Post Hole Concrete Calculator
Concrete volume and bags for round post holes and footings.
What it does
Post holes are cylinders, so volume is pi times radius squared times depth. Eight fence holes near a cubic yard total, which is why bag counts surprise first time builders.
The tool above takes diameter, depth, and hole count in imperial or metric. It returns per hole and total volume with bag counts in pounds or liters.
Belling beats depth alone against frost. A hole wider at the bottom than the top locks the footing like a nail head, while straight tubes can ride frost upward year by year.
Posts displace almost nothing in the math. A 4x4 post in a 12 inch hole fills under a tenth of the volume, so ordering to the full cylinder with a small waste share lands right.
How to use
- Switch the unit system first: imperial for inches, metric for cm.
- Enter hole diameter and depth to the bell bottom.
- Enter the hole count.
- Buy bags plus one spare for spillage.
Formula
- pi x r²
- Circle area of the hole cross section.
- depth
- Hole depth to the bell bottom.
- 0.6 ft3
- Mixed volume of one 80 lb bag.
Bell bottoms add volume the cylinder math misses. Add roughly ten percent when holes flare generously.
Worked example
Example: eight holes, 12 in wide, 36 in deep
Fence posts, 12 inch diameter, 36 inch depth, 8 holes.
- Radius: 12 / 2 = 6 in, which is 0.5 ft.
- Per hole: 3.1416 x 0.25 x 3 = 2.36 ft3.
- Total: 2.36 x 8 = 18.85 ft3.
- Bags: 18.85 / 0.6 = 31.4, so 32 bags of 80 lb.
Reference table
| Diameter | Per hole | Bags |
|---|---|---|
| 8 in | 1.05 ft3 | 2 bags |
| 10 in | 1.64 ft3 | 3 bags |
| 12 in | 2.36 ft3 | 4 bags |
| 16 in | 4.19 ft3 | 7 bags |
Limits
- Straight cylinders assumed. Bells add volume.
- Posts ignored, tiny error.
- Rocky holes vary hole to hole.
- Water in holes displaces mix upward.
Common mistakes
- Buying one bag per hole by habit.
- Digging straight tubes in frost country.
- Setting posts shallow to save concrete.
- Pouring dry mix and hoping ground water finishes it.
- Skipping crown above grade for drainage.
Try it yourself
Quick checks. Answers sit tucked under each question.
One 10 in hole, 36 in deep. Bags of 80 lb?
3.1416 x (10/24)² x 3 = 1.64 ft3. Then 1.64 / 0.6 = 2.73, so 3 bags.
Metric: 30 cm wide, 90 cm deep, 8 holes. Volume?
Per hole 3.1416 x 0.0225 x 0.9 = 0.064 m3. Total 0.51 m3, about 13 bags of 40 L.
Questions
How many bags per fence post?
A 12 by 36 in hole needs 4 bags of 80 lb. Smaller holes need 2 to 3.
Should holes bell at the bottom?
Yes in frost zones. Wider bottoms resist heave far better.
How deep should posts go?
About a third of exposed height, below the frost line in cold zones.
Can I pour dry mix in the hole?
It works in damp ground but cures weaker and unevenly. Mixing first is better.
Do posts displace much concrete?
No. A 4x4 in a 12 in hole fills under a tenth. Ignore it.
How long before building on posts?
A day or two for setting loads, a week for full cure strength.