Why "A Yard of Concrete" Means 27 Cubic Feet —and How That Translates to a Driveway
In construction, "a yard" never means a linear yard. It always means a cubic yard. "Order 10 yards of concrete" means order 10 cubic yards —270 cubic feet of ready-mix —delivered in a truck with a rotating drum. The cubic yard is the standard unit of concrete because a yard roughly matches the batch size of a concrete mixer truck, and because concrete's unit weight (roughly 4,000 lb per cubic yard, or 2 tons) makes a 10-yard load (40,000 lb, 20 tons) the maximum legal axle weight for a three-axle mixer truck on US highways. The truck's legal weight limit, not the drum capacity, determines the 10-yard standard load. A truck that carries 12 yards would be overweight on most roads. The cubic yard is the concrete industry's unit because it is the largest unit a fully loaded truck can legally carry without a special permit. The number 27 —feet³ per yard³ —was not chosen for construction. It was chosen by the yard being defined as 3 feet. The construction industry inherited the 27, and built the interstate highway system on it.
The Patio, the Foundation, and the Driveway: Every Concrete Calculation Reduced to ÷27
| Project | Dimensions | Cubic Feet | Cubic Yards | Order |
|---|---|---|---|---|
| Garden shed slab | 8×10 ft, 4 in thick | 26.7 | 0.99 | 1 yard |
| Patio | 20×20 ft, 4 in thick | 133.3 | 4.94 | 5 yards |
| Two-car driveway | 20×40 ft, 6 in thick | 400 | 14.81 | 15 yards —two trucks |
| House foundation | 40×60 ft, 8 in thick | 1,600 | 59.26 | 60 yards —six trucks |
| Sidewalk (100 ft run) | 4×100 ft, 4 in thick | 133.3 | 4.94 | 5 yards |
The formula every contractor knows: (length × width × thickness in feet) ÷ 27 = cubic yards. The thickness must be in feet, not inches. The most common concrete-thickness error is forgetting to convert inches to feet: a 4-inch slab is 0.333 feet, not 4. A homeowner who multiplies length × width × 4 and then divides by 27 gets a result 12× too small —because 4 inches is 0.333 feet, not 4 feet. The concrete dispatcher catches this when the order comes in at "0.4 yards for a 20×20 patio" and calls back to confirm the thickness. 4 inches = 1/3 foot. Remember the /12 step. The divide-by-27 comes after.
ft³ = yd³ × 27 | yd³ = (L × W × H in feet) ÷ 27
Mulch, Topsoil, and Gravel: The Landscaping Yard
In landscaping, "a yard" means a cubic yard —the same 27 cubic feet —but the material is loose and fluffy, not dense and liquid. A cubic yard of bark mulch weighs about 600—00 lb. A cubic yard of topsoil weighs about 2,000—,500 lb. A cubic yard of gravel weighs about 2,600—,000 lb. All occupy exactly 27 cubic feet. The weight varies by a factor of 5× because the material's density varies. The volume is constant. The cost per yard is based on the volume, not the weight —the landscape supplier sells by the cubic yard because the customer's project (a flower bed, a driveway base, a French drain) is dimensioned in volume, not weight. A flower bed that measures 20×5 feet filled 6 inches deep with mulch needs 20 × 5 × 0.5 = 50 cu ft ÷ 27 = 1.85 yards. The supplier delivers 2 yards —54 cubic feet of shredded bark —in a dump truck, and the extra 0.15 yards becomes mulch around the mailbox. The 27:1 ratio is the same whether the material weighs 600 lb per yard (mulch) or 3,000 lb per yard (crushed stone). The weight difference is the reason the supplier's truck can carry 15 yards of mulch (9,000 lb, well within legal limits) but only 8 yards of gravel (24,000 lb, at the legal limit). The volume is the order unit. The weight is the truck's constraint.
Frequently Asked Questions
How do I calculate cubic yards from inches?
Convert all dimensions to feet first: inches ÷ 12 = feet. Then (L × W × H) ÷ 27 = cubic yards. The most common mistake is multiplying inches directly and then dividing by some wrong factor. A concrete slab 120 inches × 240 inches × 4 inches = 115,200 cubic inches. Divide by 46,656 (inches³ per yard³, or 36³) = 2.47 cubic yards. The easier method: 120/12 = 10 ft, 240/12 = 20 ft, 4/12 = 0.333 ft. 10 × 20 × 0.333 = 66.67 cu ft ÷ 27 = 2.47 yards. Both methods give the same answer. The feet-first method avoids large numbers and is less error-prone in a field notebook. The quick-and-dirty rule for a 4-inch slab: square feet ÷ 81 = cubic yards. Because 4 inches = 1/3 foot, and (area × 1/3) ÷ 27 = area ÷ 81. A 20×20 patio = 400 sq ft ÷ 81 = 4.94 yards. For more precise concrete calculations and the unit-conversion pitfalls that have caused foundation contractors to under-order by factors of 12, see the cubic feet to cubic yards companion page.
Why is a cubic yard exactly 27 cubic feet?
Because a yard is exactly 3 feet —defined by the 1959 International Yard and Pound Agreement as 0.9144 meters, and the foot as exactly 1/3 of a yard (0.3048 meters). The relationship 1 yard = 3 feet is exact by definition. Therefore 1 yd³ = (3 ft)³ = 27 ft³, also exact. The cubic yard is the only widely used cubic volume unit that converts to its linear-foot equivalent by an integer. Every other cubic conversion —cubic feet to cubic meters, cubic inches to liters —involves a decimal. The cubic yard's 27:1 ratio to the cubic foot is a mathematical gift from the yard being defined as an integer multiple of the foot. The metric system has no equivalent clean-cube integer relationship between any of its commonly used volume units, because the SI prefixes go in steps of 1,000, and 1,000 is not a perfect cube (10³, but 1,000 cm³ = 1 L, and 1,000 L = 1 m³ —so 1 m³ = 1,000,000 cm³, a clean square of 1,000 but not a cube of a small integer). The 27 is unique. Enjoy it.
Engineering Context
The cubic yard (yd³) is the standard unit for excavation, fill, concrete, and bulk landscaping materials in the US. The conversion 1 yd³ = 27 ft³ is exact —derived from 1 yd = 3 ft, which is itself exact from the 1959 treaty. In civil engineering, earthwork quantities are computed in cubic yards from cross-section areas (in square feet) multiplied by station intervals (in feet), divided by 27 —the same ÷27 step performed at every grade stake. For landfill airspace calculations, waste volumes are compacted and measured in cubic yards —and the remaining airspace is the permitted volume in yd³ converted from surveyed cubic feet, with the ÷27 performed on the surveyor's total. In construction estimating, concrete is ordered in cubic yards, but formwork dimensions are measured in feet and inches —making the inch →foot →yard conversion the first calculation on every pour card. For adjacent converters: ft³ to yd³ (the reverse), ft³ to m³ (international earthwork), gal to ft³ (water volume to tank size), and the Volume Guide.