Concrete Formulas by Shape
Concrete is always bought by volume — cubic yards (27 cubic feet). Every shape below reduces to the same question: how many cubic feet of concrete fill the form?
Rectangular Slab / Footing: Length × Width × (Depth ÷ 12)
Circular Slab: π × (Diameter ÷ 2)² × (Depth ÷ 12)
Round Column: π × (Diameter ÷ 2)² × Height
Square Column: Width × Length × Height
Poured Wall: Length × Height × (Thickness ÷ 12)
Steps: Width × Run × Rise × [N × (N+1) ÷ 2], N = step count
Curb: Length × (Width ÷ 12) × (Height ÷ 12)
Then: Cubic Yards = Cubic Feet ÷ 27, plus a 5–10% waste margin depending on the pour.
Frequently Asked Questions
When should I order a ready-mix truck instead of mixing bags?
If your project requires more than 1.5 to 2 cubic yards (around 70 to 90 bags of 80 lb concrete), ordering a ready-mix concrete truck is significantly cheaper, faster, and ensures a uniform cure without cold joints.
How thick should a concrete slab be?
A 4-inch slab is standard for residential walkways, shed pads, and outdoor patios. Heavy-duty applications such as driveways, RV pads, and garage floors should be at least 5 to 6 inches thick with rebar reinforcement.
How deep should a footing be?
Footing depth is set by your local frost line, not by load alone — check your municipal building code. Residential strip footings are commonly 12 inches deep and 16 to 24 inches wide, poured below the frost depth for your climate.
Why do columns and walls use a higher waste margin than slabs?
Vertical pours (columns, walls, steps) lose more material to formwork seepage, overfill, and hard-to-reach corners than a flat, level slab does. This calculator applies 10% to vertical shapes and 5% to flat ones.