One cubic metre of M20 concrete with a nominal mix of 1:1.5:3 needs about 8 bags of cement (403 kg). It also needs about 0.42 m³ (14.8 cft) of sand and 0.84 m³ (29.7 cft) of coarse aggregate (gitti).
These are standard textbook quantities for nominal mix concrete. The working below uses a dry volume factor of 1.54 and a cement density of 1440 kg/m³.
Cement bags for 1 m³ of M20 concrete: step by step
- Wet volume = 1 m³.
- Dry volume = 1 × 1.54 = 1.54 m³. Dry materials lose volume once water is added and the fine particles fill the gaps between stones, so you need about 54% more dry material than the finished concrete.
- Sum of ratio = 1 + 1.5 + 3 = 5.5.
- Cement = 1.54 × 1 ÷ 5.5 = 0.28 m³. Weight = 0.28 × 1440 = 403.2 kg. Bags = 403.2 ÷ 50 = 8.06, say 8 bags.
- Sand = 1.54 × 1.5 ÷ 5.5 = 0.42 m³ = 0.42 × 35.3147 = 14.83 cft.
- Aggregate = 1.54 × 3 ÷ 5.5 = 0.84 m³ = 29.66 cft.
One 50 kg bag of cement has a volume of 50 ÷ 1440 = 0.0347 m³ (about 1.23 cft). That is why 0.28 m³ of cement works out to roughly 8 bags.
Table for M15, M20 and M25 per m³
| Grade (nominal mix) | Cement (kg) | Cement (bags) | Sand (m³ / cft) | Aggregate (m³ / cft) |
|---|---|---|---|---|
| M15 (1:2:4) | 316.8 | 6.34 | 0.44 / 15.54 | 0.88 / 31.08 |
| M20 (1:1.5:3) | 403.2 | 8.06 | 0.42 / 14.83 | 0.84 / 29.66 |
| M25 (1:1:2) | 554.4 | 11.09 | 0.385 / 13.60 | 0.77 / 27.19 |
IS 456 lists nominal mixes only up to M20. For M25 and above, a design mix from lab trials is the proper method, and it often uses less cement than the 1:1:2 rule of thumb. Treat the M25 row as a rough site check only. These figures do not include wastage; many estimators add 2 to 5%.
Worked example
A slab needs 4.5 m³ of M20 concrete. Cement = 4.5 × 8.064 = 36.3, so order 37 bags. Sand = 4.5 × 14.83 = 66.7 cft. Aggregate = 4.5 × 29.66 = 133.5 cft.
Use the concrete calculator for other volumes and mixes, and the unit converter to switch between m³ and cft.
Frequently asked questions
Why 1.54 and not 1.52 or 1.57?
Different books use values from about 1.52 to 1.57. The difference is small, around 2% in cement. 1.54 is widely used in Nepali and Indian estimating practice.
How much water does 1 m³ of M20 need?
At a water-cement ratio of 0.45 to 0.50, it is about 181 to 202 litres for 403 kg of cement. Adjust on site for wet sand.
Does M20 always mean 1:1.5:3?
No. M20 means a characteristic strength of 20 N/mm² at 28 days. The 1:1.5:3 nominal mix is one way to aim for it. Cube tests confirm whether you got it.
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