Calculate concrete volume and bags needed for slabs, footings, columns and stairs.
Concrete volume is calculated in cubic feet for the selected shape. A waste factor (typically 5–10%) is added to account for spillage, overfill and uneven sub-grade. The total is divided by 27 to get cubic yards — the standard unit used when ordering ready-mix. For bag mixes, dividing the cubic-foot total by the published per-bag yield gives the number of bags. Bags are always rounded up because you cannot use a partial bag.
Slab / Footing
Volume (cu ft) = Length × Width × (Thickness_inches ÷ 12)
Column (cylinder)
Volume (cu ft) = π × (diameter_in / 24)² × height_ft
Stairs
Volume = (n × rise_ft / 2) × (n × run_ft) × width_ft
Bags
Bags = ceiling(volume with waste ÷ per-bag yield)
10 × 10 × (4/12) = 33.33 cu ft. With 10% waste: 36.67 cu ft ÷ 27 = 1.36 cubic yards. The calculator gives you this instantly — just set Length 10 ft, Width 10 ft, Thickness 4 in.
An 80 lb bag yields 0.60 cu ft vs 0.45 cu ft for a 60 lb bag. You mix fewer bags for the same volume, reducing labor. On small pours, the weight difference is manageable.
5% for well-prepared, flat sub-grade. 10% for typical residential work. 15% for rough or sloped terrain. The default 10% is a safe choice for most jobs.
Ready-mix is typically more economical above 1 cubic yard. Below that, bags are practical. The calculator shows cubic yards so you can make that call directly.