Concrete calculator

Concrete is ordered by volume. Measure in metres (100 mm is 0.1 m) and multiply the three sizes, then add a little for wastage.

• Slab or footing: length × width × depth.
• Round post holes: π × (diameter ÷ 2)² × depth, less the post, times the number of holes.
• Steps: width × going × rise × n × (n + 1) ÷ 2, for n steps poured as a solid block.

A worked example

A 3 × 3 m slab, 100 mm thick: 3 × 3 × 0.1 = 0.90 m³. With 10% wastage, that is 0.99 m³ (1.29 yd³).

• Bags: a 25 kg bag of ready-mixed concrete makes about 0.0115 m³, so 0.99 ÷ 0.0115 = 87 bags (or 108 bags of 20 kg, at about 0.0092 m³ each). Check the yield on your bag.
• Ready-mix: round up to the supplier’s step: 1.00 m³.
• Site mix 1:2:4: dry materials take about 54% more room than the concrete they make, so 0.99 × 1.54 = 1.52 m³ of dry mix, split 1 + 2 + 4 = 7 parts: about 13 bags of cement (25 kg), 0.70 t of sharp sand and 1.31 t of gravel.

The weights assume cement at about 1,440 kg/m³, damp sharp sand at 1,600 kg/m³ and gravel at 1,500 kg/m³. These are estimates: for structural work, follow the engineer’s or building control’s specification.