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. For this numerical example only, assume dry volume ≈ wet volume × 1.54. Actual yield must come from the approved mix and materials; 1.54 is not a universal batching factor.
- 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. For this example only, assume a cement bulk density of 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
This arithmetic example assumes 1 m³ at a nominal 1:2:4 ratio. It is not a batch recipe or a specified strength 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 an assumed 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. This is total water, not necessarily water to add. Account for material moisture and other water sources in the approved batching method.
Under these assumptions, the estimate is 316.8 kg of cement (buy 16 bags of 20 kg) and 158.4 litres of total water. Buying 16 bags does not mean batching all 320 kg into this unchanged example. Sand comes in at 0.44 m³ and aggregate at 0.88 m³. Agree a suitable order allowance with the supplier for the real mix.
Example nominal proportions
| Ratio (cement : sand : aggregate) | Total parts |
|---|---|
| 1 : 1.5 : 3 | 5.5 |
| 1 : 2 : 4 | 7 |
| 1 : 3 : 6 | 10 |
These ratios illustrate counting parts. They do not specify a strength class or establish suitability for a structural application.
The water-cement ratio is one mix-design input
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. It affects strength, but so do the materials, air content, mixing, compaction and curing. It cannot establish a strength class by itself.
With other factors controlled, extra water can reduce strength. Do not use this as a reason to reduce the specified water on site.
Use the water-cement ratio and workability limits in the approved mix specification. Do not adjust a batch by adding water, cement paste or an admixture from this example. Ask the supplier for the permitted adjustment procedure. See NRMCA’s concrete-materials guidance.
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-volume factor and 1440 kg/m³ bulk density are assumptions in this arithmetic example. They have not been verified for a product. Use an approved mix and actual material data for batching.