The Math Behind Concrete: Volume, Mix Ratios, and Why You Always Order 10% Extra

I poured a patio slab last summer. Measured the area: 12 feet by 10 feet, 4 inches thick. Calculated the volume: 12 * 10 * (4/12) = 40 cubic feet = 1.48 cubic yards. Ordered 1.5 cubic yards from the concrete plant. The truck arrived, the pour started, and I ran out with about 15% of the slab still exposed. The remaining concrete was not enough to cover the area because my calculation had not accounted for the irregular subgrade, slight over-excavation in one corner, and the concrete that adhered to the chute and wheelbarrow. I called for a short load (0.5 yards) and paid a premium for the second delivery. The lesson cost me about $200 and three hours of panic. The math is simple. The margins are what matter. Basic volume calculation Concrete is sold by the cubic yard (in the US) or cubic meter. The fundamental calculation is length times width times depth, converted to the right unit. Volume (cubic feet) = Length (ft) x Width (ft) x Depth (ft) Volume (cubic yards) = Volume (cubic feet)