Free Concrete Calculator – Volume, Cubic Yards & Bags

Concrete Calculator Guide: Cubic Yards, Cubic Meters & Bag Counts

The Concrete Calculator estimates the volume of concrete needed for slabs, footings, stem walls, columns, stairs, and post holes, then converts that volume into cubic feet, cubic yards, cubic meters, and the number of pre-mix bags. Before calculating, measure every dimension in consistent units. For a flat slab you need length, width, and thickness; thickness is usually given in inches even when length and width are in feet, so the tool divides inches by 12 to keep the math aligned. For footings and walls, measure length, height, and thickness. For round columns or post holes, measure diameter and depth. For stairs, count the steps and record run width, rise height, and tread depth so the solid fill under the treads is captured. Note the placement thickness from your plans: 4 inches (about 100 mm) is typical for residential slabs and walkways, while driveways run 5 to 6 inches. Measure the subgrade carefully, because an extra half inch of depth across a large slab adds a surprising amount of volume.

The core formula is cubic yards = length(ft) x width(ft) x thickness(in) / 12 / 27. Dividing thickness by 12 converts inches to feet, giving cubic feet; dividing by 27 converts cubic feet to cubic yards (since 1 yd = 3 ft, 1 cubic yard = 27 cubic feet). Worked example: a 20 ft x 10 ft slab poured 4 inches thick = 20 x 10 x (4/12) = 66.7 cubic feet = 66.7 / 27 = 2.47 cubic yards (about 1.89 cubic meters). With a 10% allowance you would order roughly 2.7 cubic yards of ready-mix. For bag estimates, an 80 lb bag yields about 0.60 cubic feet, 60 lb about 0.45 cubic feet, and 40 lb about 0.30 cubic feet. The same slab in 80 lb bags = 66.7 / 0.60 = 112 bags. For round post holes, volume = pi x radius(ft)^2 x depth(ft); a 12 in (1 ft) diameter hole 3 ft deep = 3.14 x 0.25 x 3 = 2.36 cubic feet.

Add 5 to 10 percent to your final figure for spillage, subgrade irregularities, over-excavation, and uneven slab depth; rough or hand-dug ground often justifies the higher end. Ordering short is the costliest mistake, because a cold joint forms if a second batch arrives after the first has begun to set, creating a weak plane. Once volume exceeds roughly 1 cubic yard (0.76 m³), ready-mix delivery is usually more economical and more consistent than mixing many bags by hand. Mind the standards: ACI 318 governs structural concrete design, while ACI 332 covers residential foundations and typically calls for a minimum 2,500 psi (17 MPa) mix, with 3,000 to 4,000 psi (21 to 28 MPa) common for slabs and footings exposed to freeze-thaw. ASTM C94 specifies ready-mixed concrete requirements. The IRC sets minimum footing widths and depths below the local frost line. Concrete reaches walking strength in 24 to 48 hours but full design strength at 28 days, so plan curing and protect fresh placement from rapid drying.