Concrete Mix Ratios Explained (1:2:4, M20, and More)

What a Mix Ratio Actually Means

A mix ratio is the proportion of cement to sand to coarse aggregate, given by volume, in that order.

In 1:2:4, for every one part of cement you add two parts of sand and four parts of aggregate, the crushed stone or gravel. “Part” just means a unit of measure you keep consistent: one bucket of cement, two of sand, four of stone. Cement is the binder that hardens and glues everything together, sand fills the gaps between the larger stones, and aggregate provides the bulk and much of the strength. The ratio is by volume in these traditional mixes, which is why it is sometimes called a nominal mix. Water is added separately, and how much you add is a decision in its own right, covered further down.

What the “M” Number Means

Concrete is graded by strength, and the grade is written as an M followed by a number, such as M20.

The M stands for mix, and the number is the characteristic compressive strength in newtons per square millimetre, which is the same as megapascals, measured on a standard cube after 28 days of curing. So M20 concrete is designed to reach 20 N/mm² at 28 days, and M25 reaches 25. A higher number is stronger concrete. Strength climbs as you use more cement relative to sand and aggregate, which is why a richer ratio like 1:1:2 produces a higher grade than a leaner 1:3:6. The grade is what a structural drawing specifies, and the ratio is the traditional recipe that gets you roughly there.

Common Mix Ratios and Their Grades

These nominal mixes cover most small and medium jobs. The bag figures are approximate and explained in the calculation section below.

For most reinforced concrete around a home, M20 at 1:1.5:3 is the workhorse. Anything M25 and above is usually specified by an engineer and mixed to a tested design rather than a nominal ratio.

The Water-Cement Ratio

This is the part people underestimate, and it undoes a good ratio faster than anything else.

The water-cement ratio is the weight of water divided by the weight of cement in the mix, and it typically sits between about 0.4 and 0.6 for ordinary concrete. Less water gives stronger, more durable concrete, but it is stiffer and harder to place. More water makes the mix flow easily, which is tempting on site, but every extra splash of water lowers the final strength and invites cracking as it dries. The most common site error is adding water to make the mix easier to work with, quietly turning your M20 recipe into something much weaker. A workable M20 mix usually lands near a 0.5 water-cement ratio. If the concrete needs to be more fluid, the right fix is a plasticiser admixture, not more water.

Nominal Mixes vs Design Mixes

There are two ways to specify concrete, and knowing which you are using tells you how much to trust the ratio.

A nominal mix is the traditional fixed recipe, the 1:2:4 and 1:1.5:3 ratios above. It is simple, needs no laboratory, and is fine for small non-critical work and lower grades up to about M20. A design mix is worked out by a professional who tests the actual materials, the cement strength, the aggregate size and moisture, and calculates proportions to hit a target strength reliably. Codes such as India’s IS 456 recommend design mixes for structural concrete and for grades above M20, because the nominal ratios carry a safety margin that gets wasteful and unreliable at higher strengths. For a garden path, a nominal mix is sensible. For a load-bearing beam, the ratio should come from an engineer, not a table.

How Many Cement Bags per Cubic Metre

The bag counts in the table come from one calculation, and it is worth seeing once so the numbers are not a mystery.

Dry materials pack down when mixed with water, so you start with more loose volume than the wet concrete you end up with. The standard allowance for this is a dry-volume factor of 1.54, meaning 1 cubic metre of finished concrete needs about 1.54 cubic metres of dry material. Take M20 at 1:1.5:3, where the parts add up to 5.5. The cement fraction is 1 out of 5.5, so cement volume is 1.54 × (1 ÷ 5.5) = 0.28 m³. Cement weighs roughly 1,440 kg per cubic metre, so that is 0.28 × 1,440 = 403 kg. At 50 kg per bag, that is about 8 bags per cubic metre of M20, which matches the table. Swap in the parts for any other ratio and the same method gives its bag count.

Common Mistakes

• Adding extra water for workability. It is the fastest way to weaken concrete; use an admixture instead if you need flow.
• Treating a nominal ratio as good enough for structural, load-bearing work. Above M20 and for anything the building relies on, use an engineered design mix.
• Measuring by the shovel instead of a consistent container, so the ratio drifts batch to batch. Use the same bucket for every part.
• Forgetting the 1.54 dry-volume factor and ordering too little material, then running short mid-pour.
• Skipping curing. Even a perfect mix loses strength if it dries too fast in the first days; keep it damp.

Frequently Asked Questions

What does a 1:2:4 concrete mix mean?

It means one part cement, two parts sand, and four parts coarse aggregate, measured by volume in that order. A “part” is any consistent unit, such as a bucket. The 1:2:4 nominal mix corresponds to grade M15, giving about 15 N/mm² of compressive strength at 28 days, and suits paths, kerbs, and light foundations.

What is the difference between M20 and M25 concrete?

The number is the compressive strength in newtons per square millimetre at 28 days, so M20 reaches about 20 and M25 about 25. M25 is stronger because it uses more cement relative to sand and aggregate, roughly a 1:1:2 ratio versus 1:1.5:3 for M20. M20 covers most home reinforced concrete; M25 and above is for heavier structures.

What is the best concrete mix ratio for a slab?

For a reinforced home slab, M20 at 1:1.5:3 is the usual choice, giving good strength for columns, beams, and floor slabs. Non-structural bases can use the leaner 1:2:4 (M15). For any load-bearing structural slab, the exact mix should be specified by an engineer rather than taken from a general table.

Why does adding more water weaken concrete?

Strength depends heavily on the water-cement ratio: more water spaces the cement particles further apart and leaves tiny voids as it evaporates, reducing density and strength. Extra water also increases drying shrinkage and cracking. A workable mix usually needs a ratio near 0.5; if you need it more fluid, use a plasticiser admixture rather than more water.

How many bags of cement are in a cubic metre of M20?

About 8 bags of 50 kg cement per cubic metre. It comes from the dry-volume factor of 1.54, the cement fraction of 1 in 5.5 parts, and cement density near 1,440 kg per cubic metre: 1.54 divided by 5.5, times 1,440, gives roughly 403 kg, which is about 8 bags. Sand and aggregate scale from the same parts.

What is the water-cement ratio?

It is the weight of water divided by the weight of cement in a mix, usually between 0.4 and 0.6 for ordinary concrete. A lower ratio gives stronger, more durable concrete but a stiffer mix; a higher ratio is easier to place but weaker. Controlling it is one of the most important steps in getting the strength the grade promises.

Can I use a nominal mix for a structural beam?

It is not recommended. Nominal ratios carry a rough safety margin that becomes unreliable and wasteful at higher grades, so codes call for a tested design mix for structural elements and for grades above M20. For a load-bearing beam, the mix, reinforcement, and dimensions should all come from a structural engineer.

The Bottom Line

A mix ratio sets the proportions of cement, sand, and aggregate, and it maps to a strength grade: 1:2:4 is M15, 1:1.5:3 is M20, and richer mixes give higher grades. The water-cement ratio matters just as much, so resist adding water for workability. Use nominal ratios for small non-structural jobs and an engineered design mix for anything load-bearing or above M20. When you have your ratio and volume, the concrete calculator turns them into exact material quantities.