How to use this calculator
- 1Choose what you are pouring — a slab or floor, a beam or column, or a round column — and enter its sizes. For a slab the thickness is the concrete only, not the soling or sand under it.
- 2Enter how many of the same shape there are, and keep about 10 % extra for spillage and uneven ground.
- 3Choose the mix: 1 : 2 : 4 for floors, footings and paths, or the mix on your drawings for slabs, beams and columns.
- 4Check the cement bag size (50 kg in Pakistan, India and the Gulf) and read the bags, sand and crush. Switch any box or result to cft (ft³) with its unit menu.
The formula
- 01Wet volume = length × width × thickness × how many × (1 + extra %)
- 02Dry volume = wet volume × 1.54 (loose materials shrink when mixed)
- 03Cement = dry volume × cement parts ÷ all parts × 1,440 kg/m³ → bags = cement ÷ 50 kg, rounded up
- 04Sand = dry volume × sand parts ÷ all parts · Crush = dry volume × stone parts ÷ all parts
- 05Water ≈ cement × 0.5 (litres)
Concrete mixed on site is measured by volume: in a 1 : 2 : 4 mix there are 7 parts, so cement is 1/7 of the dry materials, sand 2/7 and crush 4/7. The dry materials take about 54 % more space than the finished concrete, because the sand fills the gaps between the stones and the cement paste fills the gaps in the sand — hence the factor 1.54.
Loose cement weighs about 1,440 kg per m³, so a 50 kg bag is about 0.035 m³ (1.23 cft). That is why 1 m³ of 1 : 2 : 4 concrete needs about 6.3 bags, and 1 : 1.5 : 3 about 8.1 bags.
For plaster and brick mortar (cement and sand only) use the plaster calculator (cement–sand plaster) and the brick calculator, which add the right allowances for those jobs.
Worked example
A 3 m × 3 m slab, 100 mm thick, with 10% extra is 0.99 m³ of concrete.
In a 1 : 2 : 4 mix that takes 7 bags of 50 kg cement (314 kg), 0.44 m³ of sand and 0.87 m³ of crush, with about 157 L of water.
Tips before you buy
- Use clean, sharp sand and washed crush: mud and silt in the sand weaken the concrete.
- Measure the parts with the same box or bucket (a 'farma'), not by shovelfuls.
- Add only as much water as you need to work the mix — extra water makes weaker concrete.
- Keep new concrete wet for at least 7 days (curing), longer in hot, dry weather.
- For a roof slab, beams and columns, follow the mix and the steel on your engineer's drawings.
Frequently asked questions
How many cement bags are in 1 m³ of concrete?
About 6.3 bags of 50 kg for a 1 : 2 : 4 mix (buy 7), 8.1 bags for 1 : 1.5 : 3 and 4.4 bags for 1 : 3 : 6 — with 0.44 m³ (15.5 cft) of sand and 0.88 m³ (31 cft) of crush for 1 : 2 : 4.
How many cement bags do I need per 100 cft of concrete?
100 cft is 2.83 m³, so about 18 bags of 50 kg in a 1 : 2 : 4 mix, with about 44 cft of sand and 88 cft of crush. Enter your own sizes to include the extra for spillage.
Which mix should I use?
1 : 2 : 4 (about M15) is the usual mix for floors, footings and paths. Roof slabs, beams and columns are usually 1 : 1.5 : 3 (about M20) or richer — use the mix your engineer specifies.
Why is the dry volume 1.54 times the concrete?
Loose sand and crush have air gaps. When mixed with water the sand fills the gaps between the stones and the cement paste fills the gaps in the sand, so the mix shrinks by about a third. 1.54 is the standard quantity-surveying figure in South Asia.
How much cement do I need for plaster or brickwork?
That depends on the thickness and the mix: a 12 mm coat of 1 : 4 plaster takes roughly 1 bag of 50 kg for every 9–10 m² (about 100 sq ft). Use our plaster calculator (cement–sand plaster) and brick calculator for exact figures.
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Sources
- Nominal mixes by volume (1 : 2 : 4 ≈ M15, 1 : 1.5 : 3 ≈ M20) and the 1.54 dry-volume factor: standard quantity-surveying practice in South Asia (IS 456 nominal mixes); cement bulk density 1,440 kg/m³ — docs/specs/mixing.md.
- Tile flooring and mortar worked examples using the same 1,440 kg/m³ and dry-volume method — Civil Sir
Formulas last reviewed: September 30, 2026 ·How we check our formulas