How to calculate concrete in cubic metres
Footing: length (m) × width (mm) ÷ 1,000 × depth (mm) ÷ 1,000
Post hole: π × (diameter ÷ 2)² × depth, in metres
Bags = cubic metres × 1,000 ÷ litres per bag
Always add a waste allowance: 5% for a formed slab on a flat base, 10% or more for uneven ground, trenches and hand mixing.
Typical thicknesses
| Job | Typical thickness |
|---|---|
| Garden path, shed slab | 75 to 100 mm on 50 to 100 mm of compacted road base |
| Patio base, light garage | 100 to 150 mm |
| Driveway for cars | 100 to 125 mm, with SL72 or SL82 reinforcing mesh |
| Strip footing for a garden wall | about 300 to 450 mm wide, 300 mm deep, more on reactive clay |
House slabs and footings are designed to AS 2870 for your site's soil class: follow your engineer's specification and your council's requirements.
Worked example
A 4 m × 3 m shed base, 100 mm thick, is 4 × 3 × 0.1 = 1.2 m³. With 10% waste you need 1.32 m³, which is about 139 bags of 20 kg concrete mix at 9.5 litres each, or a small ready-mix delivery. It weighs about 3.2 tonnes.
Bags or ready-mix?
Up to about 0.5 m³ (around 53 bags) bags are usually simplest. From about 1 m³, a ready-mix truck or a mini-mix delivery is normally cheaper and far less work, and the concrete is more consistent. Ask about minimum loads and part-load charges.
Frequently asked questions
How many 20 kg bags of concrete make 1 cubic metre?
About 105 bags (1,000 ÷ 9.5), if each bag yields about 9.5 litres of mixed concrete. Yields vary by product, so check the bag.
How much concrete do I need for a 3 m by 3 m base?
At 100 mm thick it is 0.9 m³, or about 0.99 m³ with 10% waste: roughly 105 bags of 20 kg mix.
How much does a cubic metre of concrete weigh?
About 2.4 tonnes once mixed and set.
How thick should a concrete shed slab be?
Usually 75 to 100 mm of concrete on 50 to 100 mm of well compacted road base.
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Last updated October 2026.