Fence post concrete calculator
The post displaces concrete, and on a chunky timber post that is a quarter of the hole.
Concrete needed
0.3823m³
- Concrete per post
- 0.031 86
- 25 kg bags
- 35
- Weight
- 918
- Gravel for the bases
- 0.0636
- Hole depth in metres
- 0.600
Show the working
- Number of posts = 12
- Hole diameter = 300
- Hole depth = 600
- Post width = 100
- Post thickness = 100
- Gravel base depth = 75
- hole_volume = pi * (300/2000)^2 * 600/1000 = 0.042 411 5
- gravel_volume = pi * (300/2000)^2 * 75/1000 = 0.005 301 44
- post_volume = 100/1000 * 100/1000 * (600 - 75)/1000 = 0.005 25
- Concrete per post = max(0.042 411 5 - 0.005 301 44 - 0.005 25, 0) = 0.031 860 1
- Concrete needed = 0.031 860 1 * 12 = 0.382 321
- 25 kg bags = ceil(0.382 321 / 0.011) = 35
- Weight = 0.382 321 * 2400 = 917.57
- Gravel for the bases = 0.005 301 44 * 12 = 0.063 617 3
- Hole depth in metres = 600 > 0 ? 600 / 1000 : 0 = 0.6
Values are shown to six significant figures here; the arithmetic itself runs on exact fractions and is only rounded where a number is printed.
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The formula
- per post = hole volume - gravel base - the buried part of the post
The usual rule is to bury a third of the post length, or 600 mm, whichever is more.
A gravel base lets water drain away from the end grain and roughly doubles the life of a timber post.