Fence Post Concrete Calculator
Fence post concrete volume and bag counts.
What it does
Fence posts stand in concrete cylinders: bore area times depth per hole, times holes, divided by bag yield. This page runs that chain in cubic feet with 60 lb bags or cubic meters with 25 kg bags.
The default is 8 holes at 10 in wide and 36 in deep. Each hole holds 1.64 ft3, the run needs 13.09 ft3, and 60 lb bags at 0.45 ft3 each mean 30 bags. Metric defaults convert the same dig to 25 by 91 cm and give 0.357 m3 with 29 bags.
Dig shape decides truth. Straight auger holes match the cylinder, while hand dug bells, roots, and soft shoulders all widen the pour past math. Depth should pass the local frost line so posts skip heave.
Panels differ from posts. This page sizes footings only; pickets, rails, and gates belong to the fence count page next.
How to use
- Pick imperial or metric first.
- Enter post hole count.
- Enter hole diameter and depth.
- Read concrete first, then bags.
- Dig below frost line on site.
Formula
- posts
- Post hole count.
- d
- Hole diameter across.
- h
- Hole depth down to footing.
- yield
- Concrete per bag (0.45 ft3 / 0.0127 m3).
Holes are assumed straight cylinders; belled footings, roots, and cave ins raise volume. Yields used are 0.45 ft3 per 60 lb bag and 0.0127 m3 per 25 kg bag. Depth should clear the frost line per local code.
Worked example
Example: 8 fence posts
8 holes, 10 in wide, 36 in deep, 60 lb bags (metric 25 by 91 cm).
- Per hole: 3.1416 x 25 x 36 / 1728 = 1.64 ft3.
- Total: 1.64 x 8 = 13.09 ft3.
- Bags: 13.09 / 0.45 = 29.09, up to 30 bags (60 lb).
- Metric: 25 by 91 cm gives 0.357 m3, 29 bags (25 kg).
- Tool shows 13.09 ft3 and 30 bags imperial.
Reference table
| Posts | Concrete | Bags |
|---|---|---|
| 2 | 3.27 ft3 | 8 |
| 4 | 6.54 ft3 | 15 |
| 6 | 9.82 ft3 | 22 |
| 8 | 13.09 ft3 | 30 |
| 10 | 16.36 ft3 | 37 |
Limits
- Straight cylinders assumed. Bells add volume.
- Fixed yields. Brands vary a little.
- Posts and gravel excluded.
- Frost depth per local code, not this page.
Common mistakes
- Sizing one hole and forgetting count.
- Mixing inches with feet mid chain.
- Rounding bags down.
- Digging shallow of frost line.
- Reading panel math as footing math.
Try it yourself
Quick checks. Answers sit tucked under each question.
4 holes at 10 by 36 in?
6.54 ft3 (4 x 1.64), 15 bags (6.54 / 0.45 = 14.5, up to 15).
2 holes at 10 by 36 in?
3.27 ft3 for 2 holes, 8 bags.
Questions
How much concrete?
Hole cylinder times count. Here 13.09 ft3 for 8 holes.
How many 60 lb bags?
Total over 0.45 ft3, rounded up. Here 30 bags.
What is that in metric?
0.357 m3 and 29 bags (25 kg).
Bag yields?
50 lb 0.375, 60 lb 0.45, 80 lb 0.6 ft3. This page uses 60 lb.
Cylinder holes assumed?
Yes for planning, but bell shapes and roots change dig size.
Posts included?
No. Posts, gravel, and water sit outside concrete math.