Post Hole Concrete
Cylindrical volume; ~0.6 ft³ per 80 lb bag rule-of-thumb.
About this calculator
This calculator treats each post hole as a cylinder and computes total concrete volume from hole diameter, depth, and the number of holes. Hole Ø (in) has an outsized effect on Volume compared to the other two inputs -- because cylinder volume depends on the radius squared, a given percentage change in diameter moves total volume by roughly twice that percentage, while the same percentage change in Depth (in) or Holes moves volume by only that percentage, since both enter the formula as simple linear multipliers. In practical terms, going from a 10-inch to a 12-inch hole diameter changes volume far more than adding a couple of extra inches of depth would.
80 lb bags converts the raw cubic-foot volume using the common rule-of-thumb that one 80 lb bag of concrete mix yields about 0.6 cubic feet once mixed, then rounds up to the next whole bag since partial bags still require buying a full one. This is a geometric estimate only -- it doesn't account for hole irregularity, concrete waste during mixing and pouring, or bag-yield variation between brands, so buying a small buffer beyond the calculated bag count is common practice.
Inputs
Results
Volume
16.49 ft³
≈ 5 wheelbarrows
How to Use This Calculator
- Enter the hole diameter in inches — a common post hole is 10–12 inches wide.
- Enter the hole depth in inches — fence posts typically go 1/3 of their above-ground length into the ground.
- Enter the number of post holes.
- Read the total concrete volume in cubic feet and the number of 80 lb bags needed.
How the result changes with Hole Ø (in)
| Hole Ø (in) | Volume |
|---|---|
| 6 | 4.12 ft³ |
| 9 | 9.28 ft³ |
| 18 | 37.11 ft³ |
| 30 | 103.08 ft³ |
What each input means
- Hole Ø (in)
- Diameter of each post hole in inches. Common sizes are 10-12 inches for fence posts, 14-18 for deck posts.
- Depth (in)
- Depth of each hole in inches. A common rule: set posts at least 1/3 of their above-ground height below grade, and below frost line.
- Holes
- Total number of post holes to fill with concrete.
How this is calculated
Worked example, using the default values
- Identify Input ParametersHole Ø (in) = 12, Depth (in) = 42, Holes = 6 = 3 input(s) provided
- Calculate VolumeVolume16.49 = 16.49
Engine last updated . Checked against 1 independently-derived test — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does hole diameter affect the concrete volume more than hole depth does?
Cylinder volume depends on the radius squared, so Hole Ø (in) has a roughly doubled proportional effect on Volume compared to Depth (in) or Holes -- a given percentage increase in diameter moves total volume by about twice that percentage, while depth and hole count each move volume by exactly their own percentage change, since both enter the formula as simple linear factors.
Does adding more post holes increase the concrete volume proportionally?
Yes -- Holes is a direct linear multiplier on total Volume, so adding more holes at the same diameter and depth increases the total concrete needed in direct proportion to the hole count, without the amplified effect that diameter has.
How does the calculator convert cubic feet of concrete into bag count?
It divides total Volume in cubic feet by 0.6, the widely used rule-of-thumb yield for one 80 lb bag of pre-mixed concrete once water is added, then rounds up to the next whole bag since you can't purchase a fraction of a bag for a real pour.
Should I buy extra bags beyond what the calculator recommends?
A small buffer is common practice -- this is a geometric estimate based on a perfect cylinder, and real holes are rarely perfectly uniform, plus some mix is always lost during mixing and pouring, so contractors often round up further beyond the calculator's already-rounded bag count.
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