How to Calculate Concrete for Fence Posts & Deck Holes
Securing wooden, vinyl, or metal posts in concrete requires digging cylindrical auger holes, placing a compactable gravel base at the bottom for drainage, and setting the post upright before pouring concrete. To accurately determine concrete requirements, you must calculate the cylinder volume of the hole, subtract the volume displaced by the embedded post, and subtract the gravel base height.
Rule-of-Thumb Sizing Guidelines
- Hole Diameter: Typically 3 times the width of the post (e.g., a 4x4 post measuring 3.5 in x 3.5 in requires a 10 to 12-inch diameter hole).
- Hole Depth: Minimum 1/3 to 1/2 of the total post height above ground, extending below the local frost line to prevent frost heave during freezing conditions.
- Gravel Base: A 4 to 6-inch layer of crushed stone at the bottom of the hole aids drainage and preserves wooden posts against premature rot.
Mathematical Formula
The formula for net concrete per post hole is:
Real Worked Example
Scenario: Setting 6 fence posts using 4x4 wooden posts (actual 3.5 in × 3.5 in) in 10-inch diameter holes dug 36 inches deep, with a 4-inch gravel base and 10% spill allowance.
- Hole radius: 5 inches = 0.4167 ft
- Concrete depth: 36 in - 4 in gravel = 32 in = 2.667 ft
- Hole concrete cylinder vol per hole: π × (0.4167)² × 2.667 = 1.455 cu. ft.
- Post displacement per hole: (3.5/12) × (3.5/12) × 2.667 = 0.226 cu. ft.
- Net concrete per hole: 1.455 - 0.226 = 1.229 cu. ft.
- Total net for 6 holes: 1.229 × 6 = 7.374 cu. ft.
- Total with 10% waste: 7.374 × 1.10 = 8.11 cu. ft.
- 80 lb bags required: 8.11 ÷ 0.60 = 13.52 → 14 bags of 80 lb concrete mix
Important Building Code & Safety Disclaimer
Frost line depth and soil bearing requirements vary significantly across regions. Always consult local municipal building codes or a licensed contractor before digging or pouring post footings.
Related Concrete Calculators
- Concrete Bag Calculator — Calculate exact 80lb, 60lb, and 40lb bag counts for any project.
- Concrete Slab Calculator — Calculate cubic yards for patios, shed bases, and driveways.
- Concrete Footing Calculator — Estimate volume for continuous wall footings and foundations.