Standards of Weights and Measures Act (SWMA) of 1977, as applicable to sale of packaged foods clearly stipulate that all selling and buying of food and food products must be mostly by weight. Exceptions are there for products like Ice Cream,fruit and vegetable juices and some liquid products which can be sold by volume. But edible oil has to be sold by weight only. The main objective of enacting such laws is supposed to be to protect the consumer from any economic loss due to lack of clarity and transparency. Besides the nutrition labeling is expressed in 100 g of the product though what purpose it serves is still not clear. The metric system made mandatory in India certainly makes it easier for the consumer to do quick calculations about the worth of the product in comparison to others. But insisting on presenting the product only in terms of weight may not be ideal in a country like India where people are familiar with buying in terms of numbers or volume.
Buying 1 kilo of a fruit is rather inconvenient as the number of fruits in that lot can vary depending on the size of the fruits. A consumer is much more comfortable buying half a dozen fruit rather than doing such a transaction by weight. Invariably the trader is more advantageously placed because one can never buy exactly half a kilo of fruits! Besides many consumers have some what hazy idea about the number of fruits that can come in a half kilo lot.
Of course some leeway is given to fruits only to be sold by numbers though this may not apply to vegetables like tomato, beet root, carrots, potato etc. Most of the traders prefer to sell even the fruits by weight rather than by number as they find it is advantageous to them. Similarly milk is purchased by volume though in some places buyers insist on weight as a unit of measure because of the profuse frothing and foaming during milking making it difficult to measure the volume easily. Legally milk is allowed to be sold in packed condition by volume only. Light food materials like puffed rice, cereals, poha, etc are still sold by measures in many places, especially in rural areas, though this is not legal under SWMA.
Weight units score over others when it comes to materials of irregular size and shape. Lower the specific gravity of the product higher will be the value for the volume. Food commodities vary enormously in specific gravity and within the same category of foods, the variations can be very significant. Take for example the case of red chillies. Drier the material lower will be its specific gravity. Small chillies will have higher specific gravity compared to longer and bigger chilli varieties. Consumers can be easily fooled by the volume figures and may end up getting more or less of the material depending on specific gravity and comparison becomes difficult. Same is true with flours where particle size makes a big difference in the volume scale, finer flours having lesser bulk density compared to the coarse flours.
Probably the weight units were decided as the most desirable means of trading because of its advantage of comparison amongst similar products of different manufacturers and helping the consumer in taking value decisions. But allowing sale by counts or volume in many cases also should be allowed in the interest of the consumer, especially not so well educated ones and such practices should not be branded as an offense.