4x4 Fence Post Concrete Calculator — Bags Per Post
Calculate concrete bags per 4×4 wood fence post in standard 10 and 12-inch diameter holes. Returns 60 and 80-pound bag counts for any post depth.
Enter Dimensions
Typically 1/3 of post height above grade
Results
Bags per Post
5bags
Total Bags
50bags
Volume per Hole
1.909cu ft
Concrete per post hole
Total Volume
19.09cu ft
All post holes combined
Editorial Standards
Each calculator is reviewed for formula accuracy, unit consistency, and alignment with current U.S. building practices before publication. We verify outputs against published engineering references and real-world project data. Learn more about our methodology.
4×4 wood posts are the residential fence default — strong enough for 6 ft tall privacy and chain link fences, available in 8 ft and 10 ft lengths at every lumber yard, and affordable enough to replace if a post does fail. The estimating math is simple, but the post-displacement subtraction (often skipped by online calculators) makes a real difference at scale.
For 4 × 4 nominal posts (actual 3-1/2 inch square), each post buried 32 inches displaces 0.227 cu ft of concrete that you do NOT need to order. Over a 20-post project, that's 4.5 cu ft of unnecessary concrete — 8 bags of 80 lb mix at $5/bag = $40 saved.
How to Calculate Concrete for a 4x4 Fence Post
Concrete per 4×4 post (cu ft) = π × (Hole D ÷ 24)² × Depth × 12 − (3.5 ÷ 12)² × Buried Depth × 12. Bags 80 lb = ⌈Concrete ÷ 0.6⌉.
4×4 Post Common Configurations
- 10 inch hole × 24 inch deep: 0.86 cu ft net concrete ≈ 2 bags 80 lb (≈ 1 cartridge expanding foam).
- 10 inch hole × 32 inch deep: 1.20 cu ft ≈ 2-3 bags 80 lb. The most common 6 ft fence setup.
- 12 inch hole × 36 inch deep: 1.86 cu ft ≈ 4 bags 80 lb. Used for corner and gate posts that take more lateral load.
- 12 inch hole × 48 inch deep (cold climate): 2.61 cu ft ≈ 5 bags 80 lb.
Frequently Asked Questions
How many bags of concrete per 4x4 fence post?
For a typical 4×4 wood post in a 10 inch hole 32 inches deep: 2 bags of 80 lb fast-set concrete per post. For a 12 inch × 36 inch hole: 3 bags of 80 lb.
What is the actual width of a 4x4 post?
A 4×4 wood post is nominally 4 × 4 inches but actually 3-1/2 × 3-1/2 inches. The 0.5 inch reduction is industry-standard for pressure-treated and cedar lumber.
How deep should I set a 4x4 fence post?
For a 6 ft fence: minimum 30-32 inches deep below grade. Below frost line in cold climates. Add 3-4 inches of gravel at the bottom for drainage.
Can I use Postmaster or Sika instead of Quikrete?
Yes — Sika Post Fix expanding foam (or similar) is a fast, lightweight alternative. Two cartridges (≈ $30) replace 2 bags of concrete and set in 3 minutes. Trade-off: not as ultimate strength as concrete; not approved by all jurisdictions.
How does the calculator handle the post displacement?
Post displaces concrete inside the hole. For a 4×4 buried 32 inches: 3.5″ × 3.5″ × 32″ = 392 cu in = 0.227 cu ft of displacement subtracted from hole volume to get net concrete needed.
Looking for the general calculator?
Calculate how many bags of concrete you need per fence post hole. Supports 4×4 and 6×6 wood posts plus metal pipe posts in standard hole sizes.
Open the Concrete Fence Post Calculator: Bag Count →