The method (ratio → parts → bags and water)
A nominal mix ratio — 1:2:3, 1:2:4, 1:3:6 — is always written cement : sand : aggregate, by volume. It tells you the proportion of each ingredient, not the amount. To turn that into real bags and litres for a job, you work through five steps:
- Add the parts. A 1:2:4 mix has 1 + 2 + 4 = 7 total parts.
- Allow for dry bulking. Loose, dry cement, sand and aggregate have air gaps between the grains that close up once it’s mixed and poured — so the dry materials you start with take up more space than the wet concrete you end with. The standard allowance is dry volume ≈ wet volume × 1.54.
- Find one part. Divide the dry volume by the total parts.
- Multiply out each material. Cement parts × one part, sand parts × one part, aggregate parts × one part.
- Convert cement to weight, then bags. Cement’s bulk density is about 1440 kg/m³. Divide the weight by your bag size to get a bag count.
Then work out water separately, from the water-cement ratio (see below) — it’s not part of the 1:2:4 ratio at all.
Work it out by hand
Say you’re batching 1 m³ of concrete at a general-purpose 1:2:4 mix.
1. Total parts. 1 + 2 + 4 = 7.
2. Dry volume. 1 m³ × 1.54 = 1.54 m³.
3. One part. 1.54 ÷ 7 = 0.22 m³.
4. Each material.
- Cement: 1 × 0.22 = 0.22 m³
- Sand: 2 × 0.22 = 0.44 m³
- Aggregate: 4 × 0.22 = 0.88 m³
5. Cement to bags. 0.22 m³ × 1440 kg/m³ = 316.8 kg of cement. At 20 kg a bag: 316.8 ÷ 20 = 15.84 → round up to 16 bags.
6. Water, from the water-cement ratio. At a typical w/c of 0.50: 316.8 kg × 0.50 = 158.4 litres (water weighs about 1 kg per litre, so kilograms and litres are near enough interchangeable here).
So 1 m³ of 1:2:4 concrete takes roughly 16 bags of cement, 0.44 m³ of sand, 0.88 m³ of aggregate and 158 litres of water — plus your usual 5–10% wastage allowance once you’re ordering for a real pour.
Common nominal ratios
| Ratio (cement : sand : aggregate) | Typical use |
|---|---|
| 1 : 1.5 : 3 | Higher-strength work — driveways, load-bearing footings |
| 1 : 2 : 4 | General purpose — most slabs, paths, standard footings |
| 1 : 3 : 6 | Low-strength mass fill — blinding layers, non-structural backfill |
The water-cement ratio: your strength dial
The ratio’s parts tell you the bulk of the mix. The water-cement ratio (w/c) — the weight of water divided by the weight of cement — is what actually sets the concrete’s strength once it’s cured.
Lower w/c → stronger, denser concrete, but harder to place. Higher w/c → easier to work, but weaker and more prone to shrinkage cracking.
For most site work, aim for a w/c around 0.45–0.55. Going below about 0.40 usually needs a plasticiser to keep it workable; going above about 0.60 starts costing you real strength. If a mix is too stiff to place, add more cement paste (cement and water together, kept in ratio) rather than a splash of water on its own — water alone is the fastest way to quietly weaken a pour.
Do it in 5 seconds
Related
- Pillar: Concrete Calculations: The Complete On-Site Guide — every concrete sum in one place.
- Get your volume first: How to Calculate Concrete Volume for a Slab.
- Bags and yield: How Many Bags of Concrete Per m³? — for pre-mix bags rather than batching your own.
Sources
- Portland Cement Association, Design and Control of Concrete Mixtures — proportioning principles and the water-cement ratio’s effect on strength.
- ACI 211.1, Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete — mix design method.
- The 1.54 dry-bulking allowance and 1440 kg/m³ cement bulk density are typical working values used across the industry — they shift with your actual product, so check the bag or batch sheet before you order at scale.