How to figure cubic yards is the same sum whether the material is concrete, fill, gravel or topsoil: a cubic yard is a cube three feet on a side, 27 cubic feet, and every volume a contractor orders is measured in feet and inches and sold in yards. The formula for cubic yards is length times width times depth, all in feet, divided by 27. The mistakes are always in the units, a depth left in inches or an area in square yards, so this page states the cubic yard formula, works it on three shapes, and says how to measure cubic yards on a site that is not a neat rectangle. The free concrete calculator on this site does the same arithmetic from your own dimensions, with no account.
The cubic yard formula, and the unit trap inside it
Cubic yards equals length times width times depth, each in feet, divided by 27. Depth is almost always given in inches, so divide it by 12 first: 6 inches is 0.5 feet, 4 inches is 0.333, 8 inches is 0.667. A 20 by 10 foot area at 6 inches deep is 200 square feet times 0.5 feet, 100 cubic feet, 3.7 cubic yards. Leave the 6 as 6 and the answer is twelve times too big; that is the error that orders four trucks for one. Working in feet throughout and dividing once by 27 at the end is the whole of how to figure cubic yards, and the same rule works for how to calculate yardage of anything sold by the yard.
Figuring cubic yards for a footing, a wall and a round pier
A footing is a long thin slab: 80 feet long by 2 feet wide by 1 foot deep is 160 cubic feet, 5.93 yards. A wall is the same sum stood on end: 40 feet long by 8 feet high by 8 inches thick is 40 times 8 times 0.667, 213 cubic feet, 7.9 yards. A round pier uses the circle: radius squared times pi times depth; a pier 2 feet across (radius 1 foot) and 4 feet deep is 1 times 3.1416 times 4, 12.6 cubic feet, and twelve of them are 151 cubic feet, 5.6 yards. Each is the cubic yard formula with the area worked for the shape; the calculator on this site takes the slab case and the rest follow from it.
How to measure cubic yards on a site that is not a rectangle
Break it into rectangles and add them: an L-shaped slab is two rectangles, a slab with a thickened edge is the thin slab plus the thickened strip worked as a footing. Measure the depth in several places and use the average, because a subgrade that reads 4 inches at the string line and 6 at the low corner is a 5-inch slab for ordering purposes. For fill and gravel, measure the depth you will compact to, not the loose depth, and add the compaction allowance your material needs. The National Institute of Standards and Technology's Handbook 44, linked below, is where the legal definitions of the units themselves live, for the day a supplier's yard and yours disagree.
From cubic yards to the order and the estimate
The formula gives the geometry; the order adds waste. The hub's calculator takes a percentage the contractor sets, 10% by default: 3.7 yards becomes 4.1, rounded up to the half yard the supplier sells. That yardage times the supplier's price per yard is the material line in the estimate, and the estimate's line is the value that later appears on the schedule of values for the draw. Figuring cubic yards once, correctly, in feet, is what makes those three numbers agree; the calculator on this site does it from your dimensions and shows the working, with no account.
Questions people ask about how to figure cubic yards
What is the formula for cubic yards?
Length times width times depth, all in feet, divided by 27. Convert inches to feet first: 6 inches is 0.5 feet. A 20 by 10 foot area at 6 inches deep is 3.7 cubic yards.
How do you measure a cubic yard on site?
Measure length and width in feet, measure the depth in several places in inches and average it, convert to feet, multiply and divide by 27. Break an irregular shape into rectangles and add them.
How do you figure cubic yards for a round pier?
Radius squared times 3.1416 times depth, in feet, divided by 27. A pier 2 feet across and 4 feet deep is 12.6 cubic feet, 0.47 yards; twelve of them are 5.6 yards.