One cement bori: kg, litres and cubic feet

The first figure is the sack. IS 269, the BIS specification for ordinary portland cement, says the net quantity of cement per bag shall be 50 kg, subject to the tolerance its Annex B sets out. The Legal Metrology (Packaged Commodities) Rules, 2011 reach the same sack from the legal side. Their Second Schedule lists cement in bags at 1 kg, 2 kg, 5 kg, 10 kg, 20 kg, 25 kg, 40 kg for white cement only, and 50 kg, and the rules keep cement inside the retail chapter for bags up to 50 kg where most other commodities drop out above 25.

The second figure is how much room that 50 kg takes. IS 875 (Part 1), the BIS code a designer works dead loads from, prints a unit weight for every building material a structure has to carry. Item 19 of its Table 1 gives cement at 14.10 kN per cubic metre, which is 1440 kg per cubic metre, with rapid hardening cement lower in the same row at 1280. The 1440 is cement weighed as it sits, which is what a cubic foot figure on a site is asking about.

The rest is division. 50 divided by 1440 is 0.0347 of a cubic metre, and a cubic metre is 1000 litres, so a bori is 34.72 litres. Multiply that volume by 35.3147, the cubic feet in a cubic metre, and the bag comes to 1.226204 cubic feet, the 1.226 the trade quotes and the 1.23 the table prints. Turn the division the other way, 1440 divided by 50, and a cubic metre holds 28.8 bags.

A sack that comes in a shade under 50 kg is not automatically short. Annex B of IS 269 sets out what a sample of bags at the plant has to satisfy: the average net quantity across the sample must be 50 kg or more, no more than 5 percent of the bags may be light by more than 2 percent, which is 1 kg, and no bag at all may be light by more than 4 percent, which is 2 kg. For a wagon or truck load up to 25 tonne the standard holds the overall tolerance on net quantity to between 0 and 0.5 percent.Those are the plant's figures, measured at packing. A sack that has been opened, dropped, or left standing against a damp wall through a wet week is a different question, and how a stack is kept between delivery and dhalai sits on the cement storage in the monsoon guide.

Cement leaves here two ways, by the bori and by the tonne. A tonne is twenty bags, because twenty fifties make 1000 kg, so an order said in tonnes is a bag count in fewer words. Either unit works on the call. Twenty bags also happens to be 694.44 litres and about 24.52 cubic feet, which is the row a thekedar quoting a tonne is standing on without saying so.

The unit changes hands at the drawing. An engineer or an estimator works in cubic feet and cubic metres, because concrete and mortar volumes are set out that way; the counter counts bori, because bori is what is stacked, tagged and loaded. The chart above is the bridge between those two habits and nothing more. It does not say how many bags a job needs. The mix on the drawing decides that, a nominal 1:1.5:3 and a lab design mix giving different counts for the same volume of concrete, and whether the bag should be OPC or PPC is a separate call again.