Cement quantity calculator
Cement quantity calculator
Bags of cement a volume of concrete needs, from the mix ratio on the drawing and the mass printed on the sack.
How this is calculated
Wet volume = length x width x thickness. Dry volume = wet volume x the dry-volume factor. Each material takes its share of the dry volume in the ratio, so cement fills dry volume x cement part / (cement + sand + aggregate), and its mass is that volume x the cement's mass per cubic metre.
- Bags are rounded up at the end because nobody is delivered a fraction of a sack; the exact mass above is the figure the arithmetic actually produced.
- This is quantity arithmetic, not a mix design. The ratio that reaches a target strength is settled by trial mixes and by the structural drawing, and water content, admixtures and the grading of the sand on site are outside what a page can compute.
A concrete mix is a ratio, and a ratio turned into bags is three multiplications. The numbers that make the difference between a correct order and eleven sacks standing in the rain are the ones on your drawing, not the ones on a website.
What the page deliberately leaves blank
The mix ratio, the dry-volume factor and the bulk density of the cement are all asked for rather than filled in. Each is a convention with more than one honest value: a 1:2:4 and a M20 are not the same thing, the factor between wet and dry measure is quoted differently by different manuals, and the density of a cement depends on the product and on how long the sack has sat in the yard.
A page that quietly picked one of each would produce an answer that looks authoritative and is only one author’s default. Only the mass of a bag is pre-filled, at 50 kg, because that is what an Indian sack carries — and it stays editable for the day yours does not.
Worked through
A slab 3 m by 2 m at 150 mm is 0.900 m³ of finished concrete. At a dry-volume factor of 1.54 that becomes 1.386 m³ of loose material, and in a 1:2:4 mix the cement takes one seventh of it — 0.198 m³. At 1,440 kg per cubic metre that is 285.12 kg, which is 5.70 bags, so six are ordered.
The sand and aggregate come out of the same 1.386 m³ in the ratio’s other two parts: 0.396 m³ of sand and 0.792 m³ of coarse aggregate. Those three volumes add back to the dry volume exactly, which is the check to run on any quantity sheet — if they do not, the ratio was divided against the wrong number.
Questions readers ask
Why not just publish the 1:2:4 answer for a slab?
Because the mix is a structural decision, not a convenience. A nominal 1:2:4 and an M20 concrete are close but not identical, and the one your drawing names is the one your cover, your loads and your engineer agreed on. Substituting a general default changes the strength you are buying.
What is a dry-volume factor doing in the formula at all?
Loose sand and aggregate hold air between their grains; once concrete is mixed and compacted, the same material occupies less space. Ordering against the finished volume therefore under-buys, and the factor is the allowance for that. Its exact value is a convention, which is why the field asks rather than answers.
Is the cement figure by volume or by weight?
Both lines are shown. The ratio is a volume ratio, so the cement is first found as a volume and then converted with the density you entered. A supplier who sells by the tonne and a site that counts sacks need those two figures to agree, and they only do if the density is the one on that cement’s data sheet.
Should I round the bags up or order the exact amount?
Round up — the exact figure is printed next to it so you can see how much the rounding cost. Ordering down to save a sack is a false economy on a pour that cannot be stopped halfway, because the last part of a slab made with short cement is not the same slab.
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