Gate-Post Sizing for Swing-Gate Weight and Lever Arm
Why this matters
The gate post supports the gate; the gate hinges transfer the gate weight to the post; the post transfers everything to the footing. Get the post too small, the gate sags within a season. Get the footing too shallow, the post leans and the gate drags on the latch side. Sizing the gate post is a moment calculation, not a guessing game from how the rest of the fence looks.
The moment problem
A swing gate is a lever. The gate weight acts at the gate centroid, which sits horizontally out from the post by half the gate width plus the hinge offset. That horizontal distance times the gate weight is the overturning moment the post must resist. A 4-foot wide, 80-pound gate puts a 160 foot-pound moment on the post at the top hinge level; a 6-foot wide, 200-pound gate puts a 600 foot-pound moment.
Two moments to size for:
- Overturning moment from gate dead weight
- Wind moment on the gate face (lbs per square foot times gate area times half the gate width)
For a residential 6 by 6 foot solid privacy gate in a 90 mph wind zone, the wind moment can exceed the dead-weight moment.
Post material selection by gate weight
A general rule from the major hardware manufacturers (D&D Technologies, Locinox, BMP):
- Gates under 100 pounds, 4 feet wide or less: 4x4 PT wood, 2-inch schedule 40 steel, or 4-inch square aluminum gate post
- 100 to 200 pound gates, 4 to 5 feet wide: 6x6 PT wood, 2.5-inch schedule 40 steel, or 4-inch square heavy-wall aluminum
- 200 to 350 pound gates, 5 to 6 feet wide: 6x6 PT wood post is marginal; specify 3-inch schedule 40 steel or 6-inch square steel tube
- Over 350 pounds or driveway gates: 4-inch schedule 80 steel minimum, often 6 by 6 by 1/4 inch wall HSS, set in a footing sized to the moment
A 4x4 PT post on a 5-foot wide solid privacy gate will sag the latch side 2 to 3 inches per year as the post fibers compress at the upper hinge bearing.
Footing depth and diameter
The footing must resist both vertical load and overturning. The classic civil-engineering pole-footing formula (Czerniak / IBC 1807.3) sets the embedment depth as a function of moment, allowable lateral soil pressure, and footing diameter. Practical results for typical Class 4 soil (1500 psf lateral bearing):
- 100-pound gate post: 12-inch diameter footing, 36 inches deep below frost line
- 200-pound gate post: 12 to 14 inch diameter, 42 inches deep
- 350-pound gate post: 16-inch diameter, 48 inches deep
- Driveway gate (commonly 600 to 1200 pounds): 18 to 24 inch diameter, 60 inches deep, often with rebar cage
Frost line depth in the IRC R403.1.4 controls minimum depth; the moment calculation can require deeper. Specify the deeper of the two.
Hinge selection and gate weight rating
Every hinge brand publishes a per-pair load rating. Two hinges share the gate weight, but the upper hinge takes the full overturning moment in tension (pulling out of the post) and the lower hinge takes it in compression. Use only hinges rated for the gate weight; oversize one class up for solid-wood privacy gates in windy areas.
D&D Technologies SureClose, Locinox Mammoth, and BMP G80 series all publish capacity tables. Pin-and-socket hinges out-perform strap hinges for high-cycle gates.
When to use a gate frame
A gate without an internal diagonal brace will rack. The diagonal must run from the bottom hinge corner to the top latch corner (compression brace, pushing the latch side up) or from the top hinge corner to the bottom latch corner with a cable (tension brace, pulling the latch side up). Reversing the diagonal does nothing structurally; it just looks like a brace.
For solid privacy gates over 5 feet wide, weld or screw a steel angle frame on the back side, with the diagonal as part of the frame. A wood-only 6 by 6 foot gate cannot be braced enough to hold square without a steel frame.
Common failures and what they look like
- Gate sags at the latch: post leaning toward the latch, or upper hinge pulling out. Re-plumb the post or upgrade the hinge.
- Gate drags on the ground: footing has rotated. Pull the post, enlarge the footing, reset.
- Latch will not engage in summer: gate has expanded; either trim or relocate the latch
- Gate will not stay closed in wind: missing positive latch, or latch in the wrong location (latch should be at the wind-load center of the gate, typically the geometric center)
A driveway gate over an automated operator increases all moments. Add 25 percent to the gate weight for the operator pull force, and 100 percent if the gate has a solid panel facing the prevailing wind. Driveway gates almost always require an engineered footing under IRC R401.2 if the local jurisdiction permits them.
References
- 2021 International Residential Code, Section R401.2 and R403.1 (footings)
- 2021 IRC R403.1.4 (footing depth, frost line)
- 2021 IBC Section 1807.3 (embedded posts and poles, Czerniak formula)
- D&D Technologies SureClose Hinge Capacity Tables, current revision
- Locinox Hinge and Lock Catalog, hinge load ratings
- American Wood Council National Design Specification (NDS), wood post bending and bearing values