How to Calculate Cubic Yards of Concrete

What a cubic yard is

A cubic yard is the volume of a cube three feet on each side. That is 3 × 3 × 3 = 27 cubic feet. In metric units it is 0.764555 cubic meters, so a cubic meter is 1.30795 cubic yards. Ready-mix plants in the United States quote prices and deliver loads in cubic yards. A standard mixer truck carries up to about 10 yards, though many carry a little less.

When people say “yards” of concrete they mean cubic yards, not linear yards or square yards. That mix-up causes real shortages: a square yard is an area of 9 square feet, and a linear yard is a length of 3 feet.

Conversion factors

The foot is defined as exactly 0.3048 meters, so all of the factors above follow from it and are exact to the digits shown.

From length, width and thickness in feet and inches

The most common case is a rectangle measured in feet with the thickness in inches. Convert the thickness into feet by dividing by 12, multiply the three dimensions to get cubic feet, and divide by 27.

yd³ = L (ft) × W (ft) × T (in) ÷ 12 ÷ 27, which simplifies to L × W × T ÷ 324.

The shortcut is useful on site: multiply length (ft) × width (ft) × thickness (in) and divide by 324. A 10 × 10 ft slab at 4 in is 10 × 10 × 4 ÷ 324 = 1.235 yd³.

From square footage

1. Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Area to cover 400 ft²; Thickness 5 in = 0.4167 ft
2. Volume = 400 ft² × 0.4167 ft = 166.6667 ft³
3. Net volume = 166.6667 ft³ ÷ 27 = 6.1728 yd³ (× 0.0283168 = 4.7195 m³)
4. Order quantity = 6.1728 yd³ × (1 + 10%) = 6.7901 yd³ = 183.3333 ft³
5. 80 lb bags = 183.3333 ft³ ÷ 0.6 ft³ per bag = 305.56 → 306 bags (always round up)
6. Weight = 183.3333 ft³ × 150 lb/ft³ = 27,500 lb

If you already know the area, multiply by the thickness in feet and divide by 27. For example, a 400 ft² patio 5 in thick is 6.173 yd³ net, or 6.79 yd³ with 10% waste. You can also use the coverage per yard: 81 ft² at 4 in, 64.8 ft² at 5 in, 54 ft² at 6 in. The yards calculator does exactly this from an area and a thickness.

From meters and millimeters

1. Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Length 4 m = 13.1234 ft; Width 3 m = 9.8425 ft; Thickness 120 mm = 0.3937 ft
2. Volume = 13.1234 × 9.8425 × 0.3937 × 1 = 50.8531 ft³
3. Net volume = 50.8531 ft³ ÷ 27 = 1.8834 yd³ (× 0.0283168 = 1.44 m³)
4. Order quantity = 1.8834 yd³ × (1 + 10%) = 2.0718 yd³ = 55.9384 ft³
5. 80 lb bags = 55.9384 ft³ ÷ 0.6 ft³ per bag = 93.23 → 94 bags (always round up)
6. Weight = 55.9384 ft³ × 150 lb/ft³ = 8,391 lb

A 4 × 3 m slab at 120 mm thick is 1.44 m³ net, 1.58 m³ with waste, which is 2.07 yd³. Use the cubic meter calculator for metric jobs. The most frequent metric mistake is entering the thickness in the wrong unit: 120 mm is 0.12 m.

Mixing units on a single project

It is common to have footings measured in inches, a slab in feet and a delivery quoted in meters. Convert everything into a single unit first, preferably feet or meters, calculate each part as its own volume, and add the volumes at the end. Do not round in the middle. Round once at the end, and always upward.

The site calculators let you pick the unit for each input, so you can type 12 in for a footing width next to 40 ft for its run, and the conversion happens without rounding. Behind the scenes everything is converted into feet before multiplying.

Rounding for the order

Plants sell in steps. Some sell in quarter-yard increments and others in half yards or whole yards, and nearly all have a minimum load. After you add waste, ask what the smallest step is and round up to it. If the plant sells in quarter yards and you need 2.26, order 2.5. If you need 2.74 and the plant sells in whole yards, you will order 3 and have some left over; plan a small second pour, such as a pad or a step, to use it.

Common mistakes

• Dividing by 27 before converting inches to feet.
• Using 12 in a cube: 12³ = 1,728 cubic inches per cubic foot, not 12.
• Treating a cubic meter as a cubic yard. They differ by 31%.
• Adding waste twice, once in the calculator and once in your head.
• Rounding each part down and adding them up.

Irregular shapes: break them into parts

Few projects are perfect rectangles. An L-shaped patio, for example, is two rectangles. Suppose one part is 12 × 10 ft and the other 8 × 6 ft, both 4 in thick. The first is 1.48 yd³ net and the second 0.59 yd³ net. Add them to get 2.07 yd³, then add waste. Do not try to calculate an L-shape with a single length and width.

The same approach works for curves: approximate a curved patio by a rectangle plus a half circle, or by the area measured from a plan. For a triangle, the area is half of base times height. A trapezoid is the average of its parallel sides times the distance between them.

Add the volumes of the parts. Round only once, at the end.

Quick mental checks

• Slab: length × width × thickness (in) ÷ 324.
• Area ÷ 81 for a 4 in slab, ÷ 65 for 5 in, ÷ 54 for 6 in.
• A cubic meter is a bit more than 1¼ cubic yards.
• If the answer is surprising, check the units first.

Example: rounding for a real order

Say you measure a 10 × 10 ft slab at 6 in. The net volume is 1.85 yd³ and the order is 2.04 yd³ with 10% waste. If the plant sells in quarter yards you would ask for 2.25 yd³; if it sells in half yards, 2.5 yd³; and if it sells in whole yards, 3 yd³. In each case the extra is a cushion rather than waste, because forms flex and the base is never perfectly flat.

Write the final number, the unit and the thickness on your order so that the dispatcher can check the math. Plants know what a slab of a given size should take and will often correct an obvious error.

Frequently asked questions