What the animation shows
The animation and the step-by-step video above use your numbers. Here is the same walkthrough written out for the default example.
What you enter
- Number of holes or columns: 10.
- Hole or tube diameter: 10 in.
- Hole depth / column height: 36 in.
- Post in the hole: Square post.
- Post width or diameter (actual): 3.5 in. A nominal 4×4 is 3.5 in actual; a 6×6 is 5.5 in.
- Extra for waste: 5 %.
- Bag size: 80 lb bag. Yields used: 40 lb ≈ 0.30 ft³, 50 lb ≈ 0.375 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³.
What happens, step by step
- Each hole is 10 in across and 36 in deep, filled to grade.
- The post takes up room, so one hole takes 2.42 bags.
- Only 6 holes are drawn; the count covers all 10.
- All 10 holes together need 25 bags, rounded up.
The answer: bags of concrete is 25 bags.
How it’s calculated
- Hole volume = π ÷ 4 × diameter² × depth
- Post volume = width² × depth (square) or π ÷ 4 × diameter² × depth (round)
- Concrete = (hole − post) × (1 + waste %) × holes
Worked example
10 fence posts (4×4) in 10 in diameter holes, 36 in deep, 80 lb bags, no waste.
Inputs
- Number of holes or columns
- 10
- Hole or tube diameter
- 10 in
- Hole depth / column height
- 36 in
- Post in the hole
- Square post
- Post width or diameter (actual)
- 3.5 in
- Extra for waste
- 0 %
- Bag size
- 80 lb bag
Results
- Bags of concrete
- 24 bags
- Bags per hole (exact)
- 2.3
- Total concrete volume
- 13.81 cu ft
Assumptions and limits
- Concrete fills the full hole depth. If you put gravel in the bottom, reduce the depth accordingly.
- Holes are straight-sided cylinders; belled or irregular holes use more.
- Bag counts use typical manufacturer yields for pre-mixed concrete. Check the yield printed on your bag.
Verify the result against the applicable local code, adopted code edition, manufacturer instructions and project requirements. See our methodology.
Questions
How many bags of concrete per fence post?
A 4×4 post in a 10 in hole, 3 ft deep, takes about 1.38 ft³ — roughly 2.3 bags of 80 lb mix, so buy 3 per hole or calculate the whole job to use partial bags efficiently.
How wide should a post hole be?
A common rule is a hole about three times the post width. Follow the post or fence manufacturer and local frost-depth requirements.