Post Hole Concrete Calculator
Project inputs
Use the actual hole diameter and concrete depth. If the post is already in the hole, subtracting post displacement gives a closer bag estimate.
Hole size
Post displacement
For a 4x4 nominal wood post, enter the actual width if you want tighter math. Many nominal 4x4 posts measure about 3.5 inches.
Bags, waste, and cost
Results
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Formulas and assumptions
This estimator uses geometric volume for a round cylindrical hole. It is a quantity planner, not a structural design, frost-depth, or code-compliance tool.
- Hole volume:
pi x (diameter / 2)^2 x concrete depth - Round post displacement:
pi x (post diameter / 2)^2 x embedded post length - Square post displacement:
post side x post side x embedded post length - Concrete per hole:
max(hole volume - post displacement, 0) - Total net volume:
concrete per hole x number of holes - Purchase volume:
total net volume x (1 + waste% / 100) - Bags:
ceil(purchase volume / bag yield)
Bag yield is a planning value. Use the yield printed on your concrete bag whenever it differs from the presets.
How to measure post holes
- Measure the widest practical diameter of the hole after digging, especially in loose soil.
- Measure only the depth that will actually be filled with concrete. Gravel drainage below the post is not concrete depth.
- Use post displacement when the post sits inside the wet concrete. Leave it off for empty pier forms or holes filled before a bracket is set.
- Group holes with the same size together. For mixed sizes, calculate each group separately and add the bag counts.
- Round up to whole bags and keep a margin for over-digging, spillage, and uneven hole bottoms.
Planning limits
Post hole concrete quantity is only one part of the job. Post type, gate weight, lateral loads, wind exposure, soil, water table, frost depth, uplift, footing diameter, and local code can change the required depth and diameter.
Follow project drawings, local rules, and concrete manufacturer instructions for placement, water amount, curing, and weather limits.
Post hole concrete FAQs
Should the hole diameter be three times the post width?
That rule of thumb is often used for fences, but it is not universal. Gate posts, deck posts, signs, poor soil, frost, and wind exposure can require a different footing size.
Why does subtracting the post matter?
The post occupies space inside the hole. For small holes, subtracting a 4x4 or round post can noticeably reduce the concrete volume and bag count.
Can I use this for deck footings?
You can use it for quantity planning if the footing is a simple round cylinder. Do not use it to size structural footings or verify code compliance.
What if my bag yield is listed in cubic meters or litres?
Convert to cubic feet before entering a custom yield. One cubic metre is 35.3147 cubic feet, and one litre is 0.0353147 cubic feet.