Concrete Calculator

Element

Length, m

Width, m

Thickness or height, m

How many

Concrete grade

Result

Volume, m³2.4
With 5% allowance, m³2.52
Cement, kg852
50 kg bags18
Sand, kg1,704
Gravel, kg3,408
Water, l426

From dimensions to bags of cement

Concrete volume for a slab, strip footing or piles, broken down into cement, sand, gravel and water for the grade you choose.

Volume is length times width times thickness, and that is where cubic metres go missing — a strip footing measured along the wrong line, or a slab without its overhang. Round piles are computed from diameter, and a dozen of them easily add a cubic metre.

The mix depends on the grade. Cement runs from 265 kg per cubic metre at the weakest useful grade to 420 at the strongest, sand at twice that and gravel at four times. Water is about half the cement mass: more and you lose strength, less and it will not mix.

What this calculator shows

Volume, and the same figure with a five percent allowance
Cement by mass and by 50 kg bags
Sand, gravel and water in the same proportions

What to keep in mind

Proportions assume ordinary Portland cement and 5-20 mm aggregate; other materials shift the numbers.
Reinforcement barely affects volume — typically under one percent — and is not deducted.

FAQs

Which grade should I use?

M200 for paths and screed, M250 for light structures, M300 for a house strip footing, M400 for loaded elements and floor slabs.

What does a 1 : 2 : 4 mix mean?

Parts of cement, sand and gravel by mass — a general-purpose mix around grade M300. Proportions by volume differ because the densities do.

How much water goes in?

About half the cement mass, a water-cement ratio of 0.5. Extra water makes the mix easier to place and sharply weaker once cured.

Ready-mix or mix on site?

Past three cubic metres, ready-mix almost always wins: the grade is consistent and it goes in as one pour, which matters for a footing.

Worked example

A strip footing for a house

Input: 10 × 0.4 m strip, 0.6 m deep, grade M300

Output: 2.4 m³ of concrete, 852 kg of cement, 18 bags

Note: The five percent allowance is not optional: some mix stays in the mixer and on tools, and a footing has to be poured in one continuous go.

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Result

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