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At four inches thick, 133.3 cubic feet, which is 4.94 cubic yards or 3.78 cubic metres. With a ten percent waste allowance you would order 5.5 yards, since suppliers deliver in quarter-yard steps and round up.

Because subgrade is never perfectly level and forms bow outward under the weight of wet concrete. Ten percent is the usual figure. Running short matters far more than running over: a pour cannot be topped up an hour later, and a cold joint through the middle of a slab is a permanent weakness.

Forty-five eighty-pound bags, which weigh 3,600 pounds in total. That is the point where ready-mix stops being a luxury: most suppliers deliver from one cubic yard, and mixing forty-five bags by hand is a day of work and an uneven pour.

Because a flight is a staircase of prisms, not a wedge or a block. The bottom step runs the full depth, the next is one tread shorter, and so on. Treating three steps as their outer envelope overestimates by about half, which on a real flight is most of a cubic yard.

Cubic yards in the United States, cubic metres almost everywhere else. Both are shown. Note that a cubic yard is 27 cubic feet rather than three: dividing by three instead is a common and expensive slip.