Concrete Calculator
Instant results as you type.
mm
Cement (50 kg bags)
25
Sand (m³)1.26
Aggregate (m³)2.52
Water (litres)606
Concrete volume (m³)3
What this means3 m³ of M20 concrete. Dry material is taken at 1.54 times the wet volume, which is the standard allowance for voids in loose sand and aggregate.
Concrete is ordered by volume but mixed from three materials in fixed proportions, so the useful answer is not cubic metres — it is how many cement bags, how much sand and how much aggregate to have on site. This calculator converts your slab dimensions into all three for the standard nominal mixes, using the same 1.54 dry-volume allowance a site engineer applies.
How to use it
- Enter the length and width in metres and the thickness in millimetres.
- Choose the grade — M20 for slabs and footings, M25 for columns and beams.
- Read the cement bags, sand, aggregate and approximate water.
Examples
- A 5 x 4 m slab at 150 mm in M20: 3 m³ of concrete, 25 cement bags, 1.26 m³ sand, 2.52 m³ aggregate.
- The same slab in M25 needs more cement and less aggregate, because the ratio changes to 1:1:2.
Frequently asked questions
- How many cement bags are in one cubic metre of concrete?
- For M20 at a 1:1.5:3 nominal mix, roughly eight 50 kg bags per cubic metre once the dry-volume allowance is applied. M25 needs more, around eleven. The calculator works it out for the exact volume rather than making you scale a per-cubic-metre figure by hand.
- What do M15, M20 and M25 mean?
- The number is the characteristic compressive strength in newtons per square millimetre after 28 days. M15 (1:2:4) suits levelling and non-structural work, M20 (1:1.5:3) is the everyday choice for slabs and footings, and M25 (1:1:2) is used for columns and beams. Structural work should follow an engineer's specification, not a general rule.
- Why multiply the volume by 1.54?
- Because dry sand and aggregate contain air. A cubic metre of set concrete needs about 1.54 cubic metres of loose dry material, since the voids close up when water is added and the mix is compacted. Skipping this factor is the classic way to order a third too little.
- How much water should I add?
- Around half the weight of the cement, which is where the water figure here comes from — a water-cement ratio near 0.5. More water makes the mix easier to place and measurably weaker, so it is the most common and most damaging site shortcut. Use a plasticiser if the mix is hard to work.