How the concrete calculator works
Concrete is sold by volume. Ready-mix trucks deliver in cubic yards (or cubic metres), and bagged mix lists its yield in cubic feet. This calculator multiplies your dimensions to get the volume, adds your waste allowance, then divides by each bag's published yield and rounds up to whole bags.
Formulas
- Slab or footing: length × width × thickness
- Round column or post hole: π × (diameter ÷ 2)² × height, minus the post if you enter one
- Square column: side A × side B × height
- Steps: each step is treated as a solid block; step n from the top is run × (rise × n) × width
Bag yields used
Imperial yields follow the figures printed on standard concrete mix bags: 40 lb ≈ 0.30 cu ft, 50 lb ≈ 0.375 cu ft, 60 lb ≈ 0.45 cu ft, 80 lb ≈ 0.60 cu ft. Metric yields (20 kg ≈ 9.5 L, 25 kg ≈ 12 L, 30 kg ≈ 14.2 L) are typical for general-purpose premix; check your brand's bag as yields differ slightly.
Why add waste?
Sub-base is never perfectly level, forms bow a little, and some mix stays in the wheelbarrow. 10% is the usual allowance for small jobs; use 5% for precise formwork and 15–20% for pours over rough ground.