Fence Post Calculator

Calculate fence post hole volume, concrete bags, and gravel base for each post. Enter fence length, post spacing, and hole dimensions to get a complete installation material list.

ft

Total linear feet of the fence run.

ft

Distance between post centers. Standard: 8 ft. High-wind: 6 ft.

in

Post width/diameter. 4×4 = 4", 6×6 = 6". Round posts: actual diameter.

in

1/3 of fence height. 6 ft fence → 24". 8 ft fence → 32".

in

3× post width. 4×4 post → 12" hole. 6×6 post → 18" hole.

lb

Bag size for concrete. 60 lb is most common for post holes.

%

Extra for spillage, uneven holes. Default 10%.

How this calculator works

This fence post calculator focuses on the installation phase — post-hole excavation and concrete:

  1. Posts: ceil(fence_length / post_spacing) + 1.
  2. Hole volume: π × (hole_width/2/12)² × (hole_depth/12) — cylindrical hole volume in cu ft.
  3. Post volume: π × (post_diameter/2/12)² × (hole_depth/12) — subtracted from hole volume to get concrete needed.
  4. Concrete per post: hole volume − post volume.
  5. Total concrete: per-post × posts × (1 + waste %).
  6. Bags: ceil(total concrete / bag yield). 60 lb → 0.449 cu ft, 80 lb → 0.599 cu ft.

Also estimates gravel base (6" at bottom of each hole for drainage).

Formula

posts = ceil(fence_length / post_spacing) + 1
hole_radius_ft = (hole_width / 2) / 12
hole_volume = π × hole_radius² × (hole_depth / 12)
post_radius_ft = (post_diameter / 2) / 12
post_volume = π × post_radius² × (hole_depth / 12)
concrete_per_post = max(0, hole_volume - post_volume)
total_concrete = concrete_per_post × posts × (1 + waste_pct / 100)
bags = ceil(total_concrete / bag_yield)
gravel_base = π × hole_radius² × (6 / 12) × posts

Example

For a 100 ft fence with 8 ft spacing, 4×4 posts, 24" deep × 12" wide holes, 60 lb bags, 10% waste:

  • posts = ceil(100/8) + 1 = 14 posts
  • hole_volume = π × (6/12)² × (24/12) = π × 0.25 × 2 = 1.571 cu ft
  • post_volume = π × (2/12)² × 2 = π × 0.0278 × 2 = 0.1745 cu ft
  • concrete_per_post = 1.571 - 0.175 = 1.396 cu ft
  • total_concrete = 1.396 × 14 × 1.10 = 21.50 cu ft
  • bags (60 lb) = ceil(21.50 / 0.449) = 48 bags
  • gravel_base = π × 0.25 × 0.5 × 14 = 5.50 cu ft

Important assumptions

  • Hole depth: 1/3 of fence height (6 ft fence → 24", 8 ft fence → 32").
  • Hole width: 3× post width (4×4 post → 12" hole, 6×6 post → 18" hole).
  • Post volume approximated as a cylinder of post_diameter (slightly overestimates for square posts — conservative).
  • Gravel base: 6" of gravel at the bottom of each hole for drainage.
  • Concrete density: 133.4 lb/cu ft. 60 lb bag yields 0.449 cu ft, 80 lb bag yields 0.599 cu ft.
  • Gate posts need deeper holes and more concrete — plan separately.
  • Always call 811 (Call Before You Dig) before excavating.

FAQ

How deep should fence post holes be?
1/3 of the fence height: 6 ft fence → 24" deep, 8 ft fence → 32". In frost zones, go below the frost line (typically 36-48" in cold climates).
How wide should a fence post hole be?
3× the post width: 4×4 post → 12" diameter hole, 6×6 post → 18" diameter hole. This gives enough room for concrete to surround the post completely.
How many bags of concrete per fence post?
A 4×4 post in a 12" wide × 24" deep hole needs about 1.4 cu ft of concrete per post — about 3 bags (60 lb) per post. A 100 ft fence with 14 posts needs ~48 bags total (with 10% waste).
Do I need gravel at the bottom of fence post holes?
Yes — 6" of gravel at the bottom prevents water pooling, which causes post rot. This calculator includes the gravel estimate automatically.
How is this different from the Fence Calculator?
The Fence Calculator estimates posts, rails, and pickets (the fence structure). This Fence Post Calculator focuses on the installation phase: post-hole excavation, concrete bags, and gravel base. Use both for a complete fence project.