, the opening only exists after they are set. Decide post size first, then the gate width is the gap between finished post faces minus your gaps. A common error is ordering the gate, then discovering the posts eat 100 mm each and the leaf no longer fits the driveway.

A real scenario

A homeowner measured a 4000 mm gap between two brick pillars and ordered a 4000 mm double gate. On install day the leaves overlapped the pillars with no room for hinges or a latch. The fix cost a week: the gate had to be cut and re-welded. A correct order would have been about 3960 mm total across both leaves, leaving gaps for hinges, the center, and the latch.

Common mistakes and how to fix them

• Measuring at one height only. Fix: measure bottom, middle, top; use the smallest.
• Ordering to the raw opening. Fix: subtract hinge, latch, and center gaps before ordering.
• Forgetting ground slope. Fix: check clearance at the highest point of the swing arc, not just at rest.
• Ignoring coating thickness. Fix: tell your fabricator the finish so they set tolerances.
• Measuring before posts exist. Fix: set or specify posts first, measure finished faces.

Action steps

• Use a rigid tape or laser, not a fabric one that sags.
• Record width at three heights and note the smallest.
• Check the opening is square by comparing both diagonals.
• Decide gate type and hinge style, then subtract the right gaps.
• Write down surface type and slope for the ground clearance.
• Confirm every figure with your gate maker before cutting.

Conclusion and next step

Accurate measuring is the cheapest insurance in the whole project. Spend fifteen careful minutes with a tape and a notepad, record three heights, and confirm the clearances with your fabricator. Do that today and your gate will hang clean and swing free on the first try.

FAQ

What tool should I use to measure?

A steel tape or a laser measure. Avoid soft measuring tapes that stretch and sag over wide openings.

How do I check if my opening is square?

Measure both diagonals of the opening. If they match, it is square; if not, note the difference and let your fabricator adjust the gate frame.

How much ground clearance is safe?

Around 50 mm on flat concrete, more on gravel or slopes. The gate must clear the highest point it passes over, not just where it rests.

Should I measure before or after the posts are built?

After. The true opening only exists once the posts are set and finished, so measure the final post faces.