Concrete Volume Calculator
Find out exactly how many cubic yards of concrete you need for a slab, footing, or pad. Accounts for thickness and a configurable waste allowance so you never fall short on pour day.
About this calculator
Concrete volume is calculated in cubic yards because ready-mix trucks and bags are sold by the yard (1 cubic yard = 27 cubic feet). The raw volume of a rectangular slab is length × width × thickness, but thickness must first be converted from inches to feet by dividing by 12. A waste allowance is then added to cover spillage, uneven subgrades, and over-excavation. The full formula is: Volume (yd³) = (length × width × (thickness ÷ 12)) × (1 + wastePercent ÷ 100) ÷ 27. A standard waste factor of 5–10% is recommended for flatwork; footings and walls typically warrant 10–15% due to irregular forms. Always round up to the nearest quarter yard when ordering from a ready-mix supplier.
How to use
You're pouring a driveway slab that is 20 ft long, 10 ft wide, and 4 inches thick, with a 10% waste allowance. Step 1 — convert thickness: 4 ÷ 12 = 0.333 ft. Step 2 — raw cubic feet: 20 × 10 × 0.333 = 66.67 cu ft. Step 3 — apply waste: 66.67 × (1 + 10 ÷ 100) = 66.67 × 1.10 = 73.33 cu ft. Step 4 — convert to cubic yards: 73.33 ÷ 27 ≈ 2.72 yd³. Order 3 cubic yards to be safe.
Frequently asked questions
How do I convert cubic feet of concrete to cubic yards?
Since 1 cubic yard equals 27 cubic feet, you simply divide your total cubic footage by 27. For example, 81 cubic feet of concrete equals exactly 3 cubic yards. Ready-mix suppliers price and deliver concrete by the cubic yard, so this conversion is essential for ordering. This calculator handles the conversion automatically once you enter length, width, and thickness.
What waste allowance percentage should I use for a concrete slab?
A 5–10% waste allowance is standard for residential flatwork like patios, driveways, and garage floors. Use 10% for slabs with complex shapes or if the subgrade is uneven, since the actual depth can vary. Footings and foundation walls warrant a 10–15% buffer because form irregularities and soil conditions cause greater volume variance. It's always better to order slightly more than needed—most suppliers charge a short-load fee if you order less than a minimum quantity.
How many 80 lb bags of concrete mix do I need per cubic yard?
One cubic yard of concrete requires approximately 45 bags of 80 lb pre-mix (each bag yields about 0.6 cubic feet). For smaller projects under half a yard, bagged concrete is cost-effective and convenient. For anything larger than 1–2 cubic yards, ordering ready-mix from a truck is faster, cheaper per unit, and produces a more consistent mix. Always verify the yield per bag stated on the packaging, as it can vary slightly by brand and mix type.