METHOD / 06
How the concrete estimate works
Volume, not area. Every shape reduces to a cross-section multiplied by a length, and the only real decision left is how much to over-order.
Formula
slab = length × width × thickness · footing = run × width × depth · column = π × (diameter ÷ 2)² × height × count · order = volume × (1 + over-order)
Worked example
A 4 × 3 m slab at 10 cm thick is 4 × 3 × 0.10 = 1.2 m³. With a 10% allowance that is 1.32 m³ to order. From 25 kg bags yielding about 12.5 litres each, that is 106 bags — roughly two and a half tonnes to carry and mix by hand, which is exactly why ready-mix wins at this size.
Common mistakes
- Measuring thickness in metres by accident. A slab entered as 10 m thick instead of 10 cm gives a hundredfold answer, which is why thickness is asked for in centimetres here.
- Ordering the net volume. Sub-base is never perfectly level and formwork bulges; 10% over is the standard allowance, and short is far more expensive than long.
- Forgetting that a trench is not flat-bottomed. Dug ground gives more than the drawing says, sometimes considerably.
- Mixing bags for a slab. Above about a cubic metre the labour, the consistency and the working time all argue for ready-mix.
- Ignoring the working time. Ready-mix arrives with a clock running; the pour has to be organised before the truck does.
- Pouring without reinforcement where it is needed. Volume is the easy part of a slab and not the part that fails.
FAQ
Questions
How much extra should I order?
About 10%. The sub-base is never level, formwork moves, and some concrete is always left in the barrow or the truck. On a small pour the rounding to whole bags usually covers it; on a large one the percentage is real money and still worth having.
Bags or ready-mix?
The crossover is around one cubic metre. Below that, bags are practical. Above it, you are looking at a hundred bags or more, and hand mixing that volume consistently within the working time is not realistic for most people.
How thick should a slab be?
That is a structural question, not a volume one, and it depends on the load, the ground and local rules. A garden shed base and a driveway are not the same. This calculator tells you the volume for a thickness you specify; it does not tell you what thickness is safe.
Why does it show cubic yards in imperial?
Because that is the unit ready-mix is sold in throughout the United States. Bags are still shown by their cubic-foot yield, which is what the bag itself prints.
Estimates for ordering material, based on regular shapes and even depths. They are not a structural specification: thickness, reinforcement and mix strength depend on the load and the ground, and belong to whoever is responsible for the design.
About this material
Concrete and mortar mixesStrength classes, bag yields, and the difference between concrete, mortar and screed.