Concrete mix ratios

Two different “mix ratios” get treated as the same chart. One is Table 9 of IS 456:2000, which was read for this page. The other is the dry-volume split this site’s calculator already performs. They do not use the same units. This page keeps them apart so a 1:2:4 order is not described as a grade it is not. The arithmetic stays on the calculator.

What IS 456 calls a nominal mix

Clause 9.1.1 says proportions come from a design mix or from a nominal mix. Design mix is preferred. If it cannot be used, nominal mixes may be used for M20 or lower, with the engineer-in-charge’s permission, and the clause notes that this is likely to use more cement. Clause 9.3 repeats the limit and points the proportions at Table 9.

Table 9 is headed “Proportions for Nominal Mix Concrete.” Each row is a grade, the total quantity of dry aggregates by mass per 50 kg of cement, the proportion of fine aggregate to coarse aggregate by mass, and the maximum quantity of water per 50 kg of cement. The fine-to-coarse proportion is generally 1:2, subject to an upper limit of 1:1½ and a lower limit of 1:2½. The note says to move from the upper limit toward the lower limit as the fine aggregate becomes finer and the coarse aggregate becomes larger. For average fine aggregate in Zone II of IS 383, the note gives 1:1½, 1:2, and 1:2½ for a maximum aggregate size of 10 mm, 20 mm, and 40 mm. That 1:2 is sand-to-stone by mass. It is not “one part cement, two parts sand.”

The table also has M5 and M7.5, at 800 kg and 60 litres, and 625 kg and 45 litres. They are leaner than the grades this page was asked about, and they are still nominal mixes under the same clause. M25 and above are design mixes. Calling a volume ratio “M25 nominal” is the shortcut Table 9 does not contain. If the specification says M25, the mix is the one the engineer designed and the lab tested, not a button on a volume calculator.

Water

The water column is a maximum. Thirty-four litres on 50 kg is 0.68, 32 litres is 0.64, and 30 litres is 0.60. Those are the caps, written as a ratio. Clause 9.3.1 says that if more water is needed to place and compact the concrete, the cement increases in proportion so the specified water-cement ratio is not exceeded.

Clause 10.2.5 says it is important to keep the water-cement ratio constant at its correct value, and that the moisture in both the fine and the coarse aggregate has to be determined as often as the job requires. Water already in the pile counts toward the litres in the table. The calculator on this site does not ask for that moisture, and it does not check the 34, 32, or 30 litre cap. It splits dry bulk volume. The water is still your measurement.

The volume ratios on this site

The cement, sand, and aggregate calculator is a dry-volume estimate. The ratios in its menu are 1:2:4, 1:1.5:3, 1:3:6, and 1:2:3, written as cement : sand : aggregate. The dry-volume box starts checked at 1.54. Densities start at 1,440 kg/m³ for cement, 1,600 for sand, and 1,500 for aggregate. Bags are 50 kg. The page’s own labels call 1:2:4 standard structural, 1:1.5:3 high-strength, 1:3:6 lean, and 1:2:3 slabs and general work. Those labels are the page’s. They are not Table 9, and this guide does not promote them into M-grades.

For 1 cubic metre wet, with 1.54 on, the dry batch is 1.54 cubic metres. A 1:2:4 mix has 7 parts, so cement is 1.54 × 1/7 = 0.22 cubic metres, sand is 1.54 × 2/7 = 0.44, and aggregate is 1.54 × 4/7 = 0.88. The order line prints cement as 0.22 m³, 7.77 CFT, 0.32 tonnes, and 7 bags. Sand is 0.44 m³ and 15.54 CFT. Aggregate is 0.88 m³ and 31.08 CFT. One cubic foot is 0.028316846592 m³, and the CFT line uses the unrounded volume, then two decimals. At 1,440 kg/m³ the cement mass behind the 0.22 m³ line is 316.8 kg. The tool prints 0.32 tonnes. The bag count rounds up from 316.8/50, so it reads 7, not 6.

The same 1 cubic metre, still with 1.54 on, prints the other three ratios as the calculator’s table already has them. A 1:1.5:3 mix, 5.5 parts, is cement 0.28 m³ and 9.89 CFT, 9 bags; sand 0.42 m³ and 14.83 CFT; aggregate 0.84 m³ and 29.66 CFT. A 1:3:6 mix is cement 0.15 m³ and 5.44 CFT, 5 bags; sand 0.46 m³ and 16.32 CFT; aggregate 0.92 m³ and 32.63 CFT. A 1:2:3 mix is cement 0.26 m³ and 9.06 CFT, 8 bags; sand 0.51 m³ and 18.13 CFT; aggregate 0.77 m³ and 27.19 CFT. Uncheck 1.54 on the 1:2:4 cubic metre and the cement line drops to 5 bags. Use the checked box when the 1 cubic metre is the concrete you will have placed, and you are ordering loose dry materials.

The 1.54 factor was not in the IS 456 clauses read for this article. It is this calculator’s allowance for the voids in a dry pile, so the loose materials fill a finished cubic metre. Treat it as that planning factor. Do not cite it back to Table 9.

Weighing, and what the other calculators do

Clause 10.2.4 allows volume batching only where weigh-batching is not practical, after the bulk densities of the actual materials are known, with a bulking allowance from IS 2386 (Part 3). The volume calculator is an order split. The calculator page says it does not replace a lab mix design.

A pour that is already specified as cubic yards of concrete, with no ratio to split, is the concrete calculator. It starts at 2,400 kg/m³, about 2.02 short tons per cubic yard, and its 12 by 24 foot example at 4 inches is 3.56 cubic yards and 7.19 short tons. That is the weight of the concrete, not the cement inside it. A pile of sand with no cement and no stone split is the concrete sand calculator. Bags of a premixed product, where the plant or the bag label has already done the ratio, are the concrete bag calculator and the bags versus ready-mix guide. Cement alone, once you already have a mix volume, is the cement calculator.

Sources

• IS 456:2000, Plain and Reinforced Concrete — Code of Practice. Clauses 9.1.1, 9.3, 9.3.1, 10.2.3, 10.2.4, and 10.2.5, and Table 9. The PDF opened is the law.resource.org copy, reaffirmed text as printed in that file.
• The 1:2:4 quantities and the 1.54 factor are this site’s cement, sand, and aggregate calculator, not Table 9. The four ratio lines above are the lines that tool prints for 1 cubic metre with the dry-volume box checked.

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