Nominal mixes (M5 1:5:10 up to M20 1:1.5:3) are volume proportions allowed for small works. From M25 upwards IS 456 requires a design mix — proportions established by trial, following IS 10262:2019.
Design mix starts from target strength = characteristic strength + 1.65 × standard deviation, then selects water-cement ratio, water content, cement content, and aggregate proportions, and proves them with trial cubes.
Cement per m³ of M20
≈ 8 bags (400 kg)
Sand per m³ of M20
0.42 m³
Aggregate per m³ of M20
0.84 m³
Water at w/c 0.50
≈ 200 litres
Remember on site
- M20 nominal = 1 : 1.5 : 3 with a dry-volume factor of 1.54.
- M25 and above: design mix per IS 10262:2019 is mandatory.
- Target mean strength always exceeds the grade — that margin is the standard deviation.
Check yourself
What does the number in a concrete grade like M20 represent?
As per IS 456, what does 'characteristic strength' mean?
For grades up to which level does IS 456 permit the use of nominal mix concrete?
Which IS code provides the detailed procedure for concrete mix design in India?
What is the general effect of increasing the water-cement ratio beyond the optimum on concrete strength?
For a nominal mix M20 (1:1.5:3), what is the minimum cement content typically specified per m³?
Approximately how many 50 kg cement bags are needed for M20 nominal mix concrete per m³, given about 320 kg cement content?
Why does a design mix generally result in more economical concrete than a nominal mix of the same grade?
What is the primary reason IS 456 mandates design mix for grades above M20 in most works?
If a design mix trial gives a target mean strength of 31.6 N/mm² for M25 grade, what does this extra margin above 25 N/mm² account for?