How do you figure concrete in yards?

Formula

L × W × T ÷ 27every dimension in feet

Cubic feet per yard

27 ft³exact conversion

Coverage at 4 in

81 ft²per cubic yard

Waste allowance

10%stated assumption, not a standard

Short version

• Convert every dimension to feet first, including thickness: 4 inches is 0.333 feet.
• Length × width × thickness gives cubic feet. Divide by 27 for cubic yards.
• One cubic yard covers 81 ft² at 4 inches, 54 ft² at 6 inches.
• Break odd shapes into rectangles and circles, figure each one, then add them up.
• Order about 10 percent extra. Running short mid-pour is far worse than paying for a little surplus.

What is the formula for figuring concrete in cubic yards?

The formula is length × width × thickness ÷ 27, with all three dimensions in feet. A cubic yard is 3 feet on every side, so it contains exactly 27 cubic feet, and that 27 is the only conversion number you need.

Work a driveway as an example. A 40-foot by 12-foot driveway at 5 inches thick is 40 × 12 × 0.4167 = 200 cubic feet. Divide by 27 and you get 7.4 cubic yards. With a 10 percent allowance, you would order 8.25 yards, rounded to whatever increment your supplier sells.

The same arithmetic covers footings and walls. A footing 60 feet long, 16 inches wide, and 8 inches deep is 60 × 1.333 × 0.667 = 53.4 cubic feet, or 1.98 cubic yards.

How do you convert slab thickness in inches to feet?

Divide the thickness in inches by 12. Thickness is the step most people get wrong, because they multiply a slab’s square footage by 4 instead of by 0.333 and end up ordering twelve times too much concrete.

Keep the decimals rather than rounding to one place. A 4-inch slab is 0.3333 feet, not 0.33, and on a large pour that difference is worth a fraction of a yard. Rounding at the end of the calculation is fine; rounding at the start is not.

Watch for slabs that are not one uniform thickness. Driveways often step from 4 inches at the garage apron to 6 inches where vehicles turn, and patios sometimes carry a thickened edge. Figure each thickness as its own section, then add the results together.

How much area does a cubic yard of concrete cover?

One cubic yard covers 81 square feet at 4 inches thick. Coverage drops as the slab gets thicker, because the same volume of concrete has to spread over less ground.

This table is the fastest way to sanity-check a number. Divide your square footage by the coverage figure and you get cubic yards directly: a 600 ft² patio at 4 inches is 600 ÷ 81 = 7.4 yards, which matches the long-form calculation.

Coverage figures assume the slab is placed at the stated thickness across its whole area. Where the subgrade is uneven, actual coverage drops, because the low spots take extra concrete that never shows up in the square footage. That is a second reason to carry a waste allowance.

How do you figure concrete for a circle or an irregular shape?

For a circle, find the area with π × radius², then multiply by thickness in feet and divide by 27. A 10-foot-diameter patio has a 5-foot radius, so the area is 3.14 × 25 = 78.5 ft². At 4 inches thick that is 78.5 × 0.333 = 26.2 cubic feet, or 0.97 cubic yards.

Round footings and post holes use the same cylinder math. A 12-inch-diameter hole 3 feet deep is 3.14 × 0.25 × 3 = 2.36 cubic feet.

For an L-shaped or stepped slab, draw it on paper and cut it into rectangles. Figure each rectangle separately, then add the cubic yards together. Do not try to average an irregular shape into a single rectangle; the error compounds with every foot of thickness.

Sloped or thickened sections need an average thickness. A slab that runs 4 inches at one end and 6 inches at the other averages 5 inches, so use 0.4167 feet across that section.

How much waste should you add to a concrete order?

Adding about 10 percent is the working assumption most crews use, and it is an assumption rather than a published standard. Subgrade is never perfectly flat, forms bow slightly under the weight of fresh concrete, and some material is always left in the chute and the wheelbarrow.

Thin slabs on a rough subgrade can absorb more than 10 percent. A slab over compacted gravel that was screeded true might need only 5 percent. Ordering a partial extra yard costs far less than the labor and cold-joint risk of stopping a pour to wait for a second delivery.

Once you know the yardage, see how many yards of concrete are in a truck to work out deliveries, or how many bags of concrete you need for small pours you plan to mix yourself.

Common mistakes

• Multiplying by inches instead of feet — using 4 rather than 0.333 overstates the order by a factor of twelve and is the single most common estimating error.
• Forgetting the 10 percent allowance — an exact calculation leaves you short the moment the subgrade dips or a form spreads.
• Averaging an irregular slab into one rectangle — break it into pieces instead, or the error grows with thickness.
• Ignoring thickened edges and footings — a turned-down edge or integral footing can add a yard or more that the flat-slab math never sees.
• Measuring the forms instead of the finished slab — measure inside face to inside face, not the outside of the lumber.