I spent last night filling 40-litre bags of mulch (don't ask) which got me to thinking: Why does the metric system use the same volume measurement (litres) for liquids and dry goods, while the US does not? Even worse, why does the US use the same terms for entirely different concepts?
Let's start with the ounce. As a unit of weight, it's 1/16th of a pound. As a unit of volume, it's 1.8 cubic inches. For water, 1 ounce of volume is about one ounce of weight. (And one pint of water weighs about one pound.) So that makes sense for water, but not for other liquids: One ounce of canola oil weighs 0.95 ounces while one ounce of honey weighs 1.48 ounces. To deal with this confusion, "fluid ounce" is supposed to be used for volume and "ounce" for weight, but they are still often used interchangeably.
The US (liquid) gallon is defined as 128 fluid ounces of water, which is 231 cubic inches. However, the US (dry) gallon is defined as 1/8 of a US bushel, which is 268.8 cubic inches. In other words, a dry gallon has a 16% greater volume than a liquid gallon.
"Dry" gallons have become obsolete in the US, but the "dry" pint is still used in supermarkets.
Teaspoons, tablespoons and cups have the same volume for liquid or dry measures (0.3, 0.9 and 14.4 cubic inches, respectively). That is to say, there are two cups in a "liquid" pint but 2.33 cups in a "dry" pint.
In the metric system, 1 liter is always 100 cubic centimeters, so moving between the two is very straightforward. (30 litres = 0.03 cubic meters, which is roughly 1 cubic foot.)
So I could have described my 40 litre bag as 9 "dry" gallons or 10 "liquid" gallons, but Americans would have looked at me like I was crazy.
The "official" US dry measurements are pint, quart, peck (8 quarts) and bushel (4 pecks, about 1.24 cubic feet). However, there are many other measures, usually for specific commodities:
- sack (cement, 1.15 cubic feet)
- dry barrel (4.08 cubic feet)
- dry hogshead (sugar, 8.4 cubic feet)
- bale (wool or cotton, 13.5 cubic feet)
- ton (as a measure of volume, about 60 cubic feet)
- cord (wood, 128 cubic feet)
The US liquid measures used to be based on the "minim" which was defined as one drop of water. The dram was 60 minim, the teaspoon was 80 minim, and the tablespoon was 3 teaspoons or 4 drams. Of course, this lead to large variances, depending on the temperature and water quality, so when the US system was standardized in 1893 the values were based on -- I kid you not -- the metric system. So now a drop of water is 0.05 grams in weight and 0.05ml in volume. A teaspoon is 5ml (100 drops of water) and a tablespoon is 15ml. The cup should be 236 6 milliliters but the US rounded up to 240ml. (The UK rounded to 250ml.)
One fluid ounce is defined as 2 fluid tablespoons, which is why I started by saying 1 ounce of water weighs about one ounce -- presumably they started out the same, but since the definition of fluid ounce changed, one ounce of water now weighs 1.04 ounces!
Other liquid measurements include:
- jig or shot (3 fluid tablespoons or 1.5 fluid ounces)
- gill (4 fluid ounces or half a cup)
- pottle (2 quarts or half a gallon)
- Beer barrel (31 gallons)
- Standard barrel (31.5 gallons)
- Whiskey barrel (40 gallons)
- Oil barrel (42 gallons)
- hogshead (2 barrels or 63 gallons)
By the way, the US is only one of three countries that have not adopted the metric system. The other two are Liberia and Myanmar. It isn't for lack of trying, however: In 1793, Thomas Jefferson requested a standard kilogram from France that could be used to adopt the metric system in the United States. However, the ship was blown off course by a storm and captured by pirates. The Metric Act of 1866 supplied each state with a set of standard metric weights and measures. In 1893, metric standards were officially adopted as the fundamental standards for length and mass in the United States, with the definitions of US customary units based on metric units. In 1966, Star Trek started using metric measures (albeit inconsistently). In 1975, Congress passed the Metric Conversion Act and in 1988 the Omnibus Foreign Trade and Competitiveness Act designated the metric system as "the Preferred system of weights and measures for United States trade and commerce" and required most federal agencies to use the metric system by the end of 1992. Post-1994 federal law mandates most packaged consumer goods be labeled in both customary and metric units.
On the other hand, TV and screen displays throughout the world are defined in inches. Nobody knows why.
I'll end this where it all started, a UK royal decree in the 13th century: "By consent of the whole Realm the King's Measure was made, so that an English Penny, which is called the Sterling, round without clipping, shall weigh Thirty-two Grains of Wheat dry in the midst of the Ear; Twenty pennies make an Ounce; and Twelve Ounces make a Pound."
