Codes & standards referenced
What Table 9 actually specifies
Open IS 456 at Table 9 and you will find no "1:2:4". The table gives the total dry aggregate mass per 50 kg of cement and the maximum water per 50 kg of cement. The familiar ratios are conventional volume-batching equivalents — derived from the mass basis, close enough for the leaner grades, and running noticeably richer in cement at M15 and M20. Both views are legitimate; confusing them is not.
| Grade | Dry aggregate per 50 kg cement | Water per 50 kg cement, max | Conventional equivalent |
|---|---|---|---|
| M5 | 800 kg | 60 L | 1 : 5 : 10 |
| M7.5 | 625 kg | 45 L | 1 : 4 : 8 |
| M10 | 480 kg | 34 L | 1 : 3 : 6 |
| M15 | 380 kg | 32 L | 1 : 2 : 4 |
| M20 | 320 kg | 30 L | 1 : 1½ : 3 |
Reference: IS 456:2000 Table 9 (Cl 9.3). The aggregate splits FA : CA = 1 : 2 by mass generally, adjustable within 1 : 1½ to 1 : 2½ for grading and maximum size of aggregate.
Read the M20 row as a recipe: one bag of cement, 320 kg of dry aggregate, at most 30 L of water. With the FA : CA default of 1 : 2, that 320 kg splits to roughly 107 kg of sand and 213 kg of coarse aggregate. And Cl 9.3.1 adds the discipline most sites skip: if more water than the table maximum is used, the cement content must be increased proportionately. The water caps are maxima, not targets.
Where each route is permitted
Cl 9.3 permits nominal mixes for concrete of M20 or lower, and their natural home is minor and plain (unreinforced) works. Reinforced concrete already needs at least M20 under Cl 6.1.3 — so an RCC nominal mix sits exactly at the floor, with no margin for exposure.
Everything above M20 requires a designed mix proportioned per IS 10262:2019. But durability pushes the boundary down, not just up: IS 456 Table 5 asks for minimum grade M25 and maximum w/c 0.50 in moderate exposure — a specification a nominal M20, with its implied maximum w/c of 180/300 = 0.60, cannot satisfy.
That last point deserves emphasis because it is the one most sites get wrong: "nominal up to M20" is a proportioning permission, not a durability permission. Once the exposure condition demands a grade, a w/c ceiling or a minimum cementitious content, only a designed mix can demonstrate compliance — the design route exists precisely to prove, on paper and then in cubes, that all three limits are met simultaneously.
Over-sanding and yield quirks
Compare the two routes for one cubic metre of M20. The Table 9 mass basis, converted through absolute volume, needs about 6 bags of cement per m³. The conventional 1 : 1½ : 3 equivalent, batched by volume with the usual dry-factor assumption of 1.54, calls for about 8.1 bags — roughly a third more cement for the same grade label. At M15 the gap is 6.3 bags against 5.2. Neither is "wrong"; they are different recipes, and only the mass basis is the code's.
| Grade | Table 9 mass basis, bags/m³ | Conventional ratio, bags/m³ |
|---|---|---|
| M15 | 5.2 | 6.3 |
| M20 | 6.0 | 8.1 |
Bag counts computed for 1 m³ of finished concrete; the Table 9 route uses absolute-volume yield at cement sp. gr. 3.15 and aggregate sp. gr. 2.65, the ratio route a stated dry factor of 1.54.
The yield quirks follow from the same confusion. Volume-batched mixes swell when moist sand bulk up, so a "1 m³" batch by head-pans delivers less concrete than the volumes suggest, and the leaner the mix the more the sand fraction dominates the error. A bag-based, mass-batched Table 9 recipe is immune to bulking — a scale does not care how much air the sand is carrying.
Three errors that decide what you actually batched
Moist sand occupies more volume. A recipe calling for 0.42 m³ of dry sand delivers only about 0.34 m³ of sand when you measure 0.42 m³ of moist sand bulking 25% — under-sanded, harsher mix, honeycomb risk. Measure bulking per IS 2386 (Part 3) and batch the moist sand at sand × (1 + bulking%), 0.53 m³ in this case.
Wet stockpiles carry free water that lands in the mix uninvited. Weigh aggregates at their moist stock mass and credit the free water against the batch water, or your effective w/c climbs with every load.
Cl 9.3.1: if more water is used than Table 9 allows, cement content increases proportionately. Adding water to "make it workable" without adding cement is not a nominal mix any more — it is a weaker, ungraded one.
The nominal mix calculator handles both Table 9 routes and the bulking correction from your IS 2386 (Part 3) test, and the trial mix batch book converts any batch — nominal or designed — to moist-stockpile weights with the water credit applied.
Crossing over to the design route
When the specification says M25, or an exposure condition beyond mild, the question changes from "how many head-pans per bag" to "what target mean strength and what durability ceiling". The full IS 10262:2019 procedure — target strength at fck + 1.65σ, Table 4 water, the Annex B corrected aggregate split — is worked end to end in our M25 walkthrough, and what those cubes must then prove is covered in the IS 456 acceptance explained companion. Mix-design vocabulary from characteristic strength to water–cement ratio is defined in Part 6 of our glossary.
Ready to design instead of guess?
The IS 10262:2019 mix design report calculator carries a design from target strength to batch weights with every table step shown, and the nominal mix calculator keeps Table 9 honest for the lean grades. Use the right tool for the grade you are actually pouring.
Related articles
IS 10262:2019 Mix Design Walkthrough
The design route in full: an M25 carried from target strength to batch weights.
IS 456 Cube Test Acceptance Explained
What the cubes from either route must prove under Table 11.
Concrete Grade Selection
Choosing M20 vs M25 vs M30 by exposure, element and durability, not by habit.
Amit Haridas
Founder & Proprietor, ConcreteInfo. 25+ years of experience in concrete technology, RMC plant operations, construction quality, consulting and technical training across India.