Finds the volume and mass of concrete needed from the area to be poured and its thickness. Formwork bulges and spillage mean more is used than the drawing says, so an allowance is added.
The quantity of concrete is the area to be poured multiplied by its thickness. The hard part is not the arithmetic but deciding how much to add on top.
Thickness is usually measured in centimetres and area in metres, so the units must be reconciled first.
The design volume is never enough. Formwork bulges slightly under pressure. A slab poured on the ground follows every hollow in the subgrade. Some is spilled in transport and placing.
Together these usually account for five to ten percent. Ordering short is worse than ordering long: a second delivery may not arrive the same day, and the pour then carries a visible cold joint.
The default input pours an area 5 m by 4 m at a thickness of 15 cm.
The area is 20 square metres, so the volume is , exactly 3 cubic metres. A five percent allowance brings it to 3.15.
The mass comes to about 7.245 tonnes. At roughly 2.3 tonnes per cubic metre, concrete is heavier than most people expect. Pouring onto a suspended floor means checking that it can carry the load.
Ready-mix is ordered by the cubic metre, commonly in steps of a quarter or half. Round the calculated figure up to whatever step the supplier uses.
There is a time limit between mixing and finishing the pour, typically ninety minutes to two hours depending on temperature. Ordering everything at once is not a saving; deliveries have to be staged to match the rate at which the concrete can be placed.
This calculator uses 2.3 tonnes per cubic metre, the density of plain concrete without reinforcement. For reinforced concrete, allow around 2.4.