Concrete Calculator

Concrete is ordered by volume, priced by the cubic yard or cubic metre, and once it is on site there is no going back — you cannot add more halfway through a pour without creating a cold joint. That is why a concrete estimate is worth doing twice, and why every experienced builder adds a margin rather than ordering exactly what the geometry suggests. The geometry itself is simple for a slab: length times width times thickness, with the thickness converted from inches to feet so the units agree. A four-inch slab over ten by twelve feet is forty cubic feet, which is about one and a half cubic yards. Footings, post holes and steps are the same calculation applied to a different shape; break a complicated pour into simple boxes, work out each one and add them together. The margin matters more than the maths. Concrete is lost to uneven subgrade, formwork that bows under the weight, spillage and the small amount left in the chute. Five to ten per cent extra is typical, and it is far cheaper than a short load, which may mean a second delivery charge or a weak, patched edge. Running slightly over is normal and the surplus can be used for a small pad. This calculator gives the volume in cubic feet, cubic yards and cubic metres, and estimates the number of eighty-pound bags if you are mixing by hand rather than ordering ready-mix. Mixing by hand is only sensible for small jobs — a cubic yard is well over a tonne and more than sixty bags. Confirm the subgrade depth and any reinforcement with your supplier before ordering.

This calculator returns estimates for planning and general information only. It is not engineering, architectural, structural, electrical or code advice, and it cannot replace a qualified professional who has seen the site. Material quantities depend on the actual dimensions, the product you buy and local building codes, so confirm the figures before ordering materials or starting work, and always follow local regulations.