US Weights & Measures
Page updated:
28-Jan-2007
© 1996-2008
Mike Todd
Most American weights and measures are the same as those in the UK, but there are a few notable exceptions. The one that most people know is the gallon, which means that other liquid measures (pint, fluid ounce etc) are also different. But the ton and the hundredweight are also different, as are the dry measures, such as the bushel.
However, some believe that American distances are also different - in fact American measures of distance/length are the same as the British.
Measuring volumes
At one time, the British gallon differed depending on what you
were measuring and where you were measuring it. But, by the 19th century,
two definitions had survived: the Queen Anne (or Wine) gallon,
which was 231 cubic inches, and the Winchester (or Ale) gallon,
of 282 cu in.
In 1824, the British abandoned both the Ale and the Wine gallons in favour of the Imperial gallon, based on the volume of 10 pounds of water (which works out at 277.41945 cu in). America, by this time, had already standardised on the Wine Gallon of 231 cubic inches (strictly speaking, this was defined as the volume of a cylinder 6 inches long and 7 inches in diamete, or, using the old approximation for pi, 231 cubic inches).
The result is that the US gallon is 83.267% of the British gallon. In more usable terms, the British gallon is about a fifth greater than the US gallon, and the US gallon is about 5/6 of the British gallon (or a little under 7 British pints)
And, yes, this has a knock-on to other liquid measures - like Britain, the US has 4 quarts, 8 pints or 32 gills in a gallon, so these measures are also smaller than in the UK. On the other hand, the Americans have 16 fluid ounces in their pint, whereas the British have 20.
The reason for his is that, in the British system, a gallon of water weighs 10lbs, or 160 ounces, and so there are 160 fluid ounces in a gallon, and 20 fluid ounces in a pint - from this, it follows that a fluid ounce of water does actually weigh an ounce.
Therefore, an American fluid ounce is greater than the British fluid ounce by about 4%, and an American flluid ounce of water doesn't weight an ounce.
Measuring dry volumes
Of course it isn't just liquids that are measured by volume. While the
Imperial system allows gallons to be used as both liquid or dry measures
(8 gallons = 1 bushel), the US system keeps them separate. The Americans
adopted the old British Winchester Bushel for their dry measures, and
this is defined as 2150.42 cu in (equivalent to 0.97 British Bushels).
Like the gallon, this was not an arbitrary volume, but was based on
the volume of a container of 18.5 inches diameter, and 8 inches in height.
Like the Imperial bushel, the US bushel is divided into 4 pecks, 8 gallons, 32 quarts or 64 pints - so that the US dry volumes are different in size to their liquid volumes, even though the names are the same.
So, not only are gallons different, but so are bushels, pints, and fluid ounces - and each by a different factor!
And what about weights?
Well, British and American (dry) pounds and ounces are the same, so
you might think that their tons are the same too, since a ton is 2240
pounds. Unfortunately not!
The Americans adopted what is often known as the short ton, so if you ask for a ton of something in the US, you'll get 2000 lbs of it, and not 2240 lbs. And a hundredweight in the US is 100 lbs, compared to the British cwt, which is 112 lbs. Like the volume measures, this difference arises from the Americans selecting one of several different definitions in common use in the 16th and 17th centuries.
And what about the metric system?
Many people are surprised to learn that America officially adopted the
metric system over a century ago! Not only that, the US has been officially
a "metric" country longer than the UK.
It comes as an even bigger surprise to learn that the only measurement system legally sanctioned by Congress is the metric system. This was done in a law passed in 1886 which specifically made it lawful to use the metric system and which made it unlawful to refuse to trade or deal in metric quantities. There is no such legislation for gallons and pounds!
In 1964, the American Bureau of Standards made the metric system (officially known as the SI System) its standard, "except when the use of these units would obviously impair communication or reduce the usefulness of a report". In 1967, Congress started an investigation as to whether full metrication should be embarked on and, in 1971 declared that its recommendation was to phase it in through a coordinated national programme over a period of ten years.
It was Gerald Ford, in 1975, who put this into law, under the Metric Conversion Act of 1975. However, compliance with the Act was purely voluntary. In 1988 came further legislation which declared that the metric system was the preferred system for US trade and commerce. Yet, despite all these attempts, the US population has resisted metrication and metric quantities appear in very few areas. A few roads in the southern states are marked in miles and kilometers, although you increasingly hear metric measurements used on news bulletins.
Measuring for cooking
Here the Americans steadfastly stick to a system based on cups and spoons.
Recipes usually give dry measures in cups, and liquid measures in a mixture
of cups and pints. Spoons are also used, and occasionally you'll see butter
measured in fractions of a "stick", since butter in the US comes
in long 4oz blocks, conveniently marked in quarters.
If you're cooking from an American recipe book, it's probably easier (and certainly more reliable) to get a set of US measures rather than trying to work out just how much 1¾ cups of flour weigh. For more details, see the American Cookery pages.
There are tables on the next page indicating the major measurement differences.