Concrete calculator
How much concrete a slab, beam, column or pad footing needs, and the cement, sand, aggregate and water to batch it on site. Every assumption is shown beside the answer, and you can change it.
Result
- Concrete, as measured
- 3.6 m³
- Waste allowance (5%)
- 0.18 m³
- Concrete to batch
- 3.78 m³
- Dry materials
- 5.821 m³
- Cement
- 1,197.5 kg
- Cement bags to buy
- 24 of 50 kg
- Sand
- 1.663 m³
- Aggregate
- 3.326 m³
- Water
- 599 litres
Mix 1 : 2 : 4 (7 parts). Sand and aggregate are loose volumes; buy by the supplier’s unit (a truck, a tonne) and convert with their figure.
No material cost is estimated: the site has no verified cement price observed in the last 12 months, and it does not estimate a cost from an unverified or older figure.
What this calculator is for
Ordering and checking materials for a pour: how many bags of cement to buy, how much sand and aggregate to have on site, and how much water the batch takes. It is an estimate for planning, not a structural design, and it does not replace the engineer’s specification or a mix design.
How it works
- The element’s volume from its dimensions: length × width × thickness for a slab; length × width × depth for a beam or a footing; width × depth × height, or π/4 × diameter² × height, for a column. Times how many.
- Add the waste allowance: volume × (1 + waste %).
- Convert to dry volume: × the dry volume factor, because the loose dry materials shrink when water is added.
- Split the dry volume by the mix: cement is 1 part in (cement + sand + aggregate) parts, and so on.
- Cement by weight: its volume × the cement bulk density, then divided by the bag weight and rounded up to whole bags.
- Water: cement weight × the water-cement ratio (a kilogram of water is a litre).
Assumptions
Each one is an input above. None of them is a legal or standard requirement; they are the figures sites commonly work with.
| Assumption | Value used | Where it comes from |
|---|---|---|
| Waste allowance | 5 % | Site rule of thumb. Spillage, over-excavated bases and uneven formwork. Many sites allow 2 to 10 percent; the right figure depends on the work and the crew. |
| Dry volume factor | 1.54 | Site rule of thumb. Loose dry cement, sand and aggregate take more room than the finished concrete, because water fills the voids between particles. A factor of about 1.52 to 1.57 is widely used on site; 1.54 is the common figure. |
| Cement bulk density | 1440 kg/m³ | Material property. The loose bulk density of ordinary Portland cement, commonly taken as 1,440 kg per cubic metre for estimating. It varies with how the cement is packed. |
| Cement bag | 50 kg | Product size. Cement is commonly sold in 50 kg bags; change this if your supplier's bag is different. |
| Water-cement ratio | 0.5 | Site rule of thumb. Litres of water per kilogram of cement. Less water gives stronger, more durable concrete if it can still be placed and compacted; the specification or the mix design sets the maximum. |
Worked example
A ground-floor slab 5 m long, 4 m wide and 150 mm thick, mixed at 1 : 2 : 4 with a 5% waste allowance:
- Volume: 5 × 4 × 0.15 = 3 m³; with waste, 3.15 m³.
- Dry volume: 3.15 × 1.54 = 4.851 m³, in 7 parts.
- Cement: 4.851 ÷ 7 = 0.693 m³ × 1,440 kg/m³ = 997.9 kg, or 19.96 bags of 50 kg: buy 20.
- Sand: 4.851 × 2/7 = 1.386 m³. Aggregate: × 4/7 = 2.772 m³.
- Water at a water-cement ratio of 0.5: 499 litres.
Limitations
- Nominal volume mixes do not guarantee a strength. Structural concrete should be to the engineer’s specification and checked with cube tests.
- The dry volume factor and the cement density are site rules of thumb. Damp sand bulks, and different materials pack differently.
- Openings, sloping soffits, thickenings and haunches are not in the volume unless you add them.
- No reinforcement, formwork or labour is included.
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