Post Hole Concrete Calculator
Concrete volume for fence post holes
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Post Hole Concrete Calculator turns the numbers behind concrete volume for fence post holes into a result, step by step. Just enter Number of Holes, Hole Diameter (inches) and Hole Depth (inches); Post Hole Concrete updates the result as you type. Nothing in Post Hole Concrete is a black box. Post Hole Concrete suits budgeting and planning checks. No account is needed for Post Hole Concrete. The browser does the arithmetic. Post Hole Concrete handles post hole concrete calculator fence posts bags cubic yards volume the same way as related figures — one of the Construction tools on CalcProMaster. Keep Post Hole Concrete bookmarked for next time. Post Hole Concrete stays bookmarked for next time.
What the defaults give
Eight holes of 12 inches diameter and 36 inches deep, with a 4-inch post, need 0.60 cubic yards of concrete.
That is about 16.2 cubic feet, or roughly 36 bags of a 0.45 cu ft mix.
The arithmetic
Each hole is a cylinder, and the post displaces some of it:
Hole volume = π × (diameter ÷ 24)² × (depth ÷ 12)
Post volume = (post width ÷ 12)² × (depth ÷ 12)
Net = (hole − post) × number of holes ÷ 27
Per hole: π × 0.5² × 3 = 2.356 ft³ of hole, minus 0.333 ft³ of post = 2.023 ft³. Times eight is 16.19 ft³, and 16.19 ÷ 27 = 0.60 yd³.
Worked examples
8 holes, 12" × 36", 4" post → 0.60 yd³ (≈36 bags)
4 holes, 10" × 30", 4" post → 0.21 yd³
12 holes, 12" × 42", 6" post → 1.01 yd³
8 holes, 8" × 24", 4" post → 0.18 yd³
Why the post volume is subtracted
The hole does not need to be filled solid — the post occupies the centre, and the concrete only has to surround it. Ignoring that overstates the pour by roughly 15% on a 4-inch post in a 12-inch hole, which is more than a bag on this job. The share grows with the post size, so a 6-inch post saves considerably more.
Practical fitting
- Bell the bottom. A wider base at the foot of the hole resists frost heave and uplift; the calculation assumes a straight cylinder.
- Add 10%. Ground is never a clean cylinder and spillage is real, so order slightly above the figure.
- Depth matters more than width. Frost depth and post height set the depth; widening the hole adds concrete far faster than deepening it.
- Drainage. A little gravel at the base stops the post sitting in water, which the volume calculation does not account for.
Bag yield varies by product — the 0.45 cu ft figure is a common 50 lb fast-setting mix, so check the bag before ordering.
Frequently Asked Questions
How much concrete for 8 post holes?
About 0.60 cubic yards for 12-inch holes at 36 inches deep with a 4-inch post — roughly 36 bags of a 0.45 cu ft mix.
Why subtract the post volume?
Because the concrete fills only the space around the post, not the whole cylinder. Ignoring it overstates the pour by about 15% on these proportions.
How many bags is a cubic yard?
About 60 bags of a 0.45 cu ft mix, so 0.60 cu yd is roughly 36 bags.
Does a wider hole use much more concrete?
Yes, with the square of the diameter. Going from 10 to 12 inches adds about 44% more concrete for the same depth.
How deep should a post hole be?
Below the local frost line and about a third of the post height above ground, whichever is greater. Check local code before digging.
Should the hole be belled at the base?
In frost-prone areas a wider base helps resist heave, but it takes more concrete than this straight-cylinder estimate.
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