Measure the finished clear opening between posts at top, middle, and bottom, then quote the smallest width and the full finished height to the gate supplier.
A gate that arrives two inches short cannot be fixed with a bigger hinge. Most ordering errors start with the tape measure, not the fabricator, and the gap between “the space I have” and “the size I ordered” is almost always a mix-up between the clear opening and the gate leaf itself. Get those two numbers straight before you call anyone, and the rest of the job falls into line.
What Are You Actually Measuring?
You are measuring the clear opening — the gap between the inside faces of your posts, pillars, or walls — unless the manufacturer states otherwise. Metal gate suppliers commonly quote the opening size because hinge, latch, and clearance allowances get built in at the factory, but practices vary by supplier and region, so confirm which number they want before you commit.
This is the step people skip. If you send a fabricator the panel width you pictured in your head when they asked for the opening, the gate shows up undersized. If you send the opening when they wanted the panel, it shows up too wide to hang.
How Do You Measure the Opening Correctly?
Take the width at the top, middle, and bottom of the gap, then use the narrowest reading. Posts are rarely perfectly parallel, and a gap that measures 48 inches at the top can close to 47⅝ inches halfway down. The small number is the true one.
- Identify the gate type — single swing, double swing, sliding, pedestrian, or driveway. Each has different clearance needs.
- Measure width between the inside faces of the posts at top, middle, and bottom. Record the smallest figure.
- Measure height from the finished ground surface to the top of the gate you want. Account for paving, gravel, soil, drainage channels, and thresholds.
- Check plumb and level with a spirit level. Leaning posts change the whole calculation.
- Note the hardware side — hinge side, latch side, swing direction, and whether the gate opens inward or outward.
- Measure the operating space so the leaves clear vehicles, walls, landscaping, and rising ground.
For a double gate, send the total clear opening and say which leaf is active and which is fixed. Do not assume each leaf is exactly half — center stops and latch clearances vary by manufacturer.
Height, Ground Clearance, and Slope
Height runs from the surface the gate will actually sit on, not from bare dirt you plan to pave later. A commonly cited installation allowance is about 2 inches of ground clearance for metal gates, but that figure depends on terrain, gate design, and local conditions — check the manufacturer’s instructions rather than treating it as a rule.
Slope is where measurements quietly go wrong. If the ground rises across the opening, a flat-bottomed gate will scrape at one end and float at the other. You have three options: specify a sloped bottom rail, add extra clearance, or redesign the opening. Measure the drop across the span and tell the supplier — that number changes the build.
Published Numbers Worth Knowing
Mueller’s fence installation guide spaces posts for a 3-foot gate at 3 feet, 4 inches, a manufacturer-specific figure you should never generalize. Typical domestic gate heights run around 900 to 1200 mm, with single-leaf width deductions in the 25 to 50 mm range — regional guidance, not US code. Treat every published deduction as the exception until your supplier confirms it.
| What to Measure | How to Record It |
|---|---|
| Width at top | Inside face to inside face |
| Width at middle | Inside face to inside face |
| Width at bottom | Inside face to inside face |
| Finished height | Ground level to gate top |
| Post width and depth | Mounting face dimensions |
| Ground slope | Drop across the opening |
| Hinge side and swing | Which way the gate opens |
Once your numbers are on paper, the next decision is which style and build suits the opening. Compare the top-rated wrought iron gate options after you have measurements in hand, not before.
Post, Hardware, and Clearance Checks
Verify the posts can carry the gate before you order it. Measure post width, depth, and the flat mounting faces available for hinges, latches, drop bolts, and handles. Sliding gates need a measured run-back area with room for the track, rollers, guides, end stops, and motor equipment — that space is often the forgotten number.
Mueller’s installation documentation is worth reading before you finalize a new opening. Mueller’s fence installation guide shows how post spacing is calculated, which matters if your posts go in fresh rather than already standing.
If your posts are included inside the overall span, account for post thickness separately from the clear opening. Mixing the two produces an opening that is right on paper and wrong in the yard.
Fixing the Measure Before You Order
Run this final pass and you will not be the person who reorders. Re-check the narrowest width with a second tape, confirm your supplier wants the clear opening rather than the panel, and write down the height from final grade. Send the hinge side, swing direction, slope, and clearance allowance in the same message so nothing depends on a guess.
Standard deductions do not apply to every gate. A supplier’s stated gap size, their bracket and hinge allowance, and the actual panel you receive are three different figures — the manufacturer balances them, and your job stops at accurate site measurements.
Common Questions
Should I measure to the face of the post or the inside edge?
Measure between the inside faces of the posts — the gap the gate fills. If the supplier asks for post dimensions as well, measure those separately. Sending an outside-to-outside figure when the fabricator needs the clear opening is one of the most common ordering errors.
What if my ground slopes across the gate opening?
Record the drop across the span and tell the supplier. A sloped bottom rail, extra clearance, or a redesigned opening may be needed. A level gate on sloped ground will scrape at one end and hang in the air at the other.
Do I need to account for hinges in my measurement?
Not usually — metal gate suppliers typically quote the clear opening with hinge, latch, and clearance allowances already built in. Confirm that assumption with your specific manufacturer, because practices vary and a standard deduction does not apply to every gate.
Sources
- Mueller, Inc. “Fence Installation Guide” Post spacing calculations for gate installations.
- Cannock Gates. “How to Measure” Clear opening versus gate leaf sizing and ground clearance guidance.
- Ruby Group. “Metal Gate Measuring Guide” Width, height, plumb, hardware, and operating space checks.