Post Hole Concrete Calculator
Calculate how much concrete you need for fence posts, deck posts, or mailbox posts based on hole dimensions and number of posts.
Fence post holes must be sized correctly to provide structural stability. The depth and diameter of each hole depends on post size, soil type, frost depth, and post height. Underdimensioned holes lead to leaning, frost heave, and premature fence failure.
Depth rule of thumb: Post Hole Depth = ⅓ of Total Post Length (for in-ground posts) Or for fencing specifically: Hole Depth = Fence Height × 0.40 to 0.50 (40–50% of above-ground height)
Frost depth rule (critical in cold climates): Minimum Depth = Local Frost Line Depth + 6 inches Frost line depths: Northern U.S./Canada: 36–60"; Central U.S.: 18–36"; Southern U.S.: 0–18".
Post hole diameter formula: Hole Diameter = 3 × Post Width Example: 4×4 post (3.5" actual) → 3 × 3.5 = 10.5" hole → use 10" or 12" diameter
Concrete volume per hole: Volume = π × (Hole Radius)² × Depth Subtract post volume: Post Volume = Width × Width × Depth (square posts)
Worked example: 6-foot fence with 4×4 posts on 8-foot centers, 10 posts total. Region: Chicago, IL (frost line 42"). Post length = 6 ft above + (42 + 6) in = 6 ft + 4 ft = 10 ft total post Hole depth = 48 inches (4 ft) Hole diameter: 10 inches Volume per hole = π × (5/12)² × 4 = π × 0.1736 × 4 = 2.18 ft³ Less post = 0.292 × 0.292 × 4 = 0.34 ft³ Concrete per hole = 2.18 − 0.34 = 1.84 ft³ ≈ 2.5 bags of 80 lb Quikrete 10 posts total = 25 bags
Soil type adjustments:
- Sandy/loose soil: add 2–4" to depth, add 2" to diameter
- Clay soil: standard dimensions, but add gravel base for drainage
- Rock: may reduce depth requirement but requires mechanical drilling