How much concrete for a slab, a footing or a column?
Updated October 2, 2026
Ordering too little concrete stops the pour halfway; ordering too much wastes money. The calculation takes two steps: the volume to fill, then the materials for each cubic metre. Here is the method, with the usual values and a complete example.
1. Calculate the volume
Work in metres, so the result is in cubic metres (m³):
- Slab: length × width × thickness (12 cm = 0.12 m).
- Strip footing: length × width × depth.
- Square column or beam: side × side × height (or length).
- Round column: 3.1416 × radius² × height.
- Multiply by the number of identical elements, then add a waste margin of 5 to 10 % (uneven ground, formwork, spillage).
2. Materials for 1 m³ of concrete
A common site mix for reinforced concrete is about 350 kg of cement per m³, with roughly 680 kg of sand, 1,100 kg of gravel and 175 litres of water. Lean concrete (blinding) uses less cement, often 150 to 250 kg/m³.
These are usual starting values, not a structural specification: follow your plans, your engineer or the ready-mix supplier for the strength class required.
Worked example: a 5 × 4 m slab, 12 cm thick
5 × 4 × 0.12 = 2.40 m³; with 5 % waste, 2.52 m³ to order. At 350 kg/m³:
- Cement: 2.52 × 350 = 882 kg, i.e. 26 bags of 35 kg (or 18 bags of 50 kg).
- Sand: about 1,714 kg. Gravel: about 2,772 kg.
- Water: about 441 litres, added gradually.
Site mix or ready-mix?
Beyond about 1 to 2 m³, a ready-mix truck is usually cheaper and more regular than mixing on site, and the strength class is guaranteed. Order the volume with the margin, and plan access for the truck or a pump.
For the reinforcement, the rebar calculator gives the weight of steel and the number of 12 m bars; for a thin levelling layer, use the screed calculator instead.
General information, checked at the date shown. It does not replace advice from a professional for your situation.