Concrete and steel
Concrete calculator
This calculator works out the cement, river sand and stone for concrete: a floor slab, a strip footing, columns, beams, or any volume you already know. Choose the element, enter its size and how many there are, and pick the mix.
What you need
This calculator works out the figures in your browser, which needs JavaScript switched on. The method below shows how to work it out by hand.
Estimates for planning and budgeting. Real quantities depend on the site and the workmanship, so check with your builder before you order.
Concrete in Zimbabwe is made with river sand, which is coarse and clean, and crushed stone, usually 19 mm. The mixes here are the traditional volume mixes used on site, from a lean 1:3:6 for blinding up to 1:1.5:3 for reinforced slabs and columns. Where an engineer has specified a strength (a 25 MPa slab, say), follow their mix: a volume mix on site is an approximation of a design strength, not a guarantee of it.
The figures include water at a water to cement ratio of about 0.5, which is about 25 litres per bag. Too much water is the most common reason site concrete is weaker than it should be.
How it is worked out
- Wet volume is the element size multiplied out (length × width × thickness, and so on), times how many there are.
- Dry volume = wet volume × 1.54. The sand fills the gaps between the stones and the cement fills the gaps in the sand, so far more dry material goes in than concrete comes out.
- The dry volume is split by the mix ratio. Cement is taken at 1,440 kg per m³ (28.8 bags of 50 kg), river sand at 1.6 tonnes per m³ and stone at 1.5 tonnes per m³.
- Water is 0.5 litres per kg of cement.
What it assumes
- Reinforcement and formwork are not included; the steel calculator does the bars and mesh.
- Volume batching with a wheelbarrow or gauge box, not weigh batching.
Common questions
How many bags of cement in a cubic metre of concrete?
About 6.5 bags for 1:2:4 and about 8 bags for 1:1.5:3, using 50 kg bags. A lean 1:3:6 needs a little under 5.
What mix should I use for a house slab?
A ground-floor surface bed is commonly 1:2:4. A suspended slab, columns and beams are structural and should be cast to the engineer's specification, usually 20 to 25 MPa.
Why river sand for concrete?
Concrete strength comes from a clean, well-graded mix of particles. River sand is coarser and has less clay and silt than pit sand, which would weaken the bond with the cement.
Want exact quantities for your plan?
A calculator works one wall or one slab at a time. A bill of quantities works from your actual drawings and counts everything, which is what builders price against and what stops you buying twice.