Nominal Mix vs Design Mix Concrete
“Nominal mix” and “design mix” describe two different ways of arriving at concrete proportions, and IS 456:2000 does not allow either one everywhere. This page explains the difference, when each applies, and the commonly used nominal mix proportions.
What a nominal mix is
A nominal (prescribed) mix uses fixed proportions given directly in the code, without any design calculation for the actual materials on site. IS 456:2000 permits nominal mixes only for concrete up to grade M20, and only for smaller or less critical works.
What a design mix is
A design mix has its proportions worked out for the actual materials being used, following the IS 10262:2019 method. IS 456:2000 requires a design mix for grade M25 and above. Because a design mix uses the real properties of your cement and aggregates rather than a generic ratio, it is usually more economical and more reliable than a nominal mix.
Commonly used nominal mix proportions
Proportions are cement : fine aggregate : coarse aggregate, by volume:
| Grade | Cement : Fine aggregate : Coarse aggregate |
|---|---|
| M5 | 1 : 5 : 10 |
| M7.5 | 1 : 4 : 8 |
| M10 | 1 : 3 : 6 |
| M15 | 1 : 2 : 4 |
| M20 | 1 : 1.5 : 3 |
These proportions are by volume and are deliberately conservative. Where materials are tested, the design-mix method is preferred.
What IS 456 actually allows for nominal mixes
IS 456 permits nominal mix concrete only for grades up to M20. Table 9 gives, for each nominal grade, the total quantity of dry aggregate per 50 kg of cement and the maximum water per 50 kg of cement:
| Grade | Total aggregate per 50 kg cement (kg) | Water per 50 kg cement (litres) |
|---|---|---|
| M5 | 800 | 60 |
| M7.5 | 625 | 45 |
| M10 | 480 | 34 |
| M15 | 330 | 32 |
| M20 | 250 | 30 |
The fine-to-coarse split is adjusted from about 1:1.5 to 1:2.5 by mass depending on grading. Note that a nominal M20 at 30 litres per 50 kg cement already implies a w/c of 0.60 — above the IS 456 limit for moderate exposure — which is one reason design mixes are preferred for exposed RCC.
How a design mix sets its target
A design mix is proportioned for a target mean strength above the characteristic strength, to allow for variability. IS 10262:2019 uses f′ck = fck + 1.65 s (or a minimum margin, whichever is greater), with assumed standard deviations of 3.5 N/mm² for M10–M15, 4.0 N/mm² for M20–M25 and 5.0 N/mm² for M30–M60 until site data is available. For M25 that gives at least 25 + 1.65 × 4.0 = 31.6 N/mm² before the minimum-margin check.
Volume batching and bulking of sand
Nominal mixes are often batched by volume on small sites. IS 456 prefers weigh batching and, where volume batching is allowed, requires allowance for the bulking of damp sand — moist sand can occupy considerably more volume than the same mass of dry sand. Without that correction, a volume-batched mix has less sand than intended and more water, making the nominal proportions unreliable.
Frequently asked questions
Can I use a nominal mix for M25?
No. IS 456 requires a design mix for grade M25 and above; a nominal mix is not permitted at that grade.
Why is a design mix more economical?
A design mix uses the actual properties of your materials, so it does not need to over-use cement just to stay safe the way a generic prescribed ratio does.