Hole Calculator

How the Hole Calculator Works

The hole calculator computes the volume of excavated material (dirt removed) and optionally the concrete or fill material needed. For a cylindrical hole: Volume = π × radius² × depth. For a rectangular hole: Volume = length × width × depth. Results in cubic feet, cubic inches, or cubic yards (divide cubic feet by 27).

Example: A fence post hole 10 inches diameter × 36 inches deep: Radius = 5 inches = 0.417 ft. Volume = π × (0.417)² × 3 ft = 3.14159 × 0.174 × 3 = 1.64 cubic feet of dirt removed. To fill with concrete: same volume (1.64 ft³) = about 2.5 bags of 60-lb concrete mix or 1.5 bags of 80-lb mix.

Post Hole Calculator: Fence and Deck Footings

Standard post hole dimensions by application:

• Fence posts (6 ft tall fence): 8–10 inch diameter, 24–36 inches deep. Rule: bury 1/3 of total post length + 6 inches for frost. In frost-prone areas, go below the frost line.
• Fence posts (8 ft tall fence): 10–12 inch diameter, 36–42 inches deep.
• Deck posts (up to 6 ft above grade): 12-inch diameter concrete piers, minimum 12 inches below frost line.
• Mailbox posts: 6-inch diameter, 24 inches deep (12 inches below frost in northern climates).
• Pergola/arbor posts: 12-inch diameter, 36–42 inches deep depending on structure height and wind exposure.

Frost line depths by region (must dig below to prevent frost heave): Southern states: 0–12 inches. Mid-Atlantic/Midwest: 18–36 inches. Northern states/Canada: 36–60 inches. Always check local building codes — frost depth requirements are specified per jurisdiction.

How Much Concrete Per Post Hole?

Concrete bag coverage for standard post holes:

• 50-lb bag: Covers 0.375 cubic feet (approximately 0.56 bags per cubic foot)
• 60-lb bag: Covers 0.45 cubic feet
• 80-lb bag: Covers 0.6 cubic feet

For a 10-inch diameter × 36-inch deep hole (1.64 ft³): Need about 2.7 bags of 80-lb concrete mix. For a typical fence with 20 posts at those dimensions: 54 bags of 80-lb mix. Always add 10–15% overage for waste and uneven hole walls.

Fast-setting concrete (like Quikrete Fast-Setting) can be poured dry into the hole with water added afterward — no mixing required, sets in 20–40 minutes vs. 24–48 hours for regular mix. Worth the price premium for fence posts.

Dirt Removal Calculator: Estimating Excavated Material

Excavated soil expands when removed (called "swell" or "bulking factor"). Dry soil swells approximately 15–25%; wet clay can swell 30%+. This matters for disposal planning:

• 10 cubic feet of in-place soil ~ 12–13 cubic feet of loose excavated material
• A standard contractor dump trailer holds 5–7 cubic yards (135–189 cubic feet)
• A pickup truck bed holds approximately 0.5 cubic yards loose (don't overload — 1 cubic yard of wet dirt weighs ~2,000 lbs)

For small post-hole projects, excavated dirt can fill raised beds, low spots in the yard, or be mixed with compost for fill. For larger excavations, contact a local waste hauler — many will take clean fill dirt free or at minimal cost.

Frequently Asked Questions

How deep should I dig a post hole?

The standard rule: post holes should be 1/3 to 1/2 the above-ground height of the post, plus 6 inches for a gravel drainage layer at the bottom. For a 6-foot fence (with 8-foot posts): bury at least 30–36 inches. In areas with frost, you must also dig below the local frost depth to prevent post heaving and leaning. Your local building department or frost depth map (NOAA publishes one for the US) will specify the exact depth required.

Should I use concrete or gravel for fence post holes?

Both are valid, with tradeoffs. Concrete: provides maximum stability, especially in soft or sandy soil; difficult to remove if you need to replace the post later. Gravel (crushed stone, not pea gravel): allows water drainage away from the post base (reducing rot), easier to remove posts later, slightly less rigid. For wooden posts, many professional fence installers prefer gravel for drainage. For metal posts, concrete is standard. Composite and vinyl posts typically specify concrete per manufacturer guidelines.