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Concrete Mix Ratio Calculator

A mix ratio is a shopping list waiting to be worked out. Tell the calculator how much concrete you need and which ratio you are using, and it returns cement bags, sand, gravel and water — the classic 1:2:4, the stronger 1:1.5:3, a lean 1:3:6, or your own numbers.

Mix proportions by volume124CementSandAggregate+ water — mixed to a workable consistency
Concrete mix proportions
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What the ratios mean

A ratio like 1:2:4 is one part cement, two parts sand, four parts coarse aggregate, measured by volume — buckets, not scales. The proportions set the strength class. Water is the variable nobody prints in the ratio, and it matters more than the rest.

Mix ratio UK grade EU (EN 206) US strength Typical use
1 : 4 : 8 C10 / GEN0 C12/15 ~1,500 psi Blinding, haunching, mass fill
1 : 3 : 6 C15 / GEN1 C16/20 ~2,200 psi Kerb bedding, unreinforced fill
1 : 2 : 4 C20 / GEN3 C20/25 ~3,000 psi Shed bases, footings, paths
1 : 1.5 : 3 C25 / ST2 C25/30 ~3,600 psi Driveways, slabs, foundations
1 : 1 : 2 C30+ C30/37 ~4,400 psi Structural, heavy duty

The EN 206 dual notation quotes cylinder strength then cube strength — C25/30 is the same concrete as UK C25, tested two ways. There is a fuller explanation in our guide to concrete grades.

Materials for 1 m³ of 1:2:4 (C20)

  • Cement — about 317 kg, which is 13 bags at 25 kg or 7 at 50 kg
  • Sand — 0.44 m³, roughly 0.70 tonnes
  • Gravel — 0.88 m³, roughly 1.32 tonnes
  • Water — around 160 litres

Those come from the dry-volume method: multiply the wet volume by 1.54 to account for the voids that disappear when the materials compact together. The calculator uses the same factor for any ratio you give it.

Water is the strength control

The water-to-cement ratio does more to set final strength than the cement content does. Around 0.5 by weight is the target — roughly 12 to 13 litres per 25 kg bag of cement. Every extra litre poured in to make the mix easier to shovel measurably weakens the finished concrete.

A mix that looks slightly too stiff in the barrow is usually right. If it needs to flow, use a plasticiser rather than a hose.

Frequently asked questions

What is the best all-round concrete mix ratio?

1:2:4 — one cement, two sand, four aggregate, roughly C20. It covers slabs, footings, shed bases and paths. Step up to 1:1.5:3 for driveways.

What is the mix ratio for C20 concrete?

1:2:4 by volume with a water/cement ratio near 0.55. Working from all-in ballast instead, use 1:6 cement to ballast.

How many bags of cement per cubic metre?

Around 13 bags of 25 kg for 1:2:4, 16 for 1:1.5:3, and 9 for a lean 1:3:6. The calculator gives the exact figure for your volume and bag size.

What is a 1:2:3 mix?

One cement, two sand, three gravel — a richer, stronger version of 1:2:4 at roughly C25. It is a common “strong general purpose” site mix in the US.

Does more cement always mean stronger concrete?

Only up to a point, and only if the water stays controlled. A cement-rich mix drowned in water ends up weaker than a leaner mix placed properly.

What if I only have sand and cement?

Then you are making mortar or screed, not concrete — no coarse aggregate means far lower strength. See the sand and cement calculator for those ratios.

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