Concrete Calculator
Estimate the volume of concrete for slabs, footings and columns, and the number of premix bags.
About the Concrete Calculator
Choose a slab, footing or round column and enter its dimensions, the number of pieces and extra for waste. The calculator gives the volume in cubic yards and cubic metres and the number of premix bags.
Volume is length × width × thickness for slabs and footings, or π × radius² × height for round columns. Order 5–10% extra for spillage, uneven sub-base and formwork bulging. Normal concrete weighs about 2,400 kg/m³ (150 lb/ft³). An 80 lb bag of premix yields about 0.6 ft³.
The formula
V = L × W × T; column V = π r² h
Worked example
A 10 × 10 ft slab, 4 inches thick, plus 10%
- 10 × 10 × 4 ÷ 12 = 33.3 ft³; × 1.1 = 36.7 ft³
- = 1.36 yd³ (1.04 m³)
- Or 62 bags of 80 lb premix
Questions people ask
How thick should a slab be?
Commonly 100 mm (4 in) for patios and paths, 125–150 mm (5–6 in) for driveways.
Bags or ready-mix?
Ready-mix is usually cheaper and more consistent above about 1 m³ (1 yd³).