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 base | 75 to 100 mm on 75 to 100 mm of compacted sub-base |
| Patio base, light garage | 100 to 150 mm |
| Driveway for cars | 100 to 150 mm, with reinforcement mesh |
| Strip footing for a garden wall | about 450 mm wide, 225 mm deep, more on soft ground |
Building Regulations and your local building control set the rules for house foundations and extensions: follow your engineer's specification.
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 120 bags of 25 kg concrete mix at 11 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 45 bags) bags are usually simplest. From about 1 m³, ready-mix or a mix-on-site lorry 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 25 kg bags of concrete make 1 cubic metre?
About 91 bags (1,000 ÷ 11), if each bag yields about 11 litres of mixed concrete. Check the yield printed on your bag, as it varies by product.
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 90 bags of 25 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 base be?
Usually 75 to 100 mm of concrete on 75 to 100 mm of well compacted hardcore or MOT Type 1 sub-base.
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Last updated October 2026.