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 share of each material, not the amount. To turn that into real bags and litres for a job, 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. Those gaps 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.
Water is worked out on its own, from the water-cement ratio (see below). It is 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 the same here.
So 1 m³ of 1:2:4 concrete takes roughly 16 bags of cement and 158 litres of water. Sand comes in at 0.44 m³ and aggregate at 0.88 m³. Add 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) is the weight of water divided by the weight of cement. That number 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. That means cement and water together, kept in ratio. Do not add 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.