Concrete Calculator

How the materials are worked out

The wet volume of concrete (length × width × thickness) is multiplied by 1.54 to get the dry volume of the ingredients — dry materials shrink when water fills the gaps. That dry volume is split by the mix ratio, and cement is converted to 50 kg bags at about 1,440 kg per m³. For a 20 × 15 ft slab 5 inches thick in 1:2:4, that is about 131 cft of concrete (with 5% extra), roughly 24 bags of cement, 58 cft of sand and 116 cft of crush.

Choosing a mix

• 1:1.5:3 (about M20) — columns, beams and roof slabs in many houses.
• 1:2:4 (about M15) — slabs, floors and general work.
• 1:4:8 — lean concrete (PCC) under foundations and floors.

These are nominal mixes for estimating material quantities. The strength actually needed, and the steel, must come from your structural engineer and drawings; site conditions, water quality and curing all affect strength.