How the bags are worked out
The calculator treats the hole as a cylinder and takes away the part of the post that stands in it, all the way to the bottom. What’s left is the concrete; the maker’s yield turns it into bags.
Worked example: four 90 × 90 mm posts, 300 mm holes 600 mm deep
- Hole: π × 0.15² × 0.6 = 0.0424 m³.
- Post: 0.09 × 0.09 × 0.6 = 0.0049 m³.
- Concrete: 0.0424 − 0.0049 = 0.0376 m³ a hole.
- Bags at 100 a cubic metre: 3.76 a hole, so 4 if you mix each hole on its own.
- Four posts: 0.150 m³, 15.02 bags, so 16 bags of 20 kg.
Boral’s post hole chart
Boral lists bags of its Post Hole Concrete Mix for three common posts. Worked out at 100 bags a cubic metre, the concrete in each hole lands at the top of Boral’s range, which is why the calculator adds no extra to start with.
| Post | Hole | Boral’s bags | This calculator |
|---|---|---|---|
| 75 mm | 250 mm wide, 600 mm deep | 2 to 3 | 2.61, so 3 |
| 90 mm | 300 mm wide, 600 mm deep | 3 to 4 | 3.76, so 4 |
| 100 mm | 300 mm wide, 700 mm deep | 4 to 5 | 4.25, so 5 |
Square posts, holes filled to the top. Boral’s depths put around a third of the post in the ground.
How big a hole
The makers’ guides agree on the width and differ on the depth:
- Width: Easy Mix makes the hole twice the post’s width or diameter. Boral’s chart uses holes of 250 to 300 mm for 75 to 100 mm posts.
- Depth: Bunnings sets a third of the post’s height above the ground into the ground: a 600 mm hole for a post standing 1.8 m. Boral and Easy Mix bury a third of the post’s whole length, which is half the height above the ground: 900 mm for the same post, which then needs to be 2.7 m long.
Enter the height above the ground and the calculator shows both depths, and warns if the hole is shallower than Bunnings’ figure or narrower than twice the post.
Rapid set or standard mix
Rapid-set mixes are made for posts. Cement Australia’s Rapid Set hardens in 15 minutes and lets the supports come off after an hour; it is meant for fence and pergola posts, clotheslines and letterboxes, at about two 20 kg bags a hole. Its PROSTRENGTH Rapid Set hardens in 30 minutes, reaches 40 MPa and makes about 110 bags a cubic metre. Easy Mix’s Rapid Set also puts about two bags in a hole.
Standard concrete mixes make 100 to 108 bags a cubic metre (Australian Builders, Easy Mix, Boral) and take longer to set. Choose the figure on your bag in the calculator.
Footings that carry a load
Cement Australia says its Rapid Set is not designed for any structural application. Posts that hold up a deck or a roof sit on footings sized on the plans or by an engineer, not by a bag count.
Questions people ask
How many bags of concrete for a fence post?
It depends on the hole. A 90 × 90 mm post in a 300 mm hole 600 mm deep takes 0.038 m³: 3.76 bags at 100 a cubic metre, so 4, and Boral’s chart says 3 to 4. Easy Mix’s rapid-set guide digs a narrower hole, twice the post’s width, and Cement Australia and Easy Mix put about two bags of rapid set in a post hole.
How deep should a post hole be?
The guides differ. Bunnings digs a third of the post’s height above the ground: 600 mm for a post standing 1.8 m. Boral and Easy Mix put a third of the post’s whole length in the ground, which for the same post is 900 mm. Deeper holes take more concrete, and the calculator shows both depths for your post.
How wide should the hole be?
Easy Mix makes it twice the post’s width: 180 mm for a 90 mm post, 200 mm for a 100 mm one. Boral’s chart uses wider holes, 250 mm for a 75 mm post and 300 mm for 90 and 100 mm posts, which take more concrete each.
How long does rapid set concrete take to harden?
Cement Australia’s Rapid Set hardens in 15 minutes when used as directed, and the post’s supports can come off after about an hour, depending on the temperature. Its PROSTRENGTH Rapid Set hardens in 30 minutes.
Can I use rapid set concrete for a deck or pergola footing?
Not for anything structural: Cement Australia says its Rapid Set is not designed for any structural application. Footings that hold up a deck or a roof are sized on the plans or by an engineer.
Sources for the numbers on this page
- Boral — Post Hole Concrete Mix: 20 kg bags by post size, hole diameter and depth · checked 4 October 2026
- River Sands (Easy Mix) — Easy Mix Rapid Set Concrete, data sheet (March 2020): about two bags a post hole, and hole sizes · checked 4 October 2026
- Bunnings — How to set a post (hole depth a third of the post’s height above ground) · checked 4 October 2026
- Cement Australia — Rapid Set Concrete, sell sheet (December 2021): about two 20 kg bags a post hole (PDF) · checked 4 October 2026
- Cement Australia — PROSTRENGTH Rapid Set Concrete, data sheet (April 2024): about 110 bags a cubic metre (PDF) · checked 4 October 2026
- Building Products Supplies (Australian Builders) — High Strength Concrete 20 kg, product data sheet: about 100 bags make a cubic metre (PDF) · checked 4 October 2026
- River Sands (Easy Mix) — Easy Mix Concrete, data sheet (March 2020): 0.01 m³ a 20 kg bag · checked 4 October 2026
- Boral — Concrete Mix 20 kg, product data sheet (February 2022): 108 bags make a cubic metre · checked 4 October 2026
- Bunnings — Australian Builders 20kg High Strength Concrete Mix ($9.90) · checked 4 October 2026
- Holcim Australia — Glossary: density of concrete and normal-class grades (N20 to N50) · checked 4 October 2026
Corrections and updates are logged on the sources page.