Post Hole Concrete Calculator

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Construction

Calculate concrete needed to fill a post hole (annular cylinder around a post).

Formula

V = π × (R_hole² − R_post²) × depth; 60 lb bag ≈ 0.45 cu ft; 80 lb bag ≈ 0.60 cu ft
  • Square posts are approximated as circles of equivalent cross-section for simplicity.
  • Use fast-setting concrete (Quikrete Fast-Setting or equivalent) for fence posts.
  • Minimum burial depth: 1/3 total post length or 2 ft below frost line, whichever is greater.

12 in hole, 36 in deep, 4×4 post, 8 posts

Inputs
  • Hole Diameter: 12 in
  • Hole Depth: 36 in
  • Post Size (square): 4 in
  • Number of Posts: 8 posts

Per hole: π×(0.25−0.0278)×3 = 0.834 cu ft. 8 posts = 6.67 cu ft → 15 bags of 60 lb.

Frequently asked questions

How deep should fence post holes be?
Minimum 1/3 the post length above ground, or 6–12 in below the local frost line (whichever is greater). A 6 ft fence post needs a 2 ft hole minimum.