Take out the space a square post fills.
⚙️Calculation Parameters
📊Calculated Output
How Square Post Displacement Calculator Works?
To subtract a square post, the Square Post Displacement Calculator removes the buried volume of a 4×4, 6×6 or 8×8 post from the hole. The process takes 4 steps.
Enter the hole
Type the hole or tube diameter and depth.
Pick the post size
Choose a nominal size. The tool uses actual sizes: 4×4 = 3.5 in, 6×6 = 5.5 in and 8×8 = 7.25 in.
Enter embedment
Type how deep the post sits in the concrete.
Read net concrete
The calculator subtracts the post and shows cubic feet (ft³) and bags.
Square Post Displacement Calculator Formula
The Square Post Displacement Calculator subtracts a square prism from a cylinder.
Post volume
V_p = w² × E ÷ 1,728w is actual width and E embedded length, in inches.
Net concrete
V = π × (D ÷ 24)² × (H ÷ 12) − V_pHole minus post.
Clearance
c = (D − w × 1.414) ÷ 2Concrete at the post corners; w × 1.414 is the diagonal.
Worked example: 12 in hole, 36 in deep, 6×6 post
- V_h = 2.356 ft³
- V_p = 5.5² × 36 ÷ 1,728 = 0.630 ft³
- V = 1.726 ft³ → 3 bags of 80 lb
Square Post Displacement Calculator Conversion Table
The square post displacement conversion table lists hole and post pairs at 36 in (91 cm) deep, using actual lumber sizes.
| Hole | Post | Hole ft³ | Post ft³ | Net ft³ | 80 lb bags |
|---|---|---|---|---|---|
| 10 in | 4×4 (3.5 in) | 1.636 | 0.255 | 1.381 | 3 |
| 10 in | 6×6 (5.5 in) | 1.636 | 0.630 | 1.006 | 2 |
| 12 in | 4×4 (3.5 in) | 2.356 | 0.255 | 2.101 | 4 |
| 12 in | 6×6 (5.5 in) | 2.356 | 0.630 | 1.726 | 3 |
| 12 in | 8×8 (7.25 in) | 2.356 | 1.095 | 1.261 | 3 |
| 16 in | 4×4 (3.5 in) | 4.189 | 0.255 | 3.934 | 7 |
| 16 in | 6×6 (5.5 in) | 4.189 | 0.630 | 3.559 | 6 |
| 16 in | 8×8 (7.25 in) | 4.189 | 1.095 | 3.094 | 6 |
Combinations with less than 1.5 in of corner clearance are left out.
What is Square Post Displacement Calculator?
A Square Post Displacement Calculator is a calculator that subtracts an embedded square post, such as a 4×4, 6×6 or 8×8, from the concrete needed for its hole.
Lumber is sold by nominal size but measures smaller. A 6×6 post is 5.5 in × 5.5 in, not 6 in × 6 in. Embedded 36 in deep in a 12 in hole, the post displaces 0.630 ft³, or 27% of the hole. The calculator uses actual sizes, so the displacement is exact.
Square posts set in concrete hold fences, mailboxes, pergolas and pole sheds.
What are the uses of Square Post Displacement Calculator?
There are 4 main uses of the Square Post Displacement Calculator.
Fence posts
Installers subtract 4×4 posts from each hole.
Pergola posts
Builders subtract 6×6 posts set in concrete.
Pole sheds
Crews subtract 6×6 and 8×8 posts.
Bag savings
Owners avoid buying bags for space the post fills.
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