Post Hole Concrete Calculator
Calculate exact premix concrete bags and volume per hole, accounting for wooden post displacement and frost depth.
Post Hole Concrete Calculator Interactive Tool
Enter your project dimensions below. Results update instantly.
Hole dimensions depend on your regional frost depth and fence height. To prevent frost heaving, bury posts below the frost line (commonly 36"–48" in northern climates, 24" in southern zones) and adhere to a 1/3-above-ground depth ratio.
Results
0.38 cu yd
10.13 cu ft (includes +10% extra)0.9207 cu ft
1,591 cu inches net of post17
At 0.6 cu ft/bag (rounded up)23
Or 34 bags of 40-lb mixNet Post Hole Concrete Volume Formula
Concrete Volume = π × (Diameter ÷ 2)² × Hole Depth - (Post Width × Post Thickness × Hole Depth)Formula Variables:
- Hole Diameter:Recommended at 3× nominal post width (e.g., 10" or 12" hole for a 4×4 post).
- Post Displacement:Actual post cross-section (3.5" × 3.5" for 4×4 lumber) subtracted from cylinder volume.
- Hole Depth:Must extend at least 6" below regional frost line and reach 1/3 to 1/2 of above-ground fence height.
Hole dimensions and embedment depth depend on your local frost depth and fence height. Failing to extend below the frost line causes winter frost heave.
Step-by-Step Worked Example
Example: 10 fence posts (4×4 timber) set in 10-inch holes at 24-inch depth with 10% extra
Setting 10 standard 4×4 wood posts (actual 3.5 × 3.5 in) in 10-inch diameter holes dug 24 inches deep with a 10% over-excavation allowance.
- Cylinder hole volume: π × (5 in)² × 24 in = 600π = 1,884.96 cu in.
- Post displacement volume in hole: 3.5 in × 3.5 in × 24 in = 294.00 cu in.
- Net concrete per hole: 1,884.96 - 294.00 = 1,590.96 cu in = 0.9207 cu ft.
- Total concrete for 10 posts with 10% extra: 10 × 0.9207 cu ft × 1.10 = 10.13 cu ft (0.38 cu yd).
- 80-lb bags required (yield 0.60 cu ft/bag): 10.13 ÷ 0.60 = 16.88 → 17 bags (rounded up).
Related Calculators
Related estimation tools for fencing and complementary construction planning:
Fence Post Calculator
Determine line, corner, end, and gate posts with proper shared-corner handling.
Fence Calculator
Calculate posts, horizontal rails, vertical pickets, and concrete bags for wood fencing.
Concrete Footing Calculator
Calculate volume and bag counts for round sonotubes and rectangular post footings.
Related Material & Construction Guides
In-depth guidance on measurement standards, formulas, and material depths:
Input Explanations & Measurement Guidelines
Post Width & Depth
Typical: typical value, change to match your product: 3.5 × 3.5 inchesActual cross-sectional dimensions of post (nominal 4x4 lumber is 3.5 x 3.5 in).
Hole Diameter
Typical: typical value, change to match your product: 10 inchesDiameter of augered hole (standard rule of thumb is 3× post width).
Hole Depth
Typical: typical value, change to match your product: 24 inchesDepth of excavation beneath grade, extending under frost line.
Over-Excavation Extra %
Typical: typical value, change to match your product: 10%Buffer for rough irregular hole walls and loose dirt.
80-lb Bag Yield
Typical: typical value, change to match your product: 0.60 cu ftVolume of mixed concrete produced per 80-lb bag.
Assumptions & Limitations
- Assumes straight cylindrical auger hole without severe cave-ins.
- Hole depth reaches beneath local frost penetration to resist seasonal ground heaving.
- Post sits on a 4-inch compacted gravel base for bottom drainage.
Safety & Professional Notes
- Call 811 before digging to locate underground gas, electric, and communications utilities.
- Wet concrete is caustic. Wear impermeable gloves, safety glasses, and sturdy boots when mixing and pouring.
Frequently Asked Questions
How many 80 lb bags of concrete do I need for a 4x4 post?
For a 10-inch diameter hole at 24 inches deep with a 4x4 post, you need 0.92 cubic feet of concrete per hole. With standard 10% waste, plan on ~1.7 bags of 80-lb concrete per post (buy 2 bags per hole).
Why does frost depth affect post hole depth?
When moisture in the soil freezes, it expands upwards. If post footings sit above the frost line, freezing ground grips the concrete and jacks the post out of the ground.
Do you need gravel at the bottom of a post hole?
Yes. Placing 4 to 6 inches of crushed gravel at the bottom of the hole allows rainwater to drain away from the post end grain, preventing wood decay.