What the numbers mean
A mix written as 1:2:4 is three parts measured by volume: one part cement, two parts sand, four parts coarse aggregate. The M number beside it — M15, M20, M25 — is the characteristic compressive strength in megapascals the mix should reach at 28 days, tested on a cube. M20 means 20 MPa.
The two notations describe the same mix from opposite ends. The ratio tells you what goes into the mixer. The M number tells you what should come out of it.
The four mixes and where they belong
- M10 (1:3:6): Blinding, mass fill, kerb backing. Concrete that fills space rather than carrying load.
- M15 (1:2:4): Light foundations, paths, shed and garage bases. The workhorse of domestic groundwork — carrying something, but not much.
- M20 (1:1.5:3): Reinforced slabs, ground beams, most residential structural work. The sensible default, and the one to use if you are unsure.
- M25 (1:1:2): Columns, suspended slabs, anything carrying real load or fully exposed to weather.
Richer is not automatically better. More cement means more heat of hydration and more drying shrinkage, and both of those mean cracking. Specifying M25 for a garden path buys you cracks, not strength you can use.
Why you order more dry material than you get concrete
This is where most hand estimates go wrong. Cement, sand and aggregate sitting in a heap are full of air between the particles. Add water and the fine material migrates into those voids, and the whole lot collapses to roughly two thirds of its loose volume.
The standard allowance for that collapse is 1.54. One cubic metre of finished concrete needs about 1.54 cubic metres of dry ingredients. Miss the factor and you are a third short — the single most common concrete ordering mistake there is.
From ratio to shopping list
Take a slab 4 m long, 3 m wide and 150 mm thick: 1.8 cubic metres of finished concrete. At M20, the ratio 1:1.5:3 totals 5.5 parts.
- Dry volume: 1.8 × 1.54 = 2.77 m³
- Cement: 2.77 × 1/5.5 = 0.50 m³
- Sand: 2.77 × 1.5/5.5 = 0.76 m³
- Aggregate: 2.77 × 3/5.5 = 1.51 m³
Cement is sold by the bag, not the cubic metre. At a bulk density of 1440 kg/m³ that 0.50 m³ weighs about 725 kg, or roughly nineteen 40 kg bags. Sand and aggregate are sold by weight at around 1.6 tonnes per cubic metre, so you are ordering about 1.2 tonnes of sand and 2.4 tonnes of aggregate.
Water, and why less is more
Water is the one ingredient where generosity costs you strength. Only about a quarter of the cement's weight in water is needed for the chemical reaction. The rest is there purely to make the mix workable, and it leaves voids behind as it evaporates.
A working figure is 180 litres per cubic metre of concrete, adjusted down if the sand is already wet. If the mix is too stiff to place, the answer is a plasticiser or a slightly richer mix — not another bucket of water. Every extra litre is strength you paid for and poured away.
Measure with a box, not a shovel
Mix ratios are volumetric, so a "part" is whatever container you use consistently: a bucket, a barrow, a gauge box. What it must not be is a shovel. Shovel loads vary wildly between materials and between the people wielding them, and a mix batched by shovel is a mix nobody can repeat next week.