How Much Concrete Do I Need? Slabs, Paths and Post Holes

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Concrete is sold by the cubic metre (m³), or in bags that each make a small part of one. Work out the volume, add some for waste, and convert it to what you're buying.

1. Slabs, paths and pads

Volume (m³) = length (m) × width (m) × thickness (m).

The thickness is usually given in millimetres, so divide by 1,000. For example, 100 mm is 0.1 m.
If the ground isn't level, use the average thickness.
The thickness itself should come from the plans or the shed maker's slab detail, not from a guess.

Example: a 3 m × 3 m pad, 100 mm thick: 3 × 3 × 0.1 = 0.9 m³.

2. Round post holes

Each hole is a cylinder: π × radius² × depth. Take off the space the post itself fills.

Concrete per hole = (π × (diameter ÷ 2)² × depth) − (post width² × depth).

Example: 10 holes, 300 mm across and 600 mm deep, each holding a 100 mm square post.

Hole: π × 0.15² × 0.6 = about 0.0424 m³.
Post: 0.1 × 0.1 × 0.6 = 0.006 m³.
Concrete per hole: about 0.0364 m³.
10 holes: about 0.36 m³.

Set the post size to 0 for a solid footing with no post in it.

3. Add waste

Uneven ground, spillage and overfill always use more than the sum. The calculator adds 10% as a starting point.

Slab: 0.9 × 1.1 = 0.99 m³.
Post holes: 0.364 × 1.1 = about 0.40 m³.

4. Bags or ready-mix?

Premix bags. Many 20 kg premix bags make about 0.01 m³ each, but yields vary by brand and bag size, so check the bag.

Slab: 0.99 m³ ÷ 0.01 = about 100 bags, which is about 2 tonnes to carry and mix.
Post holes: 0.40 m³ ÷ 0.01 = about 41 bags.

Ready-mix. Suppliers sell by the cubic metre and may have minimum loads or short-load fees. For a slab that's about a cubic metre or more, ring a supplier and compare before you buy 100 bags.

5. Before you pour

Approvals. Check with your council whether the slab or shed needs approval.
Before You Dig. Contact Before You Dig Australia (1100.com.au) before digging holes or footings.
Safety. Wet cement and concrete can burn skin and eyes. Wear gloves, eye protection and long sleeves, and follow the safety information on the bag.
Design. This guide works out the volume only. It isn't a slab or footing design. Reinforcement, footing size and concrete strength should come from the plans, the shed or building maker, or an engineer.

6. Common mistakes

Forgetting to convert millimetres to metres.
Leaving out waste.
Not taking the post's own volume off each hole (the calculator does it for you).
Buying bags for a job big enough for ready-mix.
Guessing slab thickness instead of using the maker's detail.

7. Quick summary

Slab volume = length × width × thickness (in metres).
Post hole = π × r² × depth, minus the post.
Add about 10% for waste.
Check the bag's yield, or price ready-mix for bigger jobs.
Call Before You Dig, check approvals, and protect your skin and eyes.

Try it with your own job: Concrete & Area Calculator.

General information only. Follow the plans, the maker's instructions and the safety information on the bag.