On EU product labels, metric units are mandatory whereas non-metric units are optional. On US product labels, both metric and US customary (USC) units are mandatory for most products. So a company that wants to sell a product in the EU and the US must use metric and USC on the label unless it produces separate labels for the two markets.
For the UK market, there is one major problem with US labelling requirements. Both the UK and US use fluid ounces, pints and gallons but their quantities differ. US labelling requirements have led to the use of US fluid ounces, US pints and US gallons on products sold in the UK. This is in parallel with the optional use of imperial fluid ounces, imperial pints and imperial gallons on labels for comparable products. This article shows some examples.
In the images shown above, we have 3 different versions of the fluid ounce used: the UK fluid ounce of 28.4 ml, the US fluid ounce of 29.6 ml and the US nutrition fluid ounce of 30 ml. The latter is often used for aftershaves sold in the UK. For more information about the mess with fluid ounces, see the Metric Views article, The use and abuse of fluid ounces.
There are a few products sold in the UK that show US pints and US gallons on the label, as shown in the following examples:
Apart from the Country Life milk label in the bottom of the first image, all the labels in the images above show USC units. These labels show conversions from fluid ounces into pints and gallons. Given that there are 20 imperial fluid ounces in a pint and only 16 US fluid ounces , it is clear that the pints and gallons shown in the other labels are USC units.
The US gallon, which is exactly 231 cubic inches, was known in Britain as the wine gallon and was one of several in use until 1824, when the Weights and Measures Act adopted a single definition – the volume occupied by ten pounds of water. EU labelling rules continue to allow supplementary indications using any measures, although the worldwide adoption of the metric system in the last century effectively limits these to imperial and USC.
Because the US insists on USC appearing on labels, imperial volume measures are rarely shown on products sold in the US. However, both USC and imperial volume measures appear on labels in the UK, though rarely side-by-side. Hence, we have ended up with more than one definition for the fluid ounce, the pint and the gallon, which is confusing and undermines transparency and consumer protection.
The safest course of action is to ignore these obsolete supplementary indications, and to focus on the primary metric quantity, which will always be shown. Those who fail to do so have only themselves to blame if they become confused.