CalcPenguin

Concrete Calculator

Enter your slab or footing size to get the concrete volume and how many 50 kg cement bags, and how much sand and crush, to buy.

Your numbers are saved in this browser.

In inches when measuring in feet (a slab is often 4 to 6 inches); in centimetres when measuring in metres.

%

Cement

24 bags of 50 kg

Concrete
3.72 m³ (131.2 cft)
Sand
58 cft (1.64 m³)
Crush (aggregate)
115 cft (3.27 m³)

A 20 × 15 ft slab 5 inches thick needs about 3.72 m³ of concrete: 24 bags of cement, 58 cft of sand and 115 cft of crush at 1 : 2 : 4.

How it's worked out

  1. Concrete volume20 ft × 15 ft × 5 in ÷ 123.54 m³
  2. Plus wastage× 1.053.717 m³
  3. Dry materials3.717 × 1.54 (dry volume shrinks when mixed)5.724 m³
  4. Cement share5.724 × 1 ÷ 7 × 1,440 kg ÷ 50 kg23.55 bags
  • Nominal-mix estimate for planning and buying. Structural work should follow your engineer's design mix.

How the materials are worked out

Dry volume = wet volume × 1.54 · Cement = dry volume × cement share × 1,440 kg ÷ 50 kg

The mix ratio splits the dry volume between cement, sand and crush. For 1 : 2 : 4, cement is 1 part in 7.

Worked example

A 20 × 15 ft slab, 5 inches thick, is 125 cubic feet, or 3.54 m³. The dry volume is 5.45 m³, and cement is one seventh of that: 0.78 m³, or about 1,121 kg. That rounds up to 23 bags of cement, with about 55 cft of sand and 110 cft of crush.

Questions people ask

How many cement bags are in one cubic metre of concrete?

About 6.3 bags of 50 kg for a 1 : 2 : 4 mix, 8.1 for 1 : 1.5 : 3, and 4.4 for 1 : 3 : 6, using the usual 1.54 dry-volume factor.

Why multiply by 1.54?

Loose cement, sand and crush shrink when water is added and the gaps fill, so you need about 54% more dry material than the finished concrete volume.

Which mix should I use?

1 : 1.5 : 3 is common for slabs, beams and columns, 1 : 2 : 4 for general work and footings, and 1 : 3 : 6 for a plain concrete base under flooring. Structural work should follow your engineer's design.

How much extra should I order?

5% to 10% covers spillage, uneven ground and formwork that bulges.

Formula and rules last checked 25 October 2026Written and tested by the CalcPenguin teamReport a problem