DIY Concrete Mix Calculator

Concrete volume calculation works out how much material fills a given space, whether that is a slab, a footing, a post hole or a step, converting the dimensions into cubic yards or cubic metres and then into pre-mixed bag counts.

Core formulas: Volume (cubic feet) = Length (ft) × Width (ft) × Depth (ft) Volume (cubic yards) = Volume (cubic feet) ÷ 27 Volume (cubic meters) = Length (m) × Width (m) × Depth (m)

For cylindrical post holes: Volume = π × radius² × depth (in consistent units)

Pre-mixed bag coverage (approximate):

Always add 10% waste buffer when calculating bags: bags needed × 1.1.

Standard concrete mix ratios (by volume):

• General purpose: 1:2:3 (cement : sand : gravel)
• High strength: 1:1.5:2.5
• Driveway/heavy load: 1:2:2 (richer in cement)
• Water:cement ratio: 0.45–0.55 for most applications (less water = stronger concrete)

Worked example, a garden shed slab: Slab: 12 ft × 10 ft × 4 inches thick (4/12 = 0.333 ft) Volume = 12 × 10 × 0.333 = 40 cubic feet Cubic yards = 40 ÷ 27 = 1.48 cubic yards With 10% buffer: 1.63 cubic yards

80 lb bags needed = 1.63 ÷ 0.022 = 74 bags (80 lb each) Alternatively: order 1.75 yards of ready-mix concrete from a supplier for this size. Ready-mix is typically cheaper than bagged once you are past about a cubic yard, and it saves you mixing forty bags by hand.

Rule of thumb: Ready-mix delivery costs $120–$180/yard but saves enormous time and labor on any project larger than 1 cubic yard.