Concrete Mix Ratios Explained: 1:2:4, Grades, and Bag Counts

A concrete mix ratio looks like a secret code — 1:2:4, 1:1.5:3 — but it's the simplest part of building. Those numbers are just the proportions of the three dry ingredients, and once you can read them you can specify the right concrete for a job and check that you're buying the right amount of cement instead of trusting a supplier's guess. This guide decodes the ratios, the grade system built on top of them, and the one factor that quietly decides whether your concrete is strong or crumbly.

What the numbers actually mean

A mix ratio is always three numbers in the same order: cement : sand : aggregate (aggregate being the crushed stone, called *crush* or *bajri* locally). They're measured by volume, not weight. So 1:2:4 means for every one part cement, you add two parts sand and four parts aggregate:

• Cement — the binder that chemically sets and glues everything together.
• Sand (fine aggregate) — fills the gaps between the stones and gives a workable mix.
• Aggregate / crush (coarse aggregate) — the cheap bulk and the compressive strength.
• Water — not in the ratio, but the most important ingredient of all (see below).

Ratios vs grades: M15, M20, M25

You'll also hear concrete described by a grade like M20. The M is for "mix" and the number is the target compressive strength in newtons per square millimetre (MPa) after 28 days. Nominal ratios map roughly onto the lower grades:

The factor that matters most: the water-cement ratio

Here's the thing most self-builders get wrong. The single biggest control on concrete strength isn't the sand-to-stone ratio — it's how much water you add relative to the cement. Too much water makes the mix easy to pour but dramatically weaker; too little makes it strong but unworkable. A water-cement ratio of around 0.4 to 0.5 (by weight) is the usual target for structural concrete.

Turning a volume into cement bags

Once you know your ratio and the volume you're pouring, you can work out the materials. The chain is: compute the wet volume, add ~50–54% for the "dry volume" (dry ingredients bulk up before water compacts them), then split by the ratio. As a rough anchor, 1 cubic metre of 1:1.5:3 (M20) concrete needs roughly 8 bags of cement (a 50 kg bag being the standard unit), plus the matching sand and aggregate.

Rather than do the dry-volume and ratio maths by hand for every pour, put your slab or column dimensions into the concrete calculator — it returns the volume and the cement-sand-aggregate split for the common mixes, so your material order is grounded in numbers instead of a supplier's estimate. If you're also walling the plot, the brick calculator does the same for brick and mortar, and the plot area calculator gets you the covered area those quantities scale from.

The bottom line

A mix ratio is just cement : sand : aggregate by volume, and smaller numbers mean richer, stronger concrete. Match the ratio to the job using the grade table — 1:1.5:3 (M20) for normal structural work — guard the water-cement ratio harder than anything else, and convert your volume to bags with the calculator instead of a guess. That's the whole science a home-builder needs.