The formula
volume = length × width × thickness · materials from the mix ratio
Where it comes from
A 1:2:3 mix means one part cement, two of sand and three of aggregate, giving concrete of around 25 MPa, which is enough for domestic slabs and foundations. Loose aggregates take up more room than cured concrete, so it takes about 1.55 m³ of dry material for every finished cubic metre.
How to work it out by hand
- Multiply length by width by thickness for the volume
- Add a 5 % allowance for an uneven base
- Multiply by 1.55 for the dry material needed
- Split that material according to the mix you chose
What is worth knowing
The water-to-cement ratio governs the final strength: 0.5 litres of water per kilo of cement is right. Extra water makes it easier to work but weakens the concrete irreversibly. Above 2 or 3 m³ it usually works out cheaper, and comes out better, to order ready-mix by the truck than to batch it on site.