Fence Post Calculator
Post count, hole depth, and concrete for wood, metal, and vinyl posts.
Deeper, more concrete
- Total posts
- 14
- Line posts
- 12
- Line post hole depth
- 36 in
- Corner/gate hole depth
- 42 in
- 60 lb concrete bags
- 16
1 per line post, 2 per corner/gate post
Post count and hole depth by fence height
The one-third rule is the shortcut every fence installer learns: one third of the post length goes below ground. A 6 foot fence uses 8 foot posts set 2 feet deep. A 4 foot fence uses 6 foot posts set 2 feet deep. A tall 8 foot privacy fence needs 10 or 12 foot posts set 3 feet deep, because the taller the fence, the more wind load transferred to the post.
In Texas clay soils that expand and contract seasonally, add 6 inches to the standard depth for line posts. Corner posts and gate posts should always go 6 inches deeper than line posts because they carry lateral load from two directions instead of one.
Hole diameter and concrete quantity
The rule for hole diameter is three times the post cross-section. A 4x4 wood post (3.5 inch actual) wants a 10 to 12 inch diameter hole. A 2 3/8 inch metal line post wants a 6 to 8 inch hole. A 4x4 vinyl post over a metal insert follows the metal insert diameter.
Concrete volume by hole size: a 10 inch diameter by 24 inch deep hole is about 0.91 cubic feet, which one 60 lb bag of fast-set concrete fills. A 12 inch diameter by 30 inch deep hole is about 1.64 cubic feet, which needs two bags. Round up. Extra concrete does not hurt; short concrete leaves the post loose.
Wood vs metal vs vinyl post selection
Pressure-treated 4x4 pine posts are the default budget choice. They rot at grade line within 15 years in wet or clay-heavy areas. Cedar 4x4 posts last 20 to 25 years but cost twice as much.
Galvanized steel posts, either T-posts driven or 2 3/8 inch tube posts set in concrete, last 40 to 50 years and are the professional standard for high-wind areas and long straight runs. Steel adapts to any fencing material (wire, wood pickets on brackets, chain link fabric).
Vinyl posts are actually metal or wood posts inside a vinyl sleeve. Choose based on the internal structure. A vinyl fence with steel inserts lasts 30+ years; a vinyl fence with wood inserts fails when the wood rots regardless of the outer sleeve.
Common installation mistakes
Setting posts in dry-mix concrete without water. The bag says fast-set with water added; skipping the water leaves loose sand around the post. Add water per the bag instructions and stir with a stake.
Skipping the gravel base. Four inches of gravel at the bottom of each hole drains water away from the post base and doubles the effective life of wood posts. Every professional fence crew does this. Every DIY install skips it.
Not sloping concrete away from the post. Concrete poured flush or with a slope toward the post traps water at the wood-concrete interface and rots the post from the outside in. Slope the concrete crown down and away from the post like a small pyramid.
For fence contractors
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Frequently asked.
- How many fence posts do I need for 100 feet?
- With 8 foot on-center spacing, that is 13 line posts plus corners and end posts. Round up to 14 total for a straight run.
- How deep should a 6 ft fence post be?
- Minimum 24 inches for line posts, 30 inches for corner and gate posts. In expansive clay soil, add 6 inches to both.
- How much concrete per fence post?
- One 60 lb bag per standard line post. Two bags per corner or gate post. Fast-set concrete is fine; regular concrete needs 48 hours before hanging fabric or panels.
- Can I use quick-set concrete for fence posts?
- Yes. Fast-set (like Quikrete Fast-Setting) sets in 20 to 40 minutes and reaches full strength in 4 hours. Add water per the bag directions; do not dry-set.
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