For small jobs mixed on site, M20 concrete is often made to the nominal mix 1 : 1.5 : 3 (cement : sand : coarse aggregate) by volume. Here is how to turn that ratio into quantities you can order.
Step 1 — Allow for dry volume
Dry materials shrink in volume once water is added and the mix is compacted. The usual allowance is a factor of 1.54, so 1 m³ of wet concrete needs about 1.54 m³ of dry materials.
Step 2 — Split by the ratio
The ratio adds up to 1 + 1.5 + 3 = 5.5 parts. Each material’s share of the 1.54 m³ is its part of the ratio divided by 5.5.
| Material | Calculation | Quantity for 1 m³ |
|---|---|---|
| Cement | 1.54 × 1 ÷ 5.5 | 0.28 m³ ≈ 403 kg ≈ 8 bags (50 kg) |
| M-Sand | 1.54 × 1.5 ÷ 5.5 | 0.42 m³ |
| 20 mm blue metal | 1.54 × 3 ÷ 5.5 | 0.84 m³ |
| Water | w/c ratio ≈ 0.5 | ≈ 200 litres |
Cement is converted using its density of about 1,440 kg/m³.
Ordering by weight
Sand and aggregate are often sold by the tonne. Loose bulk density varies with moisture and grading — roughly 1.5 to 1.7 tonnes per m³ for M-Sand and 1.45 to 1.6 tonnes per m³ for 20 mm aggregate. Confirm the conversion with your supplier before ordering, and add 5% for wastage.
Nominal mix vs design mix
The nominal mix is a rule of thumb for small site work. Structural concrete for slabs, beams and columns should follow a design mix (IS 10262) that sets exact proportions for the materials actually used. Ready mix concrete is supplied to a design mix — another reason many builders now order RMC for slabs.