On a self-build site, concrete is often ordered by eye, and the eye is wrong both ways: a pour stopped because the last bag is empty leaves a cold joint that weakens the work; a heap of sand left over after the job is money drying in the sun.
This guide explains how to go from dimensions to a list of materials. It gives quantities, not a design: the thickness of a slab, the section of a beam or column and the reinforcement of anything load-bearing are designed and validated by a qualified engineer or architect, and the concrete is worked out from their drawing.
Volume first
Everything starts from the volume in cubic metres: length × width × thickness, all in metres. A slab 5 m by 4 m and 12 cm thick is 5 × 4 × 0.12 = 2.4 m³. A strip footing 12 m long, 50 cm wide and 30 cm deep is 12 × 0.5 × 0.3 = 1.8 m³.
Add 5 to 10% for losses: concrete left in the mixer and the barrow, uneven formwork, the bottom of a trench that is never perfectly flat. In the slab example, 2.4 m³ becomes 2.6 m³ to prepare.
The mix: what one cubic metre takes
An ordinary concrete for a ground slab or a ring beam is mixed at 350 kg of cement per cubic metre. For one cubic metre you then count 7 bags of 50 kg cement, about 0.4 m³ of 0/4 sand, about 0.8 m³ of gravel (half 8/15, half 15/25 for a dense concrete) and around 175 litres of water.
A blinding concrete — the thin layer poured at the bottom of a trench so the steel sits on a clean surface — is mixed leaner, around 150 kg per cubic metre. Foundation or column concrete follows the mix the engineer specifies on the drawings; do not « strengthen » it at random by adding cement, and never stretch it with water.
The example, all the way through
For 2.6 m³ at 350: 2.6 × 7 = 18.2, so 19 bags of cement; 2.6 × 0.4 = 1.04 m³ of sand; 2.6 × 0.8 = 2.08 m³ of gravel, about 1 m³ of 8/15 and 1 m³ of 15/25; and roughly 450 litres of water, added a little at a time.
Sand and gravel are sold by the cubic metre or the tonne: a cubic metre of sand weighs about 1.6 tonnes, of gravel about 1.5 tonnes. Beyond a few cubic metres, a tipper delivery costs less than bags, and the concrete calculator does all of this conversion for your dimensions.
Water, the quiet enemy
Concrete that is too wet pours more easily and ends up much weaker. The right consistency is a paste that sits on the shovel without running. If the concrete must flow more to pass between closely spaced bars, use a plasticiser, not another bucket of water.
In hot weather, freshly poured concrete must be kept damp for several days: a light spray, a sheet or a wet cloth. In the height of an Algerian summer, a slab that dries too fast is cracked on its surface by the next morning.
Step by step
Indicative time: one hour
Read the dimensions off the drawing
Take the length, width and thickness of the element from the engineer’s or architect’s drawing, in metres.
Work out the volume
Multiply the three dimensions to get cubic metres, then add 5 to 10% for losses.
Take the specified mix
Use the cement content in kg per m³ given on the drawing or the specification; 350 kg/m³ for an ordinary slab.
Convert into materials
Multiply the volume by the quantities per cubic metre — cement bags, sand and gravel — then round the bags up.
Plan the water and the delivery
Allow about half the cement’s weight in litres of water, and choose between bags and a tipper for the sand and gravel according to the volumes.
