What Is a Concrete Post Hole Volume Calculator?
When you set a fence, deck or mailbox post in concrete, the post itself takes up some of the hole's volume — so the concrete you actually need is the hole's total volume minus the post's volume over the embedded depth. This calculator handles that subtraction automatically and converts the result into standard 60 lb and 80 lb bag counts.
It's a small correction per hole, but it adds up: on a 20-post fence line, ignoring the post's displaced volume can mean ordering several extra bags of concrete you didn't need.
How to Read Your Results
Concrete Needed
The net volume of concrete required for one hole, after subtracting the post's own volume — shown in both cubic feet and cubic yards.
Hole Volume
The full cylindrical volume of the hole before any post is placed in it, calculated from diameter and depth.
Post Volume (Subtracted)
The volume displaced by the post itself over its embedded depth, based on whether you selected a round or square post.
60 lb / 80 lb Bags Needed
A ready-to-buy bag count using the typical yield printed on bagged concrete mix (about 0.45 cubic feet per 60 lb bag and 0.60 cubic feet per 80 lb bag).
Real-World Example
A homeowner is setting a 4×4 round wood post in a 10-inch-diameter hole, 24 inches deep, with the post embedded the full 24 inches.
| Item | Value |
|---|---|
| Hole volume | ≈1.09 ft³ |
| Post volume (4 in round, 24 in embed) | ≈0.17 ft³ |
| Concrete needed | ≈0.91 ft³ |
| 60 lb bags needed | 3 bags |
Without subtracting the post's volume, this job would look like it needs 4 bags rather than the 3 it actually takes — a meaningful difference across a whole fence line.
Tips for Setting Posts in Concrete
- Slope the top of the concrete away from the post so water doesn't pool and rot or rust the base.
- Add a few inches of gravel at the bottom of the hole for drainage — this reduces the effective concrete depth, so adjust your hole depth input accordingly.
- Center the post carefully before the concrete sets; use temporary bracing on tall posts.
- Check local frost depth requirements for hole depth in cold climates to prevent frost heave.
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Last updated: August 2026 · Reviewed by: Simple Calculator Tools Editorial Team