Post Hole Concrete Calculator
Works out the concrete to set fence, deck or gate posts. Gives the volume per hole — the number you actually work to when mixing on site — as well as the total across the run.
Includes the waste allowance — this is the figure to order.
Share sends a link that reopens these numbers. Copy details pastes the full breakdown as a list.
The formula
per hole = (π × r² × depth) − (post width² × depth)
The post sits in the hole and takes up space, so its buried volume is subtracted. On a 100 mm post in a 300 mm hole that is about 14% of the concrete — small per hole, but it adds up across a fence line.
What to watch out for
Post setting is the most common small concrete job, and the rules of thumb are worth knowing:
- Hole diameter: three times the post width. A 100 mm post wants a 300 mm hole. Wider does not help; narrower does not hold.
- Depth: a third of the post above ground, and always below the frost line. A 1.8 m fence post wants roughly 600 mm in the ground.
- Gate posts are the exception. A hanging gate applies leverage, so its posts want a deeper, wider hole than the rest of the fence — often half as much again.
- Crown the top. Finish the concrete slightly above ground and sloped away, so water runs off rather than sitting against the timber.
Frequently Asked Questions
How much concrete per fence post?
A 300 mm hole 600 mm deep, less a 100 mm post, needs about 36 litres — roughly three 25 kg bags per post.
How deep should a fence post be?
About a third of the height above ground: 600 mm for a 1.8 m fence. Go deeper for gate posts, corner posts, and anywhere exposed to wind.
How wide should a post hole be?
Three times the post width. A 100 mm post wants a 300 mm hole — enough concrete around it to spread the load without wasting material.
Do I need to deduct the post from the volume?
Yes for anything more than a few posts. A 100 mm post in a 300 mm hole displaces about 14% of the concrete — across 20 posts that is most of a bag-and-a-half per post saved.