Concrete Calculator

Each shape uses a geometric formula, applied in whichever unit system you selected. A slab is length times width times thickness. A footing or pad is length times width times depth. A square column is width squared times height. A round column is pi times the radius squared, the radius being half the diameter, times height. A wall is length times height times thickness. A staircase is stair width times rise times tread depth times the number of steps, which approximates the solid mass of a flight.

The volume is then converted into all three units. One cubic foot is 0.028317 cubic metres and one cubic yard is 27 cubic feet, so cubic yards are cubic feet divided by 27.

The waste allowance is applied as a multiplier of one plus the percentage divided by 100, and that is multiplied by the number of units to give the total. Bag counts divide the total volume by each bag's yield: about 0.011 cubic metres for 25 kg, 0.017 for 40 kg, 0.45 cubic feet for 60 lb and 0.60 cubic feet for 80 lb, rounded up. Weight is estimated at 2,400 kg per cubic metre.

Take a driveway slab in imperial units: 20 feet long, 12 feet wide and 6 inches thick. Six inches is 0.5 feet, so the dimensions must be entered as 20, 12 and 0.5.

The volume is 20 multiplied by 12 multiplied by 0.5, which is 120 cubic feet. Dividing by 27 gives 4.444 cubic yards, and multiplying 120 by 0.028317 gives 3.398 cubic metres.

Now add a 10 percent waste allowance for one unit. The multiplier is 1.10, so the total becomes 132 cubic feet, 4.889 cubic yards or 3.738 cubic metres. That is the figure to order against, since ready-mix is normally sold by the cubic yard or cubic metre.

The estimated weight is 3.738 multiplied by 2,400, or roughly 8,971 kg, close to 9 tonnes, which matters when planning access and formwork loading.

In bags, 132 cubic feet divided by 0.60 gives 220 eighty-pound bags, or 294 sixty-pound bags. At that quantity, ready-mix delivery is almost always cheaper and far quicker than mixing on site, which is exactly the kind of decision the bag count is there to inform.

The waste allowance is not padding, it is a necessity. Subgrade that is not perfectly level, formwork that bows slightly under pressure, concrete left in the chute and barrow, and spillage all consume material. Five to ten percent is the usual allowance on a well-prepared flat slab, and ten to fifteen percent is more realistic for footings dug into soil, where the trench walls are rarely true and over-dig is common.

Running short is much worse than over-ordering. A second batch arriving after the first has begun to set creates a cold joint, a plane of weakness through the pour, so order to the upper end of your estimate.

Be careful with units. Thicknesses are commonly quoted in inches or millimetres while lengths are in feet or metres, and entering 6 instead of 0.5 for a six-inch slab inflates the answer twelvefold. Convert everything to the same unit before entering it.

The weight figure uses 2,400 kg per cubic metre, a standard density for normal-weight concrete; lightweight mixes differ. Bag yields vary by manufacturer, so check the yield printed on the bag. This tool estimates quantities only and is no substitute for a structural engineer's specification.