Concrete Quantity Calculator

Concrete is ordered in cubic yards, and a cubic yard is a box three feet on every side. The formula is length in feet, times width in feet, times thickness in feet, divided by 27.

A 20 by 30 foot slab poured 4 inches thick is 600 square feet times one third of a foot, which is 200 cubic feet, which is 7.4 cubic yards. If you would rather not convert the thickness, square feet times thickness in inches divided by 324 is the same sum, because 324 is 27 times 12.

Nobody pours the drawing. The subgrade is never perfectly flat, the slab runs a little thick where it meets the forms, concrete stays in the chute and the wheelbarrow, and the last yard of the truck is never quite the last yard. That is what the waste factor is for.

Five percent is the working number for flatwork over subgrade you prepared yourself. Ten is the honest number when any of these is true: the subgrade is rough or uneven, the slab has thickened edges or a turned-down perimeter, the forms were set by hand and the depth wanders, or the pour has to finish in one pass with no cold joint allowed.

Then round up to what the plant batches, which is usually a quarter yard. On the 20 by 30 slab, 5 percent waste takes 7.4 to 7.8 yards, and the quarter-yard round-up makes the order 8. That round-up is the reason to do this on paper rather than in your head at the counter.

Running short mid-pour is the most expensive thing on this page. A short load means a second truck with its own minimum charge, or a cold joint where the first pour stopped, or a crew standing in wet concrete while the clock runs. Set against that, half a yard you did not strictly need is cheap.

The other direction has a trap of its own. An extra yard you cannot place is a pile that sets in the driveway and gets broken up on Monday, and the truck still billed you for it. Order to the waste factor and the batch size, not to the drawing and not to a guess.

Under about one yard, price the bags before you call the plant. A truck carries a minimum charge and a short-load fee that dwarf the material on a small pad, and the calculator gives you the bag count for both the 60 pound and the 80 pound mix.

Yardage is the easy line to see and rarely the biggest one. On the demo driveway in our concrete job costing post, the delivered concrete is about 29 percent of the cost. The lines that get skipped are crew labor at the loaded rate and the job's share of overhead.

The concrete bid calculator carries the quantity from this page into those lines: delivered concrete, base, rebar, forms, pump, labor at the loaded rate, an overhead share, and the margin you want to keep. Get the quantity right here, then price the job on top of it there.

A yardage error does not stay in the yardage. Every quantity downstream of the slab thickness moves with it: the base depth, the rebar grid, the form length, and the labor hours somebody estimated off the same wrong number. A pour figured at 4 inches and poured at 5 is 25 percent more concrete and 25 percent more mud to move, and neither the bid nor the crew was told.

Our how many yards of concrete do I need post walks the same formula in words, with the six questions we get asked most about thickness, waste and ordering.