METHOD NOTE · VERSION 1.0 · UPDATED 17 SEPTEMBER 2026
What does M20 mean?
In the Indian concrete grade designation, M20 refers to a characteristic 28-day compressive strength of 20 N/mm², conventionally assessed using a standard cube specimen. The designation is a performance requirement, not a guarantee delivered by mixing three loose materials in a fixed proportion.
The 1:1.5:3 cement:sand:aggregate proportion is a familiar nominal estimating reference. It is useful here for showing how material quantities are divided, but it is not a substitute for a project-approved mix. Structural use requires the relevant local specification, material control, water management, placement and curing.
Start with one cubic metre
For this worked example, assume a net wet concrete volume of exactly 1 m³, no waste, a dry-volume factor of 1.54, cement bulk density of 1,440 kg/m³ and a 50 kg cement bag. Keeping waste at zero makes it easier to see the underlying arithmetic.
Total ratio parts = 1 + 1.5 + 3 = 5.5
The dry factor accounts approximately for the difference between loose ingredients and the volume of combined concrete. It is not a measured yield for every aggregate grading or a code-mandated constant. A supplier’s verified mix data is preferable for a real project.
Cement: volume, mass and bags
Cement mass = 0.28 × 1,440 = 403.2 kg
50 kg bag equivalents = 403.2 ÷ 50 = 8.064 bags
For this standalone purchase, round to 9 whole bags. The equivalent count describes theoretical consumption; the whole-bag figure describes an order. Do not replace the calculated cement mass with nine bags inside the mixture without adjusting the rest of an approved batch.
Sand and aggregate
Aggregate volume = 1.54 × 3 ÷ 5.5 = 0.84 m³
At the tool’s default loose bulk densities of 1,600 and 1,450 kg/m³, those volumes correspond to 672 kg of sand and 1,218 kg of aggregate. The mass changes if the density changes. Sand bulking, moisture and the grading of aggregate make volume and mass conversion project dependent.
| Material | Loose volume | Estimated mass |
|---|---|---|
| Cement | 0.28 m³ | 403.2 kg |
| Sand | 0.42 m³ | 672 kg |
| Aggregate | 0.84 m³ | 1,218 kg |
Add an explicit allowance
If you choose 5% waste, first increase the wet volume to 1.05 m³. The model then scales each material by 1.05: 423.36 kg cement, 0.441 m³ sand and 0.882 m³ aggregate. Do not apply waste again to these outputs. Whole cement bags still round to 9 in this example.
Changing the bag size
The material requirement does not depend on the packaging. Divide the same 403.2 kg by 25 kg and you get 16.128 equivalent bags, or 17 whole bags. For 40 kg bags, it is 10.08 equivalents, or 11 whole bags. If several pours share a stock of cement, plan the combined order rather than rounding each small pour separately.
What this example cannot establish
The estimate does not determine the water-cement ratio, workability, exposure class, durability, reinforcement, or structural suitability. Those decisions depend on the project. The Concrete Society’s mix-design guidance identifies strength, workability and water-cement limits among the parameters needed for a designed mix. A ratio alone cannot settle them.
For ready-mixed concrete, use the finished wet volume as one input to an order discussion and agree the performance specification with the producer. The NRMCA ordering and scheduling checklists provide a practical starting point for that conversation.
Related references
See the calculation methodology for defaults and rounding, or browse the concrete calculator and guide hub.