Concrete Calculator
Estimate the volume of concrete needed for a rectangular slab, footing, or pad, including a wastage allowance.
The formula
Volume = length × width × thickness (all converted to the same unit). A wastage percentage is added on top to account for uneven ground, spillage, and formwork variance — concrete is typically ordered a little over the exact calculated volume rather than under, since running short mid-pour is far more disruptive than having a small surplus.
Worked example
A 6m × 4m slab at 10cm thick: 6 × 4 × 0.10 = 2.4 m³ before wastage. With a 10% wastage allowance, that's 2.64 m³ (about 3.45 yd³) of concrete to order.
Why order extra
- Ground is rarely perfectly level, so actual thickness varies across the slab
- Some concrete is lost during mixing, transport, and pouring
- Running short mid-pour can create a weak seam where the pour is interrupted
Frequently asked questions
How much wastage allowance should I use?
10% is a common default for a straightforward slab. Uneven ground, complex formwork, or a first-time DIY pour often justify 15-20% instead, to avoid running short.
Does this account for reinforcement (rebar or mesh)?
No — this calculates the concrete volume only. Rebar or mesh reinforcement is planned and ordered separately based on your project's structural requirements.