A 1:2:4 concrete mix is one part cement, two parts sand and four parts coarse aggregate, measured by volume. It produces concrete of roughly C20 strength in UK terms, or about 3,000 psi in US terms, and it’s the standard general-purpose mix for slabs, footings, paths and shed bases.
What the three numbers mean
The ratio describes proportions by volume, not weight. One bucket of cement, two buckets of sand, four buckets of gravel. Seven buckets total, which is worth remembering because it makes the arithmetic straightforward.
Each ingredient has a job. Cement is the binder — it reacts with water and glues everything else together. Sand is the fine aggregate that fills the gaps between the stones. Coarse aggregate (gravel or crushed stone) is the bulk of the material and provides most of the compressive strength.
Water isn’t in the ratio, which trips people up. It’s specified separately as a water-cement ratio, typically around 0.5 by weight for a 1:2:4 mix.
What strength it gives you
| Mix ratio | UK grade | EU (EN 206) | US strength | Typical use |
|---|---|---|---|---|
| 1 : 4 : 8 | C10 / GEN0 | C12/15 | ~1,500 psi | Blinding, mass fill |
| 1 : 3 : 6 | C15 / GEN1 | C16/20 | ~2,200 psi | Kerb bedding, fill |
| 1 : 2 : 4 | C20 / GEN3 | C20/25 | ~3,000 psi | Slabs, footings, paths |
| 1 : 1.5 : 3 | C25 / ST2 | C25/30 | ~3,600 psi | Driveways, foundations |
| 1 : 1 : 2 | C30+ | C30/37 | ~4,400 psi | Structural, heavy duty |
There’s a fuller explanation of these designations in our guide to concrete grades.
Materials for one cubic metre
The calculation uses the dry-volume method. Loose materials contain air, and when you mix and compact them the total volume shrinks by roughly a third — so you need about 1.54 m³ of dry material to make 1 m³ of concrete.
| Material | Quantity per m³ | In practical terms |
|---|---|---|
| Cement | 317 kg | 13 bags at 25 kg |
| Sand | 0.44 m³ / 0.70 t | Roughly one bulk bag |
| Coarse aggregate | 0.88 m³ / 1.32 t | Roughly 1.5 bulk bags |
| Water | ~160 litres | Adjust to consistency |
The mix ratio calculator runs these figures for any volume and any ratio, and converts to whichever bag size you’re buying.
Measuring by bucket, not by shovel
This is where most site-mixed concrete goes wrong. A shovel load varies by 30% or more depending on who’s holding it, how tired they are, and whether the material is wet. Over a day’s mixing that variation is enormous.
Use a bucket — the same bucket for everything — and count. One cement, two sand, four aggregate. If your mixer takes half that, use one cement to two to four at half scale rather than trying to eyeball a proportion.
Fill each bucket the same way each time: loosely filled and struck level, not heaped and compressed. Consistency between batches matters more than absolute precision.
Where 1:2:4 is right, and where it isn’t
Right for: shed and garden building bases, garden paths, patios, non-structural footings, fence post haunching, general repairs, over-site concrete.
Step up to 1:1.5:3 for: driveways carrying cars, garage floors, anything with vehicle loading, exposed work in freeze-thaw climates.
Step down to 1:3:6 or leaner for: blinding layers, mass fill, haunching behind kerbs — anywhere the concrete is filling space rather than carrying load.
Don’t use it for: structural elements designed by an engineer. Those come with a specified grade and usually need ready-mix with certification.
Adjusting for wet aggregate
Sand delivered loose and left uncovered can hold 5 to 10% water by weight. That’s meaningful — a bulk bag of wet sand contains 40 to 80 litres of water you’re not accounting for.
Two adjustments help. Reduce the added water and bring the mix up to consistency slowly. And be aware of bulking: damp sand occupies up to 25% more volume than dry sand, so a bucket of wet sand contains less actual sand than a bucket of dry. If your sand is soaking, add a little extra.
Using all-in ballast instead
Ballast is pre-blended sand and gravel, which reduces three materials to two. The equivalent of 1:2:4 using ballast is 1:6 cement to ballast.
It’s easier to handle and harder to get wrong, at the cost of some control over the sand-to-stone ratio. For most domestic work that trade is worth making. Quantities are in the ballast calculator.
Why this ratio in particular
1:2:4 is not arbitrary. The proportions come from packing theory: coarse aggregate forms the skeleton, sand fills the voids between the stones, and cement paste fills what is left and binds the whole thing.
Four parts stone to two parts sand happens to fill those voids closely for typical 20mm graded aggregate. Too little sand and you get honeycombing, where gaps between stones never close. Too much and you displace stone with weaker material, losing strength and increasing shrinkage.
The ratio survived for a century because it works with the materials a builders merchant actually sells, and it is forgiving of the imprecision of site mixing.
Getting the aggregate size right
The ratio says nothing about the size of the coarse aggregate, but it matters. Standard concrete uses 20mm (¾ in) stone, which suits slabs, footings and most general work.
| Aggregate size | Use it for | Why |
|---|---|---|
| 20mm / ¾ in | Slabs, footings, general work | Standard, best strength-to-cost |
| 10mm / ⅜ in | Thin sections, congested rebar, post holes | Flows into tight spaces |
| 40mm / 1½ in | Mass fill, thick foundations | Cheaper, less cement needed |
The rule is that the largest stone should be no more than a quarter of the section thickness, and it must pass comfortably between reinforcement bars. Using 20mm stone in a 60mm topping gives you a rough surface and poor compaction; drop to 10mm.
Common mistakes with 1:2:4
Measuring by shovel. A shovel load varies by 30% between people and across a day. Use a bucket and count — the same bucket for every ingredient.
Adding water to make it easier. The single fastest way to weaken concrete. If the mix is stiff, use a plasticiser rather than the hose.
Using building sand instead of sharp sand. Soft building sand is for mortar. Concrete needs sharp (grit) sand, which has angular particles that lock together. The wrong sand costs you significant strength.
Under-mixing. Three to five minutes after the last water goes in. A batch that still shows streaks of dry cement has pockets of unbound material in it.
Not accounting for damp sand. Wet sand carries water you haven’t measured and bulks up in volume, so a bucket contains less actual sand than you think.
Common questions
Is 1:2:4 by weight or volume?
By volume — buckets, not scales. Weight-batched mixes are specified differently, and the numbers wouldn’t be the same because the materials have different densities.
How much water for a 1:2:4 mix?
Roughly 0.5 to 0.55 by weight of cement, which is about 12 to 14 litres per 25 kg bag of cement. Add it gradually and stop when the mix holds a ridge without slumping.
Can I use 1:2:4 for foundations?
For domestic strip footings and garden wall foundations, yes — it’s the standard site mix for that work. Anything designed by an engineer will specify its own grade, which you should follow rather than substituting.
What is the difference between 1:2:4 and 1:2:3?
1:2:3 has less coarse aggregate relative to cement, making it richer and stronger — around C25 rather than C20. It’s a common strong general-purpose mix in the US.
How many bags of cement for a 1:2:4 mix?
About 13 bags of 25 kg per cubic metre of finished concrete, or 7 bags at 50 kg. The mix ratio calculator gives exact figures for your volume and bag size.