Post Hole Concrete Calculator

Fence & Deck Foundation Structural Engineering

Setting wooden and steel structural posts in concrete provides lateral wind resistance and axial vertical load support. Building codes (IRC §R403) mandate digging post holes below the regional ground frost depth to eliminate cyclic freeze-thaw frost jacking.

Core Post Hole Sizing Formulas

• Gross Hole Volume: V_gross = π (D_in2)^2 × H_in (cu inches)

• 4x4 Post Volume: V_4× 4 = 3.5" × 3.5" × H_in = 12.25 × H_in

• 6x6 Post Volume: V_6× 6 = 5.5" × 5.5" × H_in = 30.25 × H_in

• Net Concrete Volume per Hole: V_net (cu ft) = V_gross - V_post1728

• Bags to Buy: N_bags = V_net (cu ft) × N_holesBag Yield (cu ft)

Construction Rules of Thumb

• Hole Diameter: Hole diameter should be 3 times the post width (e.g. 10 to 12-inch hole for a 3.5-inch 4x4 post; 12 to 14-inch hole for a 6x6 post).

• Hole Depth: Hole depth should be at least 1/3 to 1/2 of the above-ground post height (e.g. a 6-foot fence post needs a 24 to 36-inch deep hole).

• Gravel Base: Pouring 3 to 6 inches of crushed gravel at the base of the hole allows water drainage and extends post rot resistance.

How to Use This Calculator

Enter your Post Hole Diameter (e.g. 10.0 in in), Post Hole Depth (below ground) (e.g. 36.0 in in), Post Size & Profile (Deducted) — choose from 4×4 Square Post (Actual 3.5" × 3.5"), 6×6 Square Post (Actual 5.5" × 5.5"), 2-3/8" Round Metal Chainlink Pipe, Number of Post Holes (e.g. 10 in holes), and Concrete Premix Bag Size — choose from 50 lb Fast-Setting Concrete (Yield 0.37 cu ft), 60 lb Standard Premix (Yield 0.45 cu ft - Universal), 80 lb Heavy-Duty Premix (Yield 0.60 cu ft). Measure twice before entering dimensions — small measurement errors scale up significantly in material estimates.

The Formula

Total cylinder hole volume in cu inches = pi (d/2)^2 depth

Net concrete volume per hole in cu inches (1 cu ft = 1728 cu inches)

Worked Example

Setting 10 fence posts with 10-inch holes drilled 36 inches deep for 4x4 pressure-treated posts: Requires 3.2 bags of 60-lb Quikrete per hole, totaling 32 bags.

Understanding Your Result

Always add a 10–15% waste factor to concrete volume estimates for normal pours, and 15–20% for irregular forms or slabs with cutouts. Standard ready-mix concrete is ordered in cubic yards (1 cubic yard = 27 cubic feet). Minimum slab thickness recommendations: driveways 4 inches, garage floors 4–6 inches, structural slabs per engineer specification. Over-ordering concrete is generally preferable to under-ordering — a short pour creates a cold joint that weakens the structure.