Fence Post Hole & Sonotube Pier Concrete Bag Calculator (Minus Post Displacement)

Calculate 50-lb, 60-lb, and 80-lb concrete bags for post holes net of post displacement.

80-lb Concrete Bags Required
59 bags (80 lb)
60-lb Concrete Bags Alternative 79 bags (60 lb)
50-lb Fast-Setting Bags Alternative 95 bags (50 lb)
Total Net Concrete Volume (With 5% Waste) 35.3 cu ft
Inputs
⚡ Scenarios:
holes
Total number of post holes or Sonotube piers.
inches
Augered hole or Sonotube diameter in inches.
inches
Depth of concrete pour in each hole (inches).
inches
Actual width of embedded square post (3.5" for 4×4, 5.5" for 6×6, 0 for Sonotube pier).
Results & Graph
80-lb Concrete Bags Required
59 bags (80 lb)
60-lb Concrete Bags Alternative
79 bags (60 lb)
50-lb Fast-Setting Bags Alternative
95 bags (50 lb)
Total Net Concrete Volume (With 5% Waste)
35.3 cu ft

80-lb Concrete Bags Required

Graph 1 • Post Hole & Sonotube Concrete • Domain Architecture & Live Telemetry Maps Number Of Post Holes, Hole Diameter Inches, Concrete Pour Depth Inches through Post Hole & Sonotube Concrete Formula.
INPUT PARAMETERS • Number Of Post Hol: 16 holes • Hole Diameter Inch: 12 inches • Concrete Pour Dept: 36 inches ∑ F(x) Post Hole & Sonotube Concr DESIGN OUTPUT 59 bags (80 lb) 60-lb Concrete Bags Alte: 79 bags (60 lb) 50-lb Fast-Setting Bags : 95 bags (50 lb)
Graph 2 • 80-lb Concrete Bags Required Across Number Of Post Holes Sweep (40% → 160%) Evaluates HoleVol_ft³ = π·(D_hole/24)²·Depth_ft; PostVol_ft³ = PostArea_ft²·Dept across 7 operating points (current input highlighted).
0 bags ( 52.80 bags ( 105.6 bags ( 23 6.40holes 37 9.60holes 48 12.80holes 59 16holes 70 19.20holes 81 22.40holes 96 25.60holes
Graph 3 • Joint Sensitivity: Number Of Post Holes × Hole Diameter Inches Compares 80-lb Concrete Bags Required when Hole Diameter Inches is shifted +25% (solid) vs. −25% (dashed).
0 bags ( 69.30 bags ( 138.6 bags ( --- Hole Diameter Inches @ 75% 66 -30% 84 -15% 96 +0% 108 +15% 126 +30%
Summary: Across 16 holes (12" dia × 36" deep, minus 3.5" post displacement), net concrete volume is 35.3 cu ft, requiring 59 bags of 80-lb (79 of 60-lb or 95 of 50-lb) concrete.
Summary: Across 16 holes (12" dia × 36" deep, minus 3.5" post displacement), net concrete volume is 35.3 cu ft, requiring 59 bags of 80-lb (79 of 60-lb or 95 of 50-lb) concrete.
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Interactive chart & math engine powered by CalculatorGraph

Formula & Mathematical Methodology: Post Hole & Sonotube Concrete Formula

HoleVolft³ = π·(Dhole/24)²·Depthft
PostVolft³ = PostAreaft²·Depthft
NetConcrete = (HoleVol − PostVol) × Posts
HoleVol_ft³ = π·(D_hole/24)²·Depth_ft; PostVol_ft³ = PostArea_ft²·Depth_ft; NetConcrete = (HoleVol − PostVol) × Posts
  • numberOfPostHoles: Number Of Post Holes (holes)
  • holeDiameterInches: Hole Diameter Inches (inches)
  • concretePourDepthInches: Concrete Pour Depth Inches (inches)
  • embeddedPostWidthInches: Embedded Post Width Inches (inches)

Rounding: Engineering outputs round to 2–3 decimal places; discrete material bag/unit counts round up via ceiling.

Worked Example: Engineering Scenario: Post Hole & Sonotube Concrete

Calculate 50-lb, 60-lb, and 80-lb concrete bags for post holes net of post displacement.

  1. Enter your system dimensions, electrical/RF ratings, or environmental conditions.
  2. Evaluate the governing equation: HoleVol_ft³ = π·(D_hole/24)²·Depth_ft; PostVol_ft³ = PostArea_ft²·Depth_ft; NetConcrete = (HoleVol − PostVol) × Posts.
  3. Inspect the primary design output, supporting allowances, and 3 SVG schematics.
Result: Computes Post Hole & Sonotube Concrete via HoleVol_ft³ = π·(D_hole/24)²·Depth_ft; PostVol_ft³ = PostArea_ft²·Depth_ft; NetConcrete = (HoleVol − PostVol) × Posts.

Frequently Asked Questions

How does the Fence Post Hole & Sonotube Pier Concrete Bag Calculator (Minus Post Displacement) calculate results?

It evaluates HoleVol_ft³ = π·(D_hole/24)²·Depth_ft; PostVol_ft³ = PostArea_ft²·Depth_ft; NetConcrete = (HoleVol − PostVol) × Posts directly in your browser and updates three custom SVG visualizations in real time.

Can I use Goal Seek to solve backwards for a required dimension or rating?

Yes. Click "🎯 Goal Seek" in the results toolbar to solve backwards for any numeric input parameter.

Does this engineering calculator work offline?

Yes. All calculations run 100% locally in your browser and work offline via the CalculatorGraph PWA.

Construction & Home · Concrete, Masonry & Sitework

Fence Post Hole & Sonotube Pier Concrete Bag Calculator (Minus Post Displacement): inputs, method, and how to read the result

Calculate 50-lb, 60-lb, and 80-lb concrete bags for post holes net of post displacement.

Choose inputs for the case you are checking

The defaults are illustrative. Replace them with values for your case and keep the units shown beside each field.

  • Number Of Post Holes: Total number of post holes or Sonotube piers.
  • Hole Diameter Inches: Augered hole or Sonotube diameter in inches.
  • Concrete Pour Depth Inches: Depth of concrete pour in each hole (inches).
  • Embedded Post Width Inches: Actual width of embedded square post (3.5" for 4×4, 5.5" for 6×6, 0 for Sonotube pier).

What the calculation includes

With the default values shown, 80-lb Concrete Bags Required is 59 bags (80 lb). The calculation follows Post Hole & Sonotube Concrete Formula; the formula and variable definitions above show how the inputs combine.

One-input check: With the other defaults held constant, changing Number Of Post Holes from 16 holes to 18 holes changes 80-lb Concrete Bags Required from 59 bags (80 lb) to 67 bags (80 lb). This is a comparison inside the calculator's stated model, not a prediction.

Where to use caution

  • Uses standard NREL, IEEE, ASHRAE, and IRC design equations.
  • Verify final structural, electrical, and mechanical specifications against local building codes.

Scope: Global Engineering & Building Code Standard Check the assumptions and limitations against your situation before relying on the estimate.

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