A concrete mix ratio like 1:2:3 tells you the proportions of cement : sand : coarse aggregate, measured by volume. It’s the oldest and simplest way to specify a mix, and for site-batched concrete it’s still the standard. Understanding what the numbers mean — and the one correction factor everyone forgets — will keep your batches consistent and your concrete strong.
Reading the ratio
In a 1:2:3 mix, every 1 part of cement gets 2 parts of sand (fine aggregate) and 3 parts of gravel or crushed stone (coarse aggregate). “Part” can be any consistent container — a bucket, a wheelbarrow, a gauge box. What matters is that you use the same measure for all three ingredients and don’t switch mid-job.
Water isn’t usually written into the ratio, but it controls strength more than anything else. A water-cement ratio around 0.45–0.55 by weight gives a workable mix without gutting strength. More water makes placing easier and the finished concrete weaker — every extra liter is a trade.
Common ratios and where they belong
| Ratio | Approx. strength | Nominal grade | Typical use |
|---|---|---|---|
| 1:3:6 | ~10 MPa (1450 psi) | M10 | Blinding, mass fill, leveling |
| 1:2:4 | ~15–20 MPa | M15–M20 | Footings, driveways, general slabs |
| 1:1.5:3 | ~20 MPa (2900 psi) | M20 | Structural slabs, beams, columns |
| 1:2:3 | ~20–25 MPa | ~M20–M25 | Slabs, patios, general structural work |
| 1:1:2 | ~25+ MPa | M25 | Higher-strength structural elements |
The classic 1:2:4 is the workhorse for residential work: footings, garden slabs, post bases, and paths. 1:2:3 uses relatively more cement per unit of aggregate, giving a slightly richer, stronger, more workable mix — a good default for slabs you’ll finish smooth. Our 1:2:3 mix calculator and 1:2:4 mix calculator work out exact material quantities for either.
The 1.54 dry volume factor
Here’s the mistake that leaves people short of material: if you need 1 m³ of finished concrete, you can’t just batch 1 m³ of combined dry ingredients. When water is added and the mix is compacted, sand particles pack into the voids between the gravel, and cement paste fills the gaps between sand grains. The mixture shrinks.
The standard correction is the dry volume factor of 1.54:
Dry volume = wet (finished) volume × 1.54
Worked example: 1 m³ of 1:2:4 concrete
- Dry volume needed: 1 × 1.54 = 1.54 m³
- Total ratio parts: 1 + 2 + 4 = 7
- Cement: 1.54 × 1/7 = 0.22 m³ (≈ 6.3 bags of 50 kg cement, at 0.035 m³ per bag)
- Sand: 1.54 × 2/7 = 0.44 m³
- Aggregate: 1.54 × 4/7 = 0.88 m³
Skip the 1.54 factor and you’d batch 35% too little of everything. The material calculator applies this automatically for any ratio and volume.
Ratio mixes vs designed mixes
Volume-batched “nominal” mixes are convenient but conservative — the actual strength depends on aggregate moisture, grading, and how carefully you measure. For structural work where a specific strength is legally required (suspended slabs, retaining walls, anything engineered), specify a grade (like M20 or M25) from a ready-mix plant instead; the plant weighs materials and tests cubes to guarantee it. For grade-based estimating, use the M20 calculator or M25 calculator.
Practical batching tips
- Measure sand loose but consistent. Damp sand bulks up to 25% in volume; if your sand is wet, heap the measure slightly.
- Mix dry first. Blend cement, sand, and aggregate to a uniform color before adding water.
- Add water gradually. Aim for a mix that holds its shape when squeezed but still flows off a shovel.
- Small, frequent batches beat one giant one. Concrete begins stiffening in about 45–60 minutes in warm weather.
Get the ratio right, apply the 1.54 factor, and control your water, and site-mixed concrete will perform reliably for decades.