How much concrete do I need for a slab?
Multiply length × width × thickness in metres. A 3 m × 3 m shed slab at 100 mm thick is 0.9 m³ before wastage, about 1.0 m³ to order.
Concrete slab calculator
SLABSET
VOLUME0.00m³includes 10% wastage
TO ORDER0 Bags (20kg) or 0.0 m3 Premix
To order
0.00m³
Inputs
ShapeSlab
Length0 mm
Width0 mm
Thickness0 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
A slab is priced by volume: length × width × thickness. A typical Australian shed or patio slab is 100 mm thick; driveways are usually 125–150 mm with mesh. Enter your dimensions in millimetres and SlabSet converts to cubic metres, adds wastage and sizes the order.
The spec sheet also estimates reinforcing mesh sheets, formwork edge length and total concrete weight, useful for checking access and barrow counts before pour day.
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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