How Much Concrete for a Shed Base?
Quick Answer
You need 0.72m³ of concrete (~17 bags of 20kg premix)
Based on: 3m x 2.4m shed base, 100mm (4") thick concrete slab, C20 mix
How We Calculated This
The volume of a concrete slab is simply length x width x depth:
- Length: 3m
- Width: 2.4m
- Depth: 100mm = 0.1m
- Volume: 3 x 2.4 x 0.1 = 0.72m³
Bagged Premix vs Ready-Mix
A standard 20kg bag of premixed concrete yields approximately 0.009m³when mixed (about 9 litres). For a volume of 0.72m³:
- Bags needed: 0.72 ÷ 0.009 = 80 bags of 20kg premix
That is a lot of bags. In reality, most people would either:
- Order ready-mix concrete: 0.72m³ from a volumetric mixer or mini-mix lorry (minimum order typically 0.5–1m³)
- Mix on site using cement, sharp sand, and 20mm aggregate in a 1:2:3 ratio
Site-Mixed Concrete (1:2:3 C20 Mix)
If mixing on site with a cement mixer, you need approximately:
- Cement: 6 x 25kg bags (~150kg)
- Sharp sand: ~0.33m³ (~530kg)
- 20mm aggregate: ~0.5m³ (~900kg)
This is usually the most cost-effective option for a shed base. A standard cement mixer (Belle Minimix 150) takes about 90 litres per mix, so you would need approximately 8 mixes.
Do I Need a Sub-Base?
Yes. You should lay a compacted sub-base of Type 1 MOT hardcoreat least 75–100mm deep before pouring the concrete. This prevents settlement and frost heave. For a 3m x 2.4m base, you need approximately:
- Sub-base volume (100mm): 3 x 2.4 x 0.1 = 0.72m³
- Weight: approximately 1.3 tonnes of Type 1
Common Shed Base Sizes
- 6’ x 4’ (1.8m x 1.2m): 0.22m³
- 8’ x 6’ (2.4m x 1.8m): 0.43m³
- 10’ x 8’ (3m x 2.4m): 0.72m³
- 12’ x 8’ (3.6m x 2.4m): 0.86m³
Make the concrete base 50–75mm larger than the shed on each side for a proper bearing edge.
Tips
Use timber formwork to contain the pour. Set it level using a spirit level and pegs. Add a layer of damp-proof membrane (DPM) between the sub-base and concrete if the shed will be used as a workshop. For a standard garden shed, a plain concrete slab without reinforcement is fine at 100mm thick.