We look at some of the consequences when there is more than one option for supplementary units on product labels.

In the UK, metric is the primary system of measurement for all purposes except for draught beer and cider, doorstep milk, and of course road traffic signs for speed and distance. It must always appear on those product labels that are required to show quantities. Labels may also show other units, so what are the rules for these ‘supplementary indications’ (SIs)?

The late, lamented LACORS provided advice on SIs as long ago as 31 July 1995:

“Neither the (EU) Directive nor the implementing UK legislation offer guidance on either the appropriate units to be used as supplementary indications, nor upon the accuracy of the indication in absolute terms.”

In other words, SIs can be in any units except metric and do not have to be accurate. That said, the UK Quantity Marking and Abbreviations of Units Regulations require that the supplementary indication should be in an equivalent type of unit – using grams and ounces together is OK, but using grams and fluid ounces is a no-no. Furthermore, there is a general requirement that product labelling should not deliberately mislead.

Since the 1960s, the pound and the foot have had the same meaning on both sides of the Atlantic (with a few unimportant exceptions pointed out by John Steele in a recent comment on MV). But this happy state of affairs does not apply to non-metric measures of volume. The US pint is less than as the UK pint (drinkers beware), and the US gallon and fluid ounce differ from their obsolete British counterparts. Confusion over mpg frequently traps the unwary, appearing to be about 17% less in the US, but what about confusion over fluid ounces on product labelling in the UK?

Some products sold in the UK use the US definition of the fluid ounce of 29.6 mL, presumably to meet US labelling requirements. The UK fluid ounce is 28.4 mL, so labels showing US fluid ounces are acceptable in the UK as the buyer will receive at least as much as expected and possibly more. (The opposite would be true were UK labels to show US quarts and pints. This would clearly be unacceptable and accordingly such labels appear rarely in the UK, and then usually on products intended to be sold in both countries with US measures clearly distinguished).

Examples of US fluid ounce labels are not difficult to find. A quick look around the shelves at Tesco recently revealed two containers of cream next to each other where the metric quantity was the same but the SIs differed. One was labelled “300 ml 10 fl oz” and the other was labelled “300 ml 10.6 fl oz”. Consumers might ask, “Are they the same size or is one bigger than the other?”

While such product labels may be legal, the use of two different fluid ounces can limit the usefulness of the information. For example, although recipes published over the past 25 years have usually been metric or have metric alternatives, cooks of the old school, accustomed to adding so many fluid ounces, might be well advised to measure rather than rely on the label. Better still, how about replacing those old cook books and scales with metric or dual ones?

Another mistake that further reduces the value of fluid ounces on UK product labels is the fact that some products contain incorrect conversions, even after allowing for rounding, using either the UK or US definitions. There are, for example, some aftershaves sold in the UK that use a conversion factor of 30 mL per fluid ounce.

Possibly, the most annoying occurrence of the US fluid ounce is its use as a description rather than a measure. That “16 oz grande” in Starbucks – just how much coffee am I getting? Actually, it will be about 473 mL.

Clearly, multiple definitions of measurement units are not helpful. Supplementary indications are likely to be with us for many years to come, but perhaps producers can be encouraged to abandon fluid ounces and use only millilitres and litres – the litre is the same all over the world unlike the fluid ounce! However, it may take a change in US labelling laws to allow metric-only labels before we see a further decline and the eventual disappearance of supplementary indications.

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