How do I calculate concrete for footings?
Multiply trench length × width × depth in metres. Add generous wastage of 15% or more, because trenches are rarely cut clean.
Concrete footing calculator
SLABSET
VOLUME0.00m³includes 10% wastage
TO ORDER0 Bags (20kg) or 0.0 m3 Premix
To order
0.00m³
Inputs
ShapeFooting
Length0 mm
Width0 mm
Depth0 mm
Wastage+10%
Estimated cost
Recommended: –Enter dimensions to estimate cost.
Individual bags–
0 × 20 kg · 0 × 30 kg
Ready-mix delivery–
0.0 m³ + delivery (ex pump)
Typical metro prices in Australia. Confirm with your local supplier before ordering.
Materials & weight
Reinforcing mesh0 sheets
Formwork edge0.0 m
Concrete weight0.00 t
How it's calculated
Concrete is sold by volume in m³. Measure the space, work out its volume, add wastage, then convert to individual bags or a premix order.
Wastage: sites lose concrete to spillage and uneven ground. 10% is standard; raise it for rough ground, lower it for tidy formwork.
Bags or premix: a 20 kg bag yields ≈ 0.0098 m³ (~102 bags/m³). Premix sells in 0.1 m³ steps; below 0.5 m³ bags usually avoid short-load fees.
About
Strip footings are volume-hungry: a 300 mm × 600 mm trench takes 0.18 m³ per metre run. Enter the trench length, width and depth in millimetres and SlabSet gives cubic metres, bag counts and a premix order size.
Footings poured against excavated ground lose more concrete than formed work: over-dig and uneven trench bottoms add up. Bump wastage to 15–20% for rough ground.
FAQ
How much wastage should I add?
10% is a normal starting point. Use less for tidy formwork and more for rough ground, uneven excavation or over-dig.
Should I order bags or premix?
Ready-mix is cheaper per cubic metre, but delivery and short-load fees make small deliveries poor value: suppliers charge extra below about 0.5 m³. Bags cost more per m³ but scale down to any size. The switchover is roughly 0.5 m³: below it, order bags; above it, book the truck.
Does this replace a builder or engineer?
No. Use SlabSet for estimating, then confirm quantities, grade and reinforcement before ordering.
Other calculators
Estimates only. Confirm quantities and prices with your supplier.
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