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.
If your supplier says c20 and you meant m20
The M numbering above is the Indian standard, IS 456, and it is used widely across South Asia, the Middle East and parts of Africa and South East Asia. In Britain and Europe the same idea is written with a C, and the two do not line up the way the numbers suggest.
The reason is what is being crushed. An M number is a cube strength: M20 is 20 MPa measured on a 150 mm cube at 28 days. Eurocode writes its classes as a pair — C20/25 — where the first number is the strength of a 150 mm diameter cylinder and the second is the equivalent cube. A cylinder of the same concrete tests at roughly 80 percent of the cube, which is why there are two numbers rather than one.
So C20/25 is not M20. Its cube strength is 25, which puts it alongside M25. Matching the digits gets you a grade weaker than you asked for:
- C8/10 → cube strength 10 → the M10 of the two systems
- C12/15 → cube strength 15 → M15
- C16/20 → cube strength 20 → M20
- C20/25 → cube strength 25 → M25
The rule is simple once you see it: compare the SECOND number of a Eurocode pair with the M number, never the first.
The other c, and why it muddies this
A UK ready-mix supplier quoting a bare "C20" with no second number is usually not speaking Eurocode. That is the older British designation, where the single number was itself a cube strength — and that one does land near M20. Same two letters, different meaning, opposite answer.
Which is why it is worth one question at the counter: is that number a cube or a cylinder? If they give you a pair, take the second. If they give you a single number, it is almost certainly a cube strength, and it maps to the M grade with the same digits.
You may also be quoted a designated mix instead — GEN1, GEN3, ST2, RC25/30 and similar. Those are named for the job rather than the ratio, and there is no reliable way to translate them by eye. Ask which strength class the mix corresponds to and work from that.
None of this changes what the calculator does. The ratios are what go in the mixer, and 1:2:4 is 1:2:4 whatever the bag is labelled. What the letter changes is which ratio you should have picked.
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.