Supermarkets here are required by law to display "per unit" prices as well as the cost of the item, so it is easy to make a comparison of $X per litre etc. They still find ways to confuse the consumer though. For example in the fruit and veg section loose tomatoes may be displayed at $4.36 per kg while some prepacked ones are $4.24 per pack. Of course, the pack is only 750g, not a kg. It seems that it is the supermarket, not the law, that defines what unit is used.