How do you use the cement calculator?
- Pick the work: concrete, brickwork or plaster. The usual mix and thickness fill in.
- Enter the length and the width (or height, for a wall) in feet, the thickness in millimetres and the price of a bag.
- Read the bags of cement and the sand and aggregate. Change the mix, dry volume factor, bag size and wastage under “Adjust mix, factors and bag size”.
How is cement worked out from a mix ratio?
A mix such as 1:2:4 gives the parts of cement, sand and aggregate by volume. Wet concrete or mortar takes up less space than the dry materials that go into it, because the sand fills the gaps between the stones and the cement fills the gaps in the sand. Site estimates allow for this with a dry volume factor, commonly 1.54 for concrete and 1.33 for mortar. The cement’s share of the dry volume is then converted to kilograms and bags.
For brickwork, the bricks take up most of the wall and the mortar only fills the joints. The calculator takes mortar as 25% of the wall volume, a common rule of thumb you can change; counting the whole wall as mortar would overstate the cement about four times (25 bags instead of 7 for a 20 × 10 ft wall).
How the quantities are worked out
Length × width × thickness (× mortar share, for brickwork)Wet volume × dry volume factor × (1 + wastage %)Dry volume × cement parts ÷ total parts × 1,440 kg per m³ ÷ bag size, rounded upDry volume × their parts ÷ total partsWorked example: 20 × 10 ft slab, 150 mm thick
The slab holds 2.79 m³ of concrete. At 1:2:4 with a 1.54 dry factor and 5% wastage, it needs 927 kg of cement, or 19 bags of 50 kg (₹ 7,220 at ₹ 380 a bag), plus 45.5 cu ft of sand and 90.9 cu ft of aggregate.
Worked example: plastering a 12 × 10 ft wall
A 12 mm coat of 1:4 plaster on 120 sq ft is 0.134 m³. With a 1.33 dry factor and 5% wastage it needs 1.1 bags, so you buy 2, and 5.3 cu ft of sand.
How much cement per cubic metre?
| Mix | 50 kg bags | Sand, cu ft | Aggregate, cu ft |
|---|---|---|---|
| Concrete 1:2:4 | 6.34 | 15.5 | 31.1 |
| Concrete 1:1.5:3 | 8.06 | 14.8 | 29.7 |
| Mortar 1:6 | 5.47 | 40.3 | 0.0 |
| Mortar 1:4 | 7.66 | 37.6 | 0.0 |
What should you check on site?
- Structural concrete for slabs, beams, columns and footings should follow your structural engineer’s drawings and mix design, not a generic ratio.
- Check the bag’s manufacturing date and keep bags dry and off the floor.
- Order a little extra sand; it bulks when damp, so a load measures more than it holds.
Frequently asked questions
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Sources
- Dry volume factors (1.54 concrete, 1.33 mortar), cement at 1,440 kg per m³ and mortar at 25% of brickwork: common site-estimation conventions, all editable except the cement density.
- Unit conversions: 1 m³ = 35.3147 cu ft; 1 ft = 304.8 mm (exact).
- Dimensions, mix, prices and wastage: your inputs. No market prices are assumed.
Indicative quantities for planning, not a structural design. For structural work, follow your structural engineer’s drawings. For GST on materials, indicative only.
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