Concrete Calculator
Cement bags, sand and aggregate from the volume of a slab.
Recent
Formula
How the calculation works
The wet volume is length times width times thickness. That is the volume of the hole the concrete fills. The dry volume is about 1.54 times it, because dry sand and gravel carry voids that close up once water and cement are mixed in, so the dry materials have to be bought in the larger amount.
The mix ratio then splits that dry volume. On 1:2:4 there are seven parts in all -- one cement, two sand, four stone -- so cement is one seventh of the dry volume. Bags come from dividing the cement volume by 0.0347 cubic metres, the space one 50-kilogram bag fills, and that is rounded up because a shop sells whole bags.
Common mistakes
- Multiplying thickness in centimetres by length in metres without converting. 15 cm is 0.15 m, not 15.
- Ordering the wet volume of dry materials, which leaves the job short by about a third.
- Rounding the bags down. A shop sells whole bags, and running out mid-slab is worse than a spare bag.
When to use it
- Use it to order materials for a slab, footing or path, and to check a contractor's list.
- It gives material quantities, not a structural design. For anything load-bearing, have it designed.
Worked example
10 × 5 m at 0.15 m on 1:2:4: 7.5 cubic metres wet, about 56 bags of cement.
Common questions
Why the 1.54 factor?
Dry sand and gravel have voids that close up when water and cement are added, so the dry volume is about 1.54 times the wet. That is the usual rule for working out the mix quantities.
Why whole bags?
Because shops do not sell part of a bag: a sum of 55.2 means buying 56. Sand and aggregate stay in cubic metres, since those are sold by weight or volume.
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