Concrete Calculator

How concrete volume is calculated

Concrete is ordered by volume, so every calculation starts by turning the shape you are pouring into cubic metres or cubic yards. For a slab the formula is volume = length × width × thickness. A footing or strip foundation uses the same idea with its own length, width and depth, and a wall is length × height × thickness. A round column or pier is a cylinder: volume = π × (diameter ÷ 2)² × height, multiplied by the number of columns. For stairs treated as solid steps, the calculator adds up every step from the ground, which gives width × run × rise × n × (n + 1) ÷ 2 for n steps.

Large dimensions such as length and height are entered in metres or feet, while small ones such as thickness, diameter, rise and run are entered in centimetres or inches. The tool converts everything to metres internally, then reports the result in cubic metres, cubic yards and cubic feet. One cubic metre equals 1.308 cubic yards, and one cubic yard equals 27 cubic feet.

Waste allowance and premix bags

Real pours never match the drawings exactly: subgrades are uneven, forms flex and some concrete is spilled. That is why a waste percentage is applied: order volume = net volume × (1 + waste ÷ 100). Five to ten percent is typical, and more is sensible for trenches dug by hand. The calculator then divides the order volume by the yield of a single bag to get the number of bags, always rounding up. Typical yields are about 0.30, 0.45 and 0.60 cubic feet for 40, 60 and 80 lb bags, and about 0.0094, 0.0117 and 0.0187 cubic metres for 20, 25 and 40 kg bags. Brands differ slightly, so check the yield printed on your bag and keep a few spare bags.

Bagged premix suits small jobs such as fence posts, steps and small pads. As a rule of thumb, once a job needs more than about a cubic metre, ordering ready-mix from a truck saves effort and gives a more consistent result. This calculator runs entirely in your browser, so your measurements stay on your device, and the figures are estimates that should be confirmed against your plans, local codes and your concrete supplier.