I was recently looking at double-walled coffee cups - thrilling, I know! The specific gift request was for "double-walled coffee cups - large". By large, I took it to be at least 500 ml.
Double walled coffee cups are hard to find, but I did find some in the end from Argos. They had a stated volume of 350ml or 12 fluid ounces. Few people in this day and age in Britain know what an imperial fluid ounce is, and rightfully so. With the exception of draught beer and cider, and milk in returnable containers, liquids are sold in metric units and have been more and more since the late 1970s.
It therefore follows that drinking vessels (with some exceptions) should, when sold, mention only metric volumes. So for these coffee cups, there should not need to be any mention of it being 12 fluid ounces.
Whilst dual labelling is a bit cumbersome, it is not the end of the world. After all, priority was given to the metric units and it is probably a helpful step for the older generation. The problem with 'old school' liquid measurements is that it can be quite confusing precisely which version of old school measurements you are using.
In this case:
1 imperial fluid ounce = 28.4 ml
12 imp. fl oz = 341 ml
Since that's not right, it must be US fluid ounces.
1 US fluid ounce = 29.6 ml
12 US fl oz = 355 ml
Neither seems quite right, but the published value of 350 ml is closer to 12 US fluid ounces than it is to its imperial cousin.
Dual labelling liquid measurements introduces confusion and inaccuracy. Units which have the same name in the imperial and US customary systems do not have the same value. This difference comes from the fact that the US customary system is 18th century 'snapshot' of imperial measurements at the time. However the imperial units we know today were not defined until 1824. This is in a similar manner to the way that some aspects of the American language reflect English usage at the time.
The confusion comes from not knowing which version of fluid ounces or gallons is being used. I think it's reasonable to assume that a British company selling to the UK market should firstly not be referring to old school liquid measurements (except the pint), and where there is any mention of fluid ounces on Argos's website would have least been imperial. Turns out this was not the case, and imperial and US measurements can be quite different.
| Unit of Measurement | Imperial System | Metric Equivalent | U.S. System | Metric Equivalent |
|---|
| 1 ounce | 1 (fluid) oz. | 28.41 ml | 1 (fluid) oz. | 29.57 ml |
| 1 gill | 5 (fluid) oz. | 142.07 ml | Not commonly used | |
| 1 cup | n/a | n/a | 8 (fluid) oz. | 236.59 ml |
| 1 pint | 20 (fluid) oz. | 568.26 ml | 16 (fluid) oz. | 473.18 ml |
| 1 quart | 40 (fluid) oz. | 1.137 L | 32 (fluid) oz. | 946.36 ml |
| 1 gallon | 160 (fluid) oz. | 4.546 L | 128 (fluid) oz. | 3.785 L |
We've got rid of most imperial liquid measurements and have benefitted from the clarity that that brings. We've got enough Americanisms creeping into our language, we don't need American measurements sneaking in to further complicate our lives.
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