Post Hole Concrete Calculator
Concrete per hole and for the whole fence run, with the post itself and the gravel drainage base subtracted — because the hole volume alone always overshoots.
Quick answer
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Hole and post
3x the post width
Below your frost line
Actual, not nominal: a 4x4 is 3.5 in
4–6 in for drainage
concrete = pi x (hole ⌀/2)² x (depth − gravel) − post area x (depth − gravel)
Per hole
Concrete per hole
1.67cu ft
Hole holds 1.7 cu ft; post displaces 0.18 cu ft
80 lb bags per hole
3
Whole run (12 posts)
Total concrete
0.74yd³
20 cu ft with 10% waste
80 lb bags total
34
$255
Gravel for bases
0.16tons
3.1 cu ft of 3/4 in stone
Bags by size
| Bag | Per hole | Whole run |
|---|---|---|
| 40 lb | 6 | 67 |
| 50 lb | 5 | 54 |
| 60 lb | 4 | 45 |
| 80 lb | 3 | 34 |
Why the post volume matters
Most calculators quote the full hole volume. A 4x4 post in a 12 inch hole occupies about 12 percent of it, and a 6x6 in a 16 inch hole nearly 18 percent — over a 30-post fence that is several bags you did not need to carry. The gravel base takes another slice off the top, since concrete only fills the depth above it.
Keep concrete slightly above grade and crown it away from the post so water runs off rather than sitting at the joint. That single detail does more for post life than extra depth.
Source & method: Cylinder and prism geometry computed in your browser; bag counts use published nominal yields (0.60 ft³ per 80 lb bag). Gravel weight uses 105 lb/ft³ for dry 3/4 inch stone.
Post setting supplies
Links below are affiliate links. If you buy through them we may earn a commission at no extra cost to you.
- Plate compactorCompact base gravel in lifts or the slab will crack where it settles.Base prep · Check price
- Hand tamperFor trench bottoms and tight corners a plate cannot reach.Check price
- Landscape fabricSeparates base stone from subgrade so it stops pumping mud.Check price
- Laser level or line levelSet grade before the material lands, not after.Check price
Frequently asked questions
- How much concrete do I need for a fence post?
- For a 4x4 post in a 12 inch hole 30 inches deep, about 2.1 cubic feet per hole after subtracting the post — 4 bags of 80 lb mix. A 6x6 in a 16 inch hole 36 inches deep takes about 4.8 cubic feet, or 8 bags of 80 lb mix.
- How deep should a post hole be?
- The common rule is one third of the post's above-ground height, minimum 24 inches, and always below the local frost line. In cold regions that can mean 42 to 48 inches — check with your building department, because a post set above frost depth will heave.
- How wide should a post hole be?
- Three times the post width: a 12 inch hole for a 4x4, 18 inches for a 6x6. That leaves enough concrete around the post to resist the leverage of wind load on the fence.
- Should I put gravel in the bottom of a post hole?
- Yes, 4 to 6 inches of compacted gravel under the post lets water drain away from the end grain instead of pooling against it. Subtract that depth from the concrete — this calculator does it for you.
- Can I pour dry concrete in a post hole?
- Fast-setting mixes are formulated for it: set the post, pour the dry mix in, then add water on top per the bag. It works well for fence and mailbox posts, but for structural posts carrying a deck or roof, mix properly and follow the design.
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