Post hole calculator
Post Hole Concrete Calculator
Calculate concrete needed for fence posts, deck posts, and round post holes. Enter hole diameter, depth, and count to get instant estimates.
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How to Calculate Concrete for Post Holes
Post hole concrete volume is calculated using the formula for a cylinder: Volume = π × r² × depth × number of holes
For example, a fence with 20 post holes, each 12 inches in diameter and 3 feet deep, requires:
Volume = π × 0.5² × 3 × 20 = 47.1 cubic feet = 1.74 cubic yards
Post Hole Sizing Guidelines
- Hole diameter: Should be 3× the post width (e.g., 4×4 post needs 12″ hole)
- Hole depth: Minimum 1/3 of above-ground post height, or below frost line
- Frost line depth: Varies by region, typically 12-48 inches
Tips for Setting Posts
Always dig holes slightly wider than required to allow for proper concrete coverage. Use gravel at the bottom for drainage, and slope the concrete surface away from the post to prevent water pooling. For fence posts, wait at least 4 hours before attaching rails; for deck posts, wait 24-48 hours before loading.
Frequently Asked Questions
How much concrete for a 4x4 fence post?
A typical 4x4 fence post hole is 12 inches in diameter and 24-36 inches deep. This requires about 1.57-2.36 cubic feet of concrete. Before adding waste, that is 3-4 bags of 80 lb concrete per hole; a 10% allowance raises a 36-inch-deep hole to 5 bags.
How deep should fence post holes be?
Fence post holes should be at least 1/3 the height of the above-ground post, plus 6 inches for gravel base. For a 6-foot fence, dig holes at least 2 feet deep. In cold climates, dig below the frost line (typically 36-48 inches).
Should I use quick-setting concrete for posts?
Quick-setting concrete is convenient for small projects and sets in 20-40 minutes. However, standard concrete mix is stronger and more cost-effective for multiple post holes. For fence posts, quick-setting is acceptable; for structural deck posts, use standard mix.
How many bags of concrete per post hole?
A 12-inch-diameter × 24-inch-deep cylindrical hole holds about 1.57 cubic feet, which requires 3 bags of 80 lb concrete. At 36 inches deep it holds about 2.36 cubic feet, requiring 4 bags before waste or 5 bags with a 10% allowance.