Concrete Calculator
Work out how much concrete you need for a slab, footing, beam or column, then see the cement bags, sand and aggregate to order.
How the calculator works
The tool first finds the wet volume of the member, adds your wastage allowance, then converts that to a dry volume and splits it by the mix ratio.
- Slab, footing or beam:
V = length × width × depth - Rectangular column:
V = width × depth × height - Circular column:
V = π × (diameter ÷ 2)² × height - Dry volume:
wet volume × 1.54(the factor covers voids in loose materials) - Each material:
dry volume × its ratio part ÷ sum of ratio parts
Cement is converted to weight using 1,440 kg/m³, then divided into 50 kg bags. Sand and aggregate are shown by volume and approximate weight (1,600 kg/m³ and 1,500 kg/m³).
Worked example: a 5 m × 4 m slab, 150 mm thick
- Wet volume: 5 × 4 × 0.15 = 3.00 m³
- Add 5% wastage: 3.00 × 1.05 = 3.15 m³
- Dry volume: 3.15 × 1.54 = 4.851 m³
- For an M20 mix (1:1.5:3) the parts add up to 5.5.
- Cement: 4.851 × 1 ÷ 5.5 = 0.882 m³, which is about 1,270 kg, or 26 bags.
- Sand: 4.851 × 1.5 ÷ 5.5 = 1.32 m³
- Aggregate: 4.851 × 3 ÷ 5.5 = 2.65 m³
Typical mixes and where they are used
| Grade | Ratio | Common use |
|---|---|---|
| M7.5 | 1:5:10 | Lean concrete, levelling course under foundations |
| M10 | 1:4:8 | Plain concrete, non-structural bedding |
| M15 | 1:2:4 | Plain footings, pavements, light floors |
| M20 | 1:1.5:3 | Residential slabs, beams, columns |
| M25 | 1:1:2 | Heavier loads, water-retaining structures |
Check your local code and structural drawings. The design grade on the drawings always overrides a generic ratio.
Frequently asked questions
How much wastage should I allow?
5% is common for simple pours with accurate formwork. Use 7-10% for uneven ground, small pours or pumped concrete where some is lost in the line.
Why is the dry volume 1.54 times the wet volume?
Cement, sand and aggregate fill voids and compact once water is added, so you need roughly 54% more loose material than the finished volume. Some designers use 1.5 to 1.57.
Should I order ready-mix instead of mixing on site?
For pours above roughly 1 m³, ready-mix is usually more consistent and faster. Use the volume result here as your order quantity and ask the plant about their minimum load.
Does this include reinforcement?
No. Reinforcing steel displaces a very small amount of concrete, so it is ignored. Use a rebar weight calculation for steel quantities.
How accurate is the result?
It is a planning estimate. Site conditions, moisture bulking of sand, formwork tolerances and mix design can all shift real quantities, so confirm with a qualified engineer or estimator before ordering.
