Building materials
How much concrete for a shed base? m³ and bags
Work out the concrete volume for a typical shed base in m³, add a sensible allowance and turn it into bags or a ready-mix order.
Volume is length × width × depth
Convert every measurement to metres first. For an illustrative base 3 m × 2.4 m at 100 mm deep, the volume is 3 × 2.4 × 0.1 = 0.72 m³. The depth trap is the classic mistake: 100 mm is 0.1 m, not 1 m — get it wrong and your order is ten times too big.
Adding a 5% allowance for uneven ground and spillage takes the figure to 0.72 × 1.05 = 0.756 m³.
Bags or ready-mix?
Bagged mix yield varies by product, so always read the bag. As an illustration, if a 25 kg bag yields about 0.011 m³, you would need 0.756 ÷ 0.011 ≈ 68.7, so 69 bags — over 1.7 tonnes to lift and mix by hand.
At that size, many people find a small ready-mix or mix-on-site delivery more practical. Suppliers usually sell in steps such as 0.25 m³ or 0.5 m³, so an order of 0.75 m³ or 1 m³ would cover this base.
Check the ground and the build-up
A base normally sits on compacted sub-base, which is a separate quantity: the aggregate calculator covers it. Edge formwork, a damp-proof membrane and the shed maker’s recommendations all affect the final design. Estimates only. Check the manufacturer’s coverage rates.
