Fence Post Concrete Calculator
Calculates concrete volume and bag counts per post hole by subtracting the wood/metal post displacement volume from the cylindrical hole volume (standard rule: hole diameter = 3× post width; depth = 1/3 post height).
Formula
Hole Cu Ft = (π × r² × Depth) - Post Vol; Bags per Post = Ceil(Hole Cu Ft ÷ Bag Yield)
1. Hole Diameter: 4x4 = 10 in (r=0.4167 ft); 6x6 = 12 in (r=0.5 ft); Steel = 8 in (r=0.333 ft) 2. Cylinder Vol = π × r² × Depth ft 3. Post Displacement Vol = Post Area sq ft × Depth ft 4. Net Concrete Vol per Post = Cylinder Vol - Post Displacement 5. Total Cu Ft with Waste = Net Vol × Post Count × (1 + Waste %) 6. Bags to Order = Ceil(Total Cu Ft ÷ Bag Yield)
Inputs
- Total Number of Fence Posts
- Post Dimensions
- Hole Depth
- Concrete Bag Type
Worked example
Setting twenty 4x4 fence posts 24 inches deep using 50 lb Fast-Setting Quikrete.
Outcome: Each hole is 1.09 cu ft. Subtracting the 4x4 post displacement (0.17 cu ft) leaves 0.92 cu ft of concrete per hole (~2.45 bags). For 20 posts with 10% waste, you need 20.2 cu ft = 54 bags of 50 lb Fast-Setting concrete (~2.7 bags/post).
How do you use fast-setting "no-mix" concrete for fence posts?
Dig the hole, place 3 inches of gravel at the bottom, set and brace the post plumb. Fill the hole half full with water, pour in the dry concrete mix until it reaches 2-3 inches below grade, and top off with remaining water. It sets solid in 20 to 40 minutes.