About the Post Hole Concrete
A post hole isn't a pure cylinder once the post itself is sitting in it, so an accurate concrete estimate subtracts the post's own volume from the hole before converting to bags.
How It Works
The calculator finds the hole's total cylindrical volume from its diameter and depth, subtracts the volume the post itself displaces, then converts the remaining concrete volume into standard 50lb bags.
Examples
A standard fence run
10 posts with 10 inch diameter holes at 24 inches deep, minus a 4 inch square post, needs about 22 bags of 50lb quick-set concrete.
Deeper holes for frost protection
The same 10 posts at 36 inches deep instead of 24 needs about 33 bags, since depth increases volume proportionally.
Advantages
- Subtracts the post's own volume for a more accurate concrete estimate
- Shows both total volume and concrete needed per individual post
- Converts directly to standard 50lb bag counts
Common Mistakes
- Not subtracting post volume and over-ordering concrete as a result
- Using a hole depth shallower than local frost line requirements
- Assuming round and square posts displace the same volume
Edge Cases to Watch For
- This assumes a square post; round posts displace less volume than a square post of the same width, so actual concrete need would be slightly higher with a round post.
- A common rule of thumb sets hole depth at one-third to one-half of the post's above-ground height, with a 24 inch minimum for most residential fences, deeper in areas with significant frost heave.
- Quick-setting concrete mixes vary slightly in yield per bag from standard mix; check the specific product's bag label for its stated cubic footage.
Common Use Cases
- Estimating concrete bags for a fence post installation
- Budgeting a deck or mailbox post-setting project
- Comparing material needs across different hole depths for frost protection